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We study dilepton production from a quark-gluon plasma of given energy density at finite quark chemical potential μ and find that the dilepton production rate is a strongly decreasing function of μ. Therefore, the signal to background ratio of dileptons from a plasma created in a heavy-ion collision may decrease significantly.
Viscous hydrodynamic calculations of high energy heavy-ion collisions (Nb-Nb and Au-Au) from 200 to 800 MeV/nucleon are presented. The resulting baryon rapidity distributions, the in-plane transverse momentum transfer (bounce-off), and the azimuthal dependence of the midrapidity particles (off-plane squeeze out) compare well with Plastic Ball data. We find that the considered observables are sensitive both to the nuclear equation of state and to the nuclear shear viscosity η. Transverse momentum distributions indicate a high shear viscosity (η≊60 MeV/fm2 c) in the compression zone, in agreement with nuclear matter estimates. The bulk viscosity ζ influences only the entropy production during the expansion stage; collective observables like flow and dN/dY do not depend strongly on ζ. The recently observed off-plane (φ=90°) squeeze-out, which is found in the triple-differential rapidity distribution, exhibits the strongest sensitivity to the nuclear equation of state. It is demonstrated that for very central collisions, b=1 fm, the squeeze-out is visible even in the double-differential cross section. This is experimentally accessible by studying azimuthally symmetric events, as confirmed recently by data of the European 4π detector collaboration at Gesellchaft für Schwerionforschung Darmstadt.
If density isomers exist they can be detected by measuring the excitation function of subthreshold kaon production. When the system reaches the density where the density isomer has influence on the equation of state (which depends on the beam energy and on the optical potential), we observe a jump in the cross section of the kaons whereas other observables change little. Above threshold Λ¯’s or p¯’s may be used to continue the search. This is the result of microscopic Boltzman-Uehling-Uhlenbeck calculations.
This thesis presents a model for the dynamical description of deconfined quark matter created in ultra-relativistic heavy ion collisions, treating quarks and antiquarks as classical point particles subject to a colour-dependent, Cornell-type potential interaction. The model provides a dynamical handle for hadronization via the recombination of quarks and antiquarks in colour neutral clusters. Gluons are not included explicitly in the model,but are described in an effective manner by the means of the potential interaction. The model includes four different quark flavours (up, down, strange and charm) and uses current masses for the quarks. The dynamical evolution of a system of colour charges subject to the Hamiltonian equations of motion of the model yields the formation of colour neutral clusters of quarks and antiquarks, which are subject only to a small remaining interaction, the strong interquark potential notwithstanding. These clusters can be mapped onto hadrons and hadronic resonances. Thus, the model allows a dynamical description of quarks degrees of freedom in heavy ion collisions, including a recombination scheme for hadronization. The thermal properties of the model turn pout to be very satisfying. The model shows a transition from a confining phase to a deconfined phase with rising temperature, going hand in hand with a softest point in the equation of state and a rise of energy density and pressure to the Stefan-Boltzmann limit of a gas of quarks and antiquarks. Moreover, the potential interaction is screened in the deconfined phase. For the dynamical description of ultra-relativistic heavy ion collision, the qMD model is coupled to UrQMD as a generator for its initial conditions. In this way, a fully dynamical description of the expansion and hadronization of the fireball created in such collisions can be achieved. Non-equilibrium aspects of the expansion dynamics and hadronization by recombination of quarks and antiquarks are discussed in detail, and a comparison with experimental data of collisions at the CERN-SPS is presented. The big advantage of the qMD model is the possibility to study cluster formation, including exotic clusters, and fluctuations in a dynamical manner. As an example, event-by-event fluctuations in electric charge are studied. Such fluctuations have been proposed as a clear criterion to distinguish a deconfined system from a hadrons gas. However, experimental data show hadron gas fluctuation measures even at RHIC, where deconfinement is taken for granted. We will see how the dynamics of quark recombination washes out the quark-gluon plasma signal in the fluctuation criterion. Moreover, we will discuss briefly the problem of entropy at recombination. In a second application, the formation of exotic hadronic clusters, larger than usual mesons and baryons, is studied. Such clusters could provide new measures for the thermalization and homogenization of a deconfined gas of colour charges. Moreover, number estimates for exotic clusters from recombination are considerably lower than corresponding predictions from thermal models, providing a clear difference between statistical hadronization and hadronization via quark recombination. A detailed analysis is provided for pentaquark candidates such as the Theta-Plus. It turns out that the distribution of exotic states over strangeness, isospin, and spin could provide a sensitive measure for thermalization and decorrelation in the deconfined quark phase, if it could be measured.
We demonstrate the occurrence of canonical suppression associated with the conservation of an U(1)-charge in current transport models. For this study a pion gas is simulated within two different transport approaches by incorporating inelastic and volume-limited collisions pi pi leftrightarrow K bar-K for the production of kaon pairs. Both descriptions can dynamically account for the suppression in the yields of rare strange particles in a limited box, being in full accordance with a canonical statistical description.
The transverse momentum dependence of the anisotropic flow v_2 for pi, K, nucleon, Lambda, Xi and Omega is studied for Au+Au collisions at sqrt s_NN = 200 GeV within two independent string-hadron transport approaches (RQMD and UrQMD). Although both models reach only 60% of the absolute magnitude of the measured v_2, they both predict the particle type dependence of v_2, as observed by the RHIC experiments: v_2 exhibits a hadron-mass hierarchy (HMH) in the low p_T region and a number-of-constituent-quark (NCQ) dependence in the intermediate p_T region. The failure of the hadronic models to reproduce the absolute magnitude of the observed v_2 indicates that transport calculations of heavy ion collisions at RHIC must incorporate interactions among quarks and gluons in the early, hot and dense phase. The presence of an NCQ scaling in the string-hadron model results suggests that the particle-type dependencies observed in heavy-ion collisions at intermediate p_T are related to the hadronic cross sections in vacuum rather than to the hadronization process itself, as suggested by quark recombination models.
Several observables of unbound nucleons which are to some extent sensitive to the medium modifications of nucleon-nucleon elastic cross sections in neutron-rich intermediate energy heavy ion collisions are investigated. The splitting effect of neutron and proton effective masses on cross sections is discussed. It is found that the transverse flow as a function of rapidity, the Q_zz as a function of momentum, and the ratio of halfwidths of the transverse to that of longitudinal rapidity distribution R_t/l are very sensitive to the medium modifications of the cross sections. The transverse momentum distribution of correlation functions of two-nucleons does not yield information on the in-medium cross section.
Elliptic flow analysis at RHIC with the Lee-Yang Zeroes method in a relativistic transport approach
(2006)
The Lee-Yang zeroes method is applied to study elliptic flow (v_2) in Au+Au collisions at sqrt s =200 A GeV, with the UrQMD model. In this transport approach, the true event plane is known and both the nonflow effects and event-by-event v_2 fluctuations exist. Although the low resolutions prohibit the application of the method for most central and peripheral collisions, the integral and differential elliptic flow from the Lee-Yang zeroes method agrees with the exact v_2 values very well for semi-central collisions.
We propose to measure correlations of heavy-flavor hadrons to address the status of thermalization at the partonic stage of light quarks and gluons in high-energy nuclear collisions, shown on the example of azimuthal correlations of D-Dbar pairs. We show that hadronic interactions at the late stage can not disturb these correlations significantly. Thus, a decrease or the complete absence of these initial correlations indicates frequent interactions of heavy-flavor quarks in the partonic stage. Therefore, early thermalization of light quarks is likely to be reached. PACS numbers: 25.75.-q
In this paper we derive a formula for the energy loss due to elastic N to N particle scattering in models with extra dimensions that are compactified on a radius R. In contrast to a previous derivation we also calculate additional terms that are suppressed by factors of frequency over compactification radius. In the limit of a large compactification radius R those terms vanish and the standard result for the non compactified case is recovered.
The pion source as seen through HBT correlations at RHIC energies is investigated within the UrQMD approach. We find that the calculated transverse momentum, centrality, and system size dependence of the Pratt-HBT radii R_L and R_S are reasonably well in line with experimental data. The predicted R_O values in central heavy ion collisions are larger as compared to experimental data. The corresponding quantity sqrt R_O^2-R_S^2 of the pion emission source is somewhat larger than experimental estimates.
Gravitational radiation from ultra high energy cosmic rays in models with large extra dimensions
(2006)
The effects of classical gravitational radiation in models with large extra dimensions are investigated for ultra high energy cosmic rays (CRs). The cross sections are implemented into a simulation package (SENECA) for high energy hadron induced CR air showers. We predict that gravitational radiation from quasi-elastic scattering could be observed at incident CR energies above 10^9 GeV for a setting with more than two extra dimensions. It is further shown that this gravitational energy loss can alter the energy reconstruction for CR energies E_CR > 5 10^9 GeV.
No black holes at IceCube
(2006)
This work is dedicated to the investigation of nuclear matter at non-zero temperatures within an effective hadronic model based on the Walecka model. It includes fermions as well as a vector omega meson and a scalar sigma meson where for the latter a quartic self-interaction has been considered. The coupling constants have been adapted to the saturation properties of infinite nuclear matter. A set of self-consistent Schwinger-Dyson equations has been set up for all included particles within the Cornwall-Jackiw-Tomboulis formalism. This has been expanded to non-zero temperatures via the imaginary time formalism. Beside tree-level two different stages of approximations have been considered: the Hartree approximation which takes into account the double-bubble diagram for the scalar meson, and an improved approximation where in addition two-particle irreducible sunset diagrams for all fields were included. In the Hartree-approximation the Schwinger-Dyson equations can be solved by quasi-particle ansaetze, while in the improved approximation spectral functions with non-zero widths have to be introduced. The Schwinger-Dyson equations are solved by the fully dressed propagators. Comparing the two levels of approximation shows the influence of finite widths on the temperature dependence of the particle properties. The consideration of finite widths in fact has a significant influence on the transition from a phase of heavy nucleons to a transition of light nucleons, observed in the Walecka-model. The temperature dependence is weakend when finte widths are taken into account.
Das HADES-Experiment (High Acceptance DiElectron Spectrometer) am SIS der GSI wurde zur Messung der e+e- - Paare dileptonischer Zerfälle der leichten Vektormesonen im Energiebereich von 1 - 2 AGeV entwickelt. Im Rahmen dieser Arbeit wurden die Eigenschaften des Spurverfolgungssystems de HADES-Spektrometers untersucht. Das Spurverfolgungssystem besteht aus vier Ebenen mit Vieldrahtdriftkammern (Mini Drift Chambers (MDCs)) niedriger Massenbelegung (low-mass), die aus je 6 Auslesedrahtebenen bestehen. Eine der Hauptanforderungen an das Spurverfolgungssystem ist eine Ortsauflösung von 100 µm (hauptsächlich in y-Richtung), die benötigt wird, um die geforderte Massenauflösung von 1 % im Bereich der w-Masse zu erzielen. Gleichzeitig muss die Nachweiseffizienz für schwach ionisierende Elektronen/Positronen hoch sein. Die primäre Messgröße von Driftkammern ist die Driftzeit der entlang einer Teilchenspur generierten Elektronen der Primärionisation zum Auslesedraht. Um die gemessene Driftzeit in eine Ortskoordinate umrechnen zu können, ist eine genaue Kenntnis der Ort-Zeit-Korrelation der Driftzellen nötig. Es wurden detaillierte Simulationen der He/i - Butan Zählgasmischung mit GARFIELD, MAGBOLTZ und HEED vorgenommen. Dabei wurden Gastemperatur, Gasdruck, sowie die Kontamination des Zählgases mit O2 und H20 und die Konzentration des Löschgases variiert und die Auswirkung auf die Driftgeschwindigkeit der Elektronen und damit auf Ort-Zeit-Korrelation der Driftzellen studiert Des Weiteren wurden die Auswirkung der Höhe der Diskriminatorschwelle der Ausleseelektronik und der Einfluss des magnetischen Feldes auf die Driftzeitmessung untersucht. Ein zweidimensionales Modell der Driftzellen, das die Abhängigkeit der Ort-Zeit-Korrelation vom Einfallswinkel des Teilchens in die Driftzelle berücksichtigt, wurde in die Spurrekonstruktionssoftware integriert. Das realistische Ansprechverhalten der Driftkammern wurde in die GEANT-Simulation des HADES-Experimentes implementiert. In der vorliegenden Arbeit wird das Ansprechverhalten der inneren Driftkammern anhand von C + C Daten analysiert, die bei einer Einschussenergie von 2 AGeV im November 2001 gemessen wurden. Es wurde eine neue Methode entwickelt, die aus der Breite des am Auslesedraht influenzierten Signals (time above threshold) eine dem Energieverlust eines Teilchens korrelierte Größe bestimmt, die sich zur Identifikation von Teilchen eignet. Die vorgestellte Methode der Energieverlustmessung besitzt eine Auflösung von etwa 10 % für minimal ionisierende Teilchen und etwa 7, 2 % für stark ionisierenden Teilchen. Die Ortsauflösung der Driftzellen betrug 128 - 154 µm für minimal ionisierende Teilchen und 84 - 116µm für stark ionisierende Teilchen. Für minimal ionisierende Teilchen wurde die Ortsauflösung der Driftkammern in x- und y-Richtung zu x = 181 - 195µm und y = 87 - 104 µm bestimmt. Für stark ionisierende Teilchen wird eine Ortsauflösung von x = 119 - 148 µm und y = 57 - 79 µm erreicht. Eine Teilchenspur wird redundant in den 6 Drahtebenen einer Driftkammer nachgewiesen. Die Nachweiseffizienz der Drahtebenen einer Driftkammer lag für minimal ionisierende Teilchen bei 90 - 96 % und für stark ionisierende Teilchen bei 94 - 98 %. Es konnte somit gezeigt werden, dass die Driftkammern des HADES-Experiment über die geforderte Ortsauflösung und Nachweiseffizienz für e+|e- verfügen und aufgrund der Messung des Energieverlustes in den Driftkammern zur Teilchenidentifikation und Reduktion des Untergrundes beitragen können.
We introduce a smooth mapping of some discrete space-time symmetries into quasi-continuous ones. Such transformations are related with q-deformations of the dilations of the Euclidean space and with the non-commutative space. We work out two examples of Hamiltonian invariance under such symmetries. The Schrodinger equation for a free particle is investigated in such a non-commutative plane and a connection with anyonic statistics is found. PACS: 03.65.Fd, 11.30.Er
Results from various theoretical approaches and ideas presented at this exciting meeting (summary talk at the 5th International Conference on Physics and Astrophysics of Quark Gluon Plasma (ICPAQGP - 2005)) are reviewed. I also point towards future directions, in particular hydrodynamic behaviour induced by jets traveling through the quark-gluon plasma, which might be worth looking at in more detail.
Im Rahmen der vorliegenden Arbeit wurde ein Spracherkennungssystem realisiert, das sowohl phonembasierte als auch wortbasierte Modelle zur sprecherunabhängigen Schlüsselworterkennung im Kontext fließender Sprache verwenden kann. Das System erlaubt dabei die Wahl zwischen zwei grundlegend verschiedenen Verfahren: Entweder kann die Bewertung von Äußerungen durch Schlüsselwortmodelle mit gewählten Schwellenwerten verglichen werden, wobei eine Schwellenwertüberschreitung die Erkennung eines Schlüsselwortes signalisiert, oder es werden beliebige Phonemfolgen als Füllmodelle verwendet, die mit den Schlüsselwortmodellen konkurrieren. Der Schlüsselworterkenner kann sowohl zur 1-Schlüsselwort- Erkennung, bei der vorausgesetzt wird, dass sich in jeder Äußerung exakt ein Schlüsselwort befindet, als auch zur n-Schlüsselwort-Erkennung verwendet werden, bei der sich eine beliebige Anzahl Schlüsselwörter in jeder Äußerung befinden kann. Durch eine effiziente Implementation wurde die Fähigkeit zur Echtzeitverarbeitung auf verfügbaren Arbeitsplatzrechnern erreicht.....
Der Ursprung der Masse bekannter Teilchen und der Einschlu der Quarks in Hadronen ist einer der grundlegendsten Fragestellungen der modernen Physik. Die Kenntnis des Verhaltens von Kernmaterie unter extremen Bedingungen ist unabdingbar zum Verstandnis der Evolution des Universums und zur Theoriebildung von stellaren Objekten wie Neutronensternen und schwarzen Löchern. Einen experimentellen Zugang zur Untersuchung dieser Problematik stellt die Erzeugung heier und dichter Kernmaterie in ultrarelativistischen Schwerionenkollisionen dar. Hierzu untersucht das NA49 Experiment seit Herbst 1994 am 208-Pb-Strahl des CERN-SPS Pb+Pb Kollisionen bei 158 GeV pro Nukleon. Ein Schwerpunkt des Forschungsprogrammes liegt in der Untersuchung des Zustandes der Materie in der frühen Phase der Reaktion. Nach gegenwartem Stand der Theorie wird bei genugent hoher Energiedichte der Einschlu der Quarks in Hadronen aufgebrochen und ein Zustand der Materie erzeugt, in welchen die eektiven Freiheitsgrade von Hadronen und Hadronen-Resonanzen in die von Quarks und Gluonen übergehen - das sogenannte Quark-Gluon-Plasma (QGP). Die Honung ist nun, da sich die Formation eines solchen QGP im hadronischen Endzustand wiederspiegelt. Es wird erwartet, da die Seltsamkeitsproduktion in einem QGP sich in ihrer Rate und ihren Gleichgewichtswerten von der in einem hadronischen Feuerball-Szenario unterscheidet und sich somit als Signatur fur die Erzeugung eines GQP eignet. Von besonderen Interesse ist hier die Produktion von Hyperonen. Schwerpunkt dieser Arbeit ist die Untersuchung der Produktion von doppelt seltsamen geladenen -Hyperonen in zentralen Pb+Pb Kollisionen. Zu diesem Zweck wurden 58000 zentrale Pb+Pb Ereignisse der im Herbst 1995 aufgezeichneten Reaktionen untersucht. Die Analyse der Daten wurde auschlielich mit der zweiten Spurendriftkammer (VTPC2) durchgeführt. Zur Rekonstruktion der -Hyperonen muten Verfahren entwickelt werden, um die typischen Zerfalls-Topologien der doppelt seltsamen Hyperonen aus der Vielzahl von ca. 700 in der Vertex-TPC gemessenen geladenen Teilchenspuren herauszulösen. Aus den in der kombinatorischen Analyse rekonstruierten 720 und 138 + - Hyperonen konnten Spektren des Transversalimpulses und Rapiditatsverteilungen ermittelt werden. Die gewonnene Phasenraum-Akzeptanz fur die in der VTPC2 gemessenen und + - Hyperonen beträgt ....
Jet physics in ALICE
(2005)
This work aims at the performance of the ALICE detector for the measurement of high-energy jets at mid-pseudo-rapidity in ultra-relativistic nucleus-nucleus collisions at LHC and their potential for the characterization of the partonic matter created in these collisions. In our approach, jets at high energy with E_{T}>50 GeV are reconstructed with a cone jet finder, as typically done for jet measurements in hadronic collisions. Within the ALICE framework we study its capabilities of measuring high-energy jets and quantify obtainable rates and the quality of reconstruction, both, in proton-proton and in lead-lead collisions at LHC conditions. In particular, we address whether modification of the jet fragmentation in the charged-particle sector can be detected within the high particle-multiplicity environment of the central lead-lead collisions. We comparatively treat these topics in view of an EMCAL proposed to complete the central ALICE tracking detectors. The main activities concerning the thesis are the following: a) Determination of the potential for exclusive jet measurements in ALICE. b) Determination of jet rates that can be acquired with the ALICE setup. c) Development of a parton-energy loss model. d) Simulation and study of the energy-loss effect on jet properties.
Im Rahmen dieser Arbeit wurde der Aufbruchsmechanismus des Projektilspektators im relativistischen Energiebereich untersucht. Es zeigte sich dabei, daß die in vorherigen Experimenten beobachtete Targetunabhängigkeit der Fragmentproduktion bei 600 AMeV sich als universelle Eigenschaft des Zerfalls von angeregter und expandierter Kernmaterie erweist. Die Untersuchung von Ladungskorrelationen zeigte ebenfalls weder eine Energie- noch Projektilabhängigkeit im Rahmen der experimentellen Auflösung. Diese Ergebnisse sind im wesentlichen auch zu höheren und niedrigeren Energien von anderen Experimenten bestätigt worden. Mit diesem experimentellen Befund kann eindeutig der Beweis für die Existenz einer Multi-Fragmentproduktion bei relativistischen Energien gegeben werden. Im Rahmen von Modellen können die beobachteten Ladungsobservablen mit einem statistisch dominierten Zerfall erklärt werden. Die sich daran anschließende Frage nach dem Aufbruchsmechanismus und dessen Eigenschaften wurde weiterführend mit Ausrichtung auf kinematische und thermodynamische Eigenschaften des Systems untersucht. Dabei ergab sich, daß die kinematischen Observablen der Projektilquelle einen thermisch äquilibrierten Zustand widerspiegeln, unabhängig vom Stoßparameter und der Einschußenergie. Die hierbei beobachtete Emission von leichten Teilchen, die nicht eindeutig einer intermediären oder Projektilquelle zugeordnet werden konnten, ist hierbei Hinweis auf Nicht-Gleichgewichtsanteile, die in der frühen Phase der Reaktion gebildet werden. Mit der Untersuchung von kollektiven Eigenschaften des zerfallenden Systems wurde versucht, einen quantitativen Einblick in die Reaktionskinematik und den damit zusammenhängenden Energietransfer in den Projektilspektator zu erhalten. Diese Analysen ergaben, daß es bei gleichem Stoßparameter eine starke Abhängigkeit des "Bounce Off" von der Targetmasse gibt, während zu höheren Energien, beim gleichen System, nur ein kleiner Effekt zu höheren Impulsüberträgen (5-10 MeV/c) beobachtet wird. Die Energiebilanz des Systems und die hieraus extrahierten Anregungsenergien zeigten zum ersten Mal in experimentellen Daten ohne Zuhilfenahme von theoretischen Modellen, daß für die stark asymmetrischen Systeme nicht der gleiche Zusammenhang zwischen Anregungsenergie und Z bounce; erhalten wird wie bei den symmetrischen Systemen. Dies zeigt sich bei den asymmetrischen Systemen durch eine Saturation der Anregungsenergie mit kleiner werdendem Z bounce, im Gegensatz zu den symmetrischen Systemen, die einen weiteren Anstieg zeigen. Die absoluten Werte der maximalen Anregungsenergie von <E0/A0> ~ 21-23 MeV bei halbzentralen Reaktionen von 197 Au + 197 Au bei 800 AMeV und <E0/A0> ~27 MeV bei 238 U + 238 U 1000 AMeV sind verschieden bei gleichem Z bound. Es stellt sich jedoch heraus, daß mit Ausnahme der stark asymmetrischen Systeme die Anregungsenergie pro herausgeschlagenem Nukleon (<E Knock/A>) in Abhängigkeit von der prozentualen Größe des Prefragments zu peripheren Reaktionen monoton und energieunabhängig steigt.Werden die experimentell bestimmten Anregungsenergien verglichen mit denen aus theoretischen Modellen, so sind diese immer deutlich geringer. Im statistischen Modell von Botvina und Mitarbeitern, das von D´esesquelles mit unseren Daten verglichen wurde [D´ese 96] [D´ese 95], ergaben sich maximale Anregungsenergien von <E0/A0> ~ 7-8 MeV in zentralen Reaktionen. Ein Vergleich mit QMD + SMM ergibt, daß für die asymmetrischen Systeme (197AU + 12C)mit einer Anregungsenergien von maximal <E0/A0> ~ 8-9 MeV eine Beschreibung der Daten möglich ist. Für die symmetrischen Systeme zeigt sich eine zunehmende Diskrepanz mit zunehmender Targetmasse zwischen den experimentellen und theoretischen Anregungsenergien. Die Ladungsobservablen der Daten werden von der verwendeten QMD + SMM-Rechnung und der QMD (SACA)-Rechnung gut wiedergegeben. Durch Anpassung von Rechnungen mit dem Quantenstatischen Modell [Hahn 88b] (QSM) an die experimentellen Daten ergaben sich Aufbruchsdichten bei zentralen Reaktionen von rho/rho 0 ~ 0.3 bis 0.4, diese sind konsistent mit dem Aufbruch eines äquilibrierten und expandierten Systems. Die aus QSM erhaltenen Temperaturen des Quellsystems in Abhängigkeit von der Anregungsenergie geben im wesentlichen den von Pochodzalla und Mitarbeitern beobachteten Verlauf der kalorischen Kurve wieder. Die Frage, ob dieser Verlauf einen Phasenübergang von flüssig zu gasförmig darstellt, ist anhand dieser Methode nicht zu entscheiden. Die Ergebnisse der Ladungsobservablen in Verbindung mit den kollektiven Eigenschaften zeigen, daß die Anregungsenergie, die zum Erreichen des Maximums der M Fragmentproduktion ( <M IMF> ~ 4.4) nötig ist <E0/A0> ~ 11 MeV, einen geringeren absoluten Wert hat als der Bereich des möglichen Phasenübergangs <E0/A0> ~17 MeV. Im Gegensatz dazu stellt sich das Maximum der mittleren IMF -Produktion energie-, target- und projektilunabhängig bei <E0/A0> ~11 MeV ein. Mit diesemVergleich wird deutlich, daß zur näheren Untersuchung des Phasenübergangs von Kernmaterie nicht die in der Anregungsenergie saturierenden asymmetrischen Projektil-Target Kombinationen benutzt werden können. Die physikalische Fragestellung einer neuen Generation von Experimenten mit dem ALADIN-Detektor müßte in der Quantifizierung des Phasenübergangs und seiner dynamischen Observablen liegen. Dabei ist Beantwortung der Frage nach der zeitlichen Entwicklung der Fragmentproduktion über Korrelationen der Fragmente im Bereich des Phasenübergangs im Vergleich zum Maximum der universellen Kurve sowie die ereignisweise Bestimmung von dynamischen Observablen anzustreben.
In the present work, the Heidelberg electron beam ion trap (EBIT) at the Max-Planck-Institute für Kernphysik (MPIK) has been used to produce, trap highly charged argon ions and study their magnetic dipole (M1) forbidden transitions. These transitions are of relativistic origin and, hence, provide unique possibilities to perform precise studies of relativistic effects in many electron systems. In this way, the transitions energies of the 1s22s22p for the 2P3/2 - 2P1/2 transition in Ar13+ and the 1s22s2p for the 3P1 - 3P2 transition in Ar14+, for 36Ar and 40Ar isotopes were compared. The observed isotopic effect has confirmed the relativistic nuclear recoil effect corrections due to the finite nuclear mass in a recent calculation made by Tupitsyn [TSC03], in which major inconsistencies of earlier theoretical methods have been corrected for the first time. The finite mass, or recoil effect, composed of the normal mass shift (NMS), and the specific mass shift (SMS) were corrected for relativistic contributions, RNMS and RSMS. The present experimental results have shown that the recoil effects on the Breit level are indeed very important, as well as the effects of the correlated relativistic dynamics in a many electron ion.
This a review of the present status of heavy-ion collisions at intermediate energies. The main goal of heavy-ion physics in this energy regime is to shed some light on the nuclear equation of state (EOS), hence we present the basic concept of the EOS in nuclear matter as well as of nuclear shock waves which provide the key mechanism for the compression of nuclear matter. The main part of this article is devoted to the models currently used for describing heavy-ion reactions theoretically and to the observables useful for extracting information about the EOS from experiments. A detailed discussion of the flow effects with a broad comparison with the avaible data is presented. The many-body aspects of such reactions are investigated via the multifragmentation break up of excited nuclear systems and a comparison of model calculations with the most recent multifragmentation experiments is presented.
In the framework of the relativistic quantum dynamics approach we investigate antiproton observables in Au-Au collisions at 10.7A GeV. The rapidity dependence of the in-plane directed transverse momentum p(y) of p's shows the opposite sigh of the nucleon flow, which has indeed recently been discovered at 10.7A GeV by the E877 group. The "antiflow" of p's is also predicted at 2A GeV and at 160 A GeV and appears at all energies also for pi's and K's. These predicted p anticorrelations are a direct proof of strong p annihilation in massive heavy ion reactions.
The quantum statistical model (QSM) is used to calculate nuclear fragment distributions in chemical equilibrium. Several observable isotopic effects are predicted for intermediate energy heavy ion collisions. It is demonstrated that particle ratios for different systemsdo not depend on the breakup density-the only free parameter in our model.The importance of entropy measurements is discussed. Specific particle ratios for the system Au-Au are predicted, which can be used to determine the chemical potentials of the hot midrapidity fragment source in nearly central heavy ion collisions. Pacs-Nr. 25.70 Pq
The Monte Carlo parton string model for multiparticle production in hadron-hadron, hadron-nucleus, and nucleus-nucleus collisions at high energies is described. An adequate choice of the parameters in the model gives the possibility of recovering the main results of the dual parton model, with the advantage of treating both hadron and nuclear interactions on the same footing, reducing them to interactions between partons. Also the possibility of considering both soft and hard parton interactions is introduced.
The properties of pions from the hot and dense reaction stage of relativistic heavy ion collisions are investigated with the quantum molecular dynamics model. Pions originating from this reaction stage stem from resonance decay with enhanced mass. They carry high transverse momenta. The calculation shows a direct correlation between high pt pions, early freeze-out times and high freeze-out densities.
Dilepton spectra for p+p and p+d reactions at 4.9GeV are calculated. We consider electromagnetic bremsstrahlung also in inelastic reactions. N* and Delta* decay present the major contributions to the pho and omega meson yields.Pion annihilation yields only 1.5% of all pho's in p+d. The pho mass spectrum is strongly distorted due to phase space effects, populating dominantly dilepton masses below 770MeV.
We calculate thermal photon and neutral pion spectra in ultrarelativistic heavy-ion collisions in the framework of three-fluid hydrodynamics. Both spectra are quite sensitive to the equation of state used. In particular, within our model, recent data for S + Au at 200 AGeV can only be understood if a scenario with a phase transition (possibly to a quark-gluon plasma) is assumed. Results for Au+Au at 11 AGeV and Pb + Pb at 160 AGeV are also presented.
We predict the formation of highly dense baryon-rich resonance matter in Au+Au collisions at AGS energies. The final pion yields show observable signs for resonance matter. The Delta1232 resonance is predicted to be the dominant source for pions of small transverse momenta. Rescattering e ects consecutive excitation and deexcitation of Delta's lead to a long apparent life- time (> 10 fm/c) and rather large volumina (several 100 fm3) of the Delta-matter state. Heavier baryon resonances prove to be crucial for reaction dynamics and particle production at AGS.
Strong mean meson fields, which are known to exist in normal nuclei, experience a violent deformation in the course of a heavy-ion collision at relativistic energies. This may give rise to a new collective mechanism of the particle production, not reducible to the superposition of elementary nucleon-nucleon collisions.
We investigate the sensivity of pionic bounce-off and squeeze-out on the density and momentum dependence of the real part of the nucleon optical potential. For the in-plane pion bounce-off we find a strong sensivity on both the density and momentum dependence whereas the out-of-plane pion squeeze-out shows a strong sensivity only towards the momentum dependence but little sensivity towards the density dependence.
We demonstrate the importance of the Bose-statistical effects for pion production in relativistic heavy-ion collisions. The evolution of the pion phase-space density in central collisions of ultrarelativistic nuclei is studied in a simple kinetic model taking into account the effect of Bose-simulated pion production by the NN collisions in a dense cloud of mesons.
Triple differential cross sections of pions in heavy ion collisions at 1 GeV/nucl. are studied with the IQMD model. After discussing general properties of resonance and pion production we focus on azimuthal correlations: At projectile- and target-rapidities we observe an anticorrelation in the in-plane transverse momentum between pions and protons. At c.m.-rapidity, however, we find that high pt pions are being preferentially emitted perpendicular to the event-plane. We investigate the causes of those correlations and their sensitivity on the density and momentum dependence of the real and imaginary part of the nucleon and pion optical potential.
The rapidity distribution of thermal photons produced in Pb+Pb collisions at CERN-SPS energies is calculated within scaling and three- fluid hydrodynamics. It is shown that these scenarios lead to very different rapidity spectra. A measurement of the rapidity dependence of photon radiation can give cleaner insight into the reaction dynamics than pion spectra, especially into the rapidity dependence of the temperature.
Spectra of various particle species have been calculated with the Quantum Molecular Dynamics (QMD) model for very central collisions of Au+Au. They are compatible with the idea of a fully stopped thermal source which exhibits a transversal expansion besides the thermal distribution of an ideal gas. How- ever, the microscopic analyses of the local flow velocities and temperatures indicate much lower temperatures at densities associated with the freeze-out. The results express the overall impossibility of a model-independent determi- nation of nuclear temperatures from heavy ion spectral data, also at other energies (e.g. CERN) or for other species (i.e. pions, kaons, hyperons)
In the framework of RQMD we investigate antiproton observables in massive heavy ion collisions at AGS energies and compare to preliminary results of the E878 collaboration. We focus here on the considerable influence of the real part of an antinucleon nucleus optical potential on the ¯p momentum spectra. Pacs-numbers: 14.20 Dh, 25.70.-z
In the framework of the relativistic quantum molecular dynamics approach (RQMD) we investigate antideuteron (d) observables in Au+Au collisions at 10.7 AGeV. The impact parameter dependence of the formation ratios d/p2 and d/p2 is calculated. In central collisions, the antideuteron formation ratio is predicted to be two orders of magnitude lower than the deuteron formation ratio. The d yield in central Au+Au collisions is one order of magnitude lower than in Si+Al collisions. In semicentral collisions di erent configuration space distributions of p s and d s lead to a large squeeze out e ect for antideuterons, which is not predicted for the p s.
Different numerical approaches and algorithms arising in the context of modelling of cellular tissue evolution are discussed in this thesis. Being suited in particular to off-lattice agent-based models, the numerical tool of three-dimensional weighted kinetic and dynamic Delaunay triangulations is introduced and discussed for its applicability to adjacency detection. As there exists no implementation of a code that incorporates all necessary features for tissue modelling, algorithms for incremental insertion or deletion of points in Delaunay triangulations and the restoration of the Delaunay property for triangulations of moving point sets are introduced. In addition, the numerical solution of reaction-diffusion equations and their connection to agent-based cell tissue simulations is discussed. In order to demonstrate the applicability of the numerical algorithms, biological problems are studied for different model systems: For multicellular tumour spheroids, the weighted Delaunay triangulation provides a great advantage for adjacency detection, but due to the large cell numbers the model used for the cell-cell interaction has to be simplified to allow for a numerical solution. The agent-based model reproduces macroscopic experimental signatures, but some parameters cannot be fixed with the data available. A much simpler, but in key properties analogous, continuum model based on reaction-diffusion equations is likewise capable of reproducing the experimental data. Both modelling approaches make differing predictions on non-quantified experimental signatures. In the case of the epidermis, a smaller system is considered which enables a more complete treatment of the equations of motion. In particular, a control mechanism of cell proliferation is analysed. Simple assumptions suffice to explain the flow equilibrium observed in the epidermis. In addition, the effect of adhesion on the survival chances of cancerous cells is studied. For some regions in parameter space, stochastic effects may completely alter the outcome. The findings stress the need of establishing a defined experimental model to fix the unknown model parameters and to rule out further models.
We calculate the evolution of quark-gluon-plasma droplets during the hadronization in a thermodynamical model. It is speculated that cooling as well as strangeness enrichment allow for the formation of strangelets even at very high initial entropy per baryon S/Ainit H 500 and low initial baryon numbers of Ainit B H 30. It is shown that the droplet with vanishing initial chemical potential of strange quarks and a very moderate chemical potential of up/down quarks immediately charges up with strangeness. Baryon densi- ties of H 2 0 and strange chemical potentials of µs > 350 MeV are reached if strangelets are stable. The importance of net baryon and net strangeness fluctuations for the possible strangelet formation at RHIC and LHC is em- phasized. Pacs-Classif.: 25.15.tr, 12.38.Mh, 24.85.tp
The extension of the Periodic System into hitherto unexplored domains - anti- matter and hypermatter - is discussed. Starting from an analysis of hyperon and single hypernuclear properties we investigate the structure of multi-hyperon objects (MEMOs) using an extended relativistic meson field theory. These are contrasted with multi-strange quark states (strangelets). Their production mechanism is stud- ied for relativistic collisions of heavy ions from present day experiments at AGS and SPS to future opportunities at RHIC and LHC. It is pointed out that abso- lutely stable hypermatter is unlikely to be produced in heavy ion collisions. New attention should be focused on short lived metastable hyperclusters ( / 10 10s) and on intensity interferometry of multi-strange-baryon correlations.
We demonstrate that the creation of strange matter is conceivable in the midrapidity region of heavy ion collisions at Brookhaven RHIC and CERN LHC. A finite net-baryon density, abundant (anti)strangeness production, as well as strong net-baryon and net-strangeness fluctuations, provide suitable initial conditions for the formation of strangelets or metastable exotic multistrange ( baryonic) objects. Even at very high initial entropy per baryon SyAinit ¯ 500 and low initial baryon numbers of Ainit B ¯ 30 a quark-gluon-plasma droplet can immediately charge up with strangeness and accumulate net-baryon number. PACS numbers: 25.75.Dw, 12.38.Mh, 24.85.+
Abstract: An accurate impact parameter determination in a heavy ion collision is crucial for almost all further analysis. The capabilities of an artificial neural network are investigated to that respect. A novel input generation for the network is proposed, namely the transverse and longitudinal momentum distribution of all outgoing (or actually detectable) particles. The neural network approach yields an improvement in performance of a factor of two as compared to classical techniques. To achieve this improvement simple network architectures and a 5 × 5 input grid in (pt, pz) space are suffcient.
We want to draw the attention to the dynamics of a (finite) hadronizing quark matter drop. Strange and antistrange quarks do not hadronize at the same time for a baryon-rich system1. Both the hadronic and the quark matter phases enter the strange sector fs 6= 0 of the phase diagram almost immediately, which has up to now been neglected in almost all calculations of the time evolution of the system. Therefore it seems questionable, whether final particle yields reflect the actual thermodynamic properties of the system at a certain stage of the evolution. We put special interest on the possible formation of exotic states, namely strangelets (multistrange quark clusters). They may exist as (meta-)stable exotic isomers of nuclear matter 2. It was speculated that strange matter might exist also as metastable exotic multi-strange (baryonic) objects (MEMO s 3). The possible creation in heavy ion collisions of long-lived remnants of the quark-gluon-plasma, cooled and charged up with strangeness by the emission of pions and kaons, was proposed in 1,4,5. Strangelets can serve as signatures for the creation of a quark gluon plasma. Currently, both at the BNL-AGS and at the CERN-SPS experiments are carried out to search for MEMO s and strangelets, e. g. by the E864, E878 and the NA52 collaborations9,
The microscopic phasespace approach URQMD is used to investigate the stopping power and particle production in heavy systems at SPS and RHIC energies. We find no gap in the baryon rapidity distribution even at RHIC. For CERN energies URQMD shows a pile up of baryons and a supression of multi-nucleon clusters at midrapidity.
The stopping behaviour of baryons in massive heavy ion collisions ( s k 10AGeV) is investigated within di erent microscopic models. At SPS-energies the predictions range from full stopping to virtually total transparency. Experimental data are indicating strong stopping. The initial baryo-chemical potentials and temperatures at collider energies and their impact on the formation probability of strange baryon clusters and strangelets are discussed.
Nuclear clusters as a probe for expansion flow in heavy ion reactions at 10-A/GeV - 15-A/GeV.
(1996)
A phase space coalescence description based on the Wigner-function method for cluster formation in relativistic nucleus-nucleus collisions is presented. The momentum distributions of nuclear clusters d,t and He are predicted for central Au(11.6AGeV)Au and Si(14.6AGeV)Si reactions in the framework of the RQMD transport approach. Transverse expansion leads to a strong shoulderarm shape and di erent inverse slope parameters in the transverse spectra of nuclear clusters deviating markedly from thermal distributions. A clear bounce-o event shape is seen: the averaged transverse flow velocities in the reaction plane are for clusters larger than for protons. The cluster yields particularly at low pt at midrapidities and the in-plane (anti)flow of clusters and pions change if suitably strong baryon potential interactions are included. This allows to study the transient pressure at high density via the event shape analysis of nucleons, nucleon clusters and other hadrons.
We investigate the properties of di erent modifications to the linear -model (including a dilaton field associated with broken scale invariance) at finite baryon density and nonzero temperature T. The explicit breaking of chiral symmetry and the way the vector meson mass is generated are significant for the appearance of a phase of nearly vanishing nucleon mass besides the solution describing normal nuclear matter. The elimination of the abnormal solution prohibits the onset of a chiral phase transition but allows to lower the compressibility to a reasonable range. The repulsive contributions from the vector mesons are responsible for the wide range of stability of the normal phase in the (µ, T)-plane. The abnormal solution becomes not only energet- ically preferable to the normal state at high temperature or density, but also mechanically stable due to the inclusion of dilatons. PACS number:12.39.F
We discuss the properties of two distinct forms of hypothetical strange matter, small lumps of strange quark matter (strangelets) and of hyperon matter (metastable exotic multihypernuclear objects: MEMOs), with special empha- sis on their relevance for present and future heavy ion experiments. The masses of small strangelets up to AB = 40 are calculated using the MIT bag model with shell mode filling for various bag parameters. The strangelets are checked for possible strong and weak hadronic decays, also taking into account multiple hadron decays. It is found that strangelets which are stable against strong decay are most likely highly negative charged, contrary to previous findings. Strangelets can be stable against weak hadronic decay but their masses and charges are still rather high. This has serious impact on the present high sensitivity searches in heavy ion experiments at the AGS and CERN facilities. On the other hand, highly charged MEMOs are predicted on the basis of an extended relativistic mean field model. Those objects could be detected in future experiments searching for short lived, rare composites. It is demonstrated that future experiments can be sensitive to a much wider variety of strangelets.
According to the Walecka mean field theory of nuclear interaction the collective mutual deceleration of the colliding nuclei gives rise to the bremsstrahlung of real and virtual ! mesons. It is shown that decays of these mesons may give a noticeable contribution to the observed yields of the baryon antibaryon pairs, dileptons and pions. Excitation functions and rapidity distributions of particles produced by this mechanism are calculated under some simplifying assumptions about the space time variation of meson fields in nuclear collisions. The calculated multiplicities of coherently produced particles grow fast with the bombarding energy, reaching a saturation above the RHIC bombarding energy. In the case of central Au+Au collisions the bremsstrahlung mechanism becomes comparable with particle production in incoherent hadron hadron collisions above the AGS energies. The rapidity spectra of antibaryons and pions exhibit a characteristic two hump structure which is a consequence of incomplete projectile target stopping at the initial stage of the reaction. The predicted distribution of e+e pairs has a strong peak at invariant masses Me+e < 0.5 GeV.
Measured hadron yields from relativistic nuclear collisions can be equally well understood in two physically distinct models, namely a static thermal hadronic source versus a time-dependent, non-equilibrium hadronization off a quark gluon plasma droplet. Due to the time-dependent particle evaporation off the hadronic surface in the latter approach the hadron ratios change (by factors of / 5) in time. The overall particle yields then reflect time averages over the actual thermodynamic properties of the system at a certain stage of evolution.
We study the time scale for pressure equilibration in heavy ion collisions at AGS energies within the three-fluid hydrodynamical model and a microscopic cascade model (UrQMD). We find that kinetic equilibrium is reached in both models after a time of 5 fm/c (center-of-mass time). Thus, observables which are sensitive to the early stage of the reaction differ considerably from the expectations within the instant thermalization scenario (one-fluid hydrodynamical model).
Basic problems of the semiclassical microscopic modelling of strongly interacting systems are discussed within the framework of Quantum Molecular Dynamics (QMD). This model allows to study the influence of several types of nucleonic interactions on a large variety of observables and phenomena occur- ring in heavy ion collisions at relativistic energies. It is shown that the same predictions can be obtained with several numerically completely di erent and independently written programs as far as the same model parameters are employed and the same basic approximations are made. Many observ- ables are robust against variations of the details of the model assumptions used. Some of the physical results, however, depend also on rather technical parameters like the preparation of the initial configuration in phase space. This crucial problem is connected with the description of the ground state of single nuclei, which di ers among the various approaches. An outlook to an improved molecular dynamics scheme for heavy ion collisions is given.
Abstract: We study transverse expansion and directed flow in Au(11AGeV)Au reactions within a multi-fluid dynamical model. Although we do not employ an equation of state (EoS) with a first order phase transition, we find a slow increase of the transverse velocities of the nucleons with time. A similar behaviour can be observed for the directed nucleon flow. This is due to non-equilibrium e ects which also lead to less and slower conversion of longitudinal into transverse momentum. We also show that the proton rapidity distribution at CERN energies, as calculated within this model, agrees well with the preliminary NA44-data.
To describe ultrarelativistic heavy-ion collisions we construct a three-fluid hydrodynamical model. In contrast to one-fluid hydrodynamics, it accounts for the finite stopping power of nuclear matter, i.e. for nonequilibrium e ects in the early stage of the reaction. Within this model, we study baryon dynamics in the BNL-AGS energy range. For the system Au+Au we find that kinetic equilibrium between projectile and target nucleons is established only after a time teq CM H 5 fm/c C 2RAu/³CM. Observables which are sensitive to the early stage of the collision (like e.g. nucleon flow) therefore di er considerably from those calculated in the one-fluid model.
We examine the properties of both forms of strange matter, small lumps of strange quark matter (strangelets) and of strange hadronic matter (Metastable Exotic Multihypernuclear Objects: MEMOs) and their relevance for present and future heavy ion searches. The strong and weak decays are discussed separately to distinguish between long-lived and short-lived candidates where the former ones are detectable in present heavy ion experiments while the latter ones in future heavy ion experiments, respectively. We find some long-lived strangelet candidates which are highly negatively charged with a mass to charge ratio like a anti deuteron (M/Z 2) but masses of A=10 to 16. We predict also many short-lived candidates, both in quark and in hadronic form, which can be highly charged. Purely hyperonic nuclei like the (2 02 ) are bound and have a negative charge while carrying a positive baryon number. We demonstrate also that multiply charmed exotics (charmlets) might be bound and can be produced at future heavy ion colliders.
Measured hadron yields from relativistic nuclear collisions can be equally well understood in two physically distinct models, namely a static thermal hadronic source vs. a time-dependent, nonequilibrium hadronization o a quark-gluon plasma droplet. Due to the time-dependent particle evapora- tion o the hadronic surface in the latter approach the hadron ratios change (by factors of <H 5) in time. Final particle yields reflect time averages over the actual thermodynamic properties of the system at a certain stage of the evolution. Calculated hadron, strangelet and (anti-)cluster yields as well as freeze-out times are presented for di erent systems. Due to strangeness distillation the system moves rapidly out of the T, µq plane into the µs-sector. Classif.: 25.75.Dw, 12.38.Mh, 24.85.+p
We discuss the early evolution of ultrarelativistic heavy-ion collisions within a multi- fluid dynamical model. In particular, we show that due to the finite mean-free path of the particles compression shock waves are smeared out considerably as compared to the one-fluid limit. Also, the maximal energy density of the baryons is much lower. We discuss the time scale of kinetic equilibration of the baryons in the central region and its relevance for directed flow. Finally, thermal emission of direct photons from the fluid of produced particles is calculated within the three-fluid model and two other simple expansion models. It is shown that the transverse momentum and rapidity spectra of photons give clue to the cooling law and the early rapidity distribution of the photon source.
Hadron and hadron cluster production in a hydrodynamical model including particle evaporation
(1997)
We discuss the evolution of the mixed phase at RHIC and SPS within boostinvariant hydrodynamics. In addition to the hydrodynamical expansion, we also consider evaporation of particles o the surface of the fluid. The back-reaction of this evaporation process on the dynamics of the fluid shortens the lifetime of the mixed phase. In our model this lifetime of the mixed phase is d 12 fm/c in Au + Au at RHIC and d 6.5 fm/c in Pb + Pb at SPS, even in the limit of vanishing transverse expansion velocity. Strong separation of strangeness occurs, especially in events (or at rapidities) with relatively high initial net baryon and strangeness number, enhancing the multiplicity of MEMOs (multiply strange nuclear clusters). If antiquarks and antibaryons reach saturation in the course of the pure QGP or mixed phase, we find that at RHIC the ratio of antideuterons to deuterons may exceed 0.3 and even 4He/4He > 0.1. In S + Au at SPS we find only N/N H 0.1. Due to fluctuations, at RHIC even negative baryon number at midrapidity is possible in individual events, so that the antibaryon and antibaryon-cluster yields exceed those of the corresponding baryons and clusters.
After a brief introduction on QCD and effective models in the first chapter, I analyze the dependence of the QCD transition temperature on the quark (or pion) mass in the second chapter. I found that a linear sigma model, which links the transition to chiral symmetry restoration, predicts a much stronger dependence of T_c on m_pi than seen in present lattice data for m_pi >~ 0.4 GeV. On the other hand, an effective Lagrangian for the Polyakov loop requires only small explicit symmetry breaking to describe T_c(m_pi) in the above mass range. In the third and fourth chapter, I study the linear sigma model with O(N) symmetry at nonzero temperature in the framework of the Cornwall-Jackiw-Tomboulis formalism. Extending the set of two-particle irreducible diagrams by adding sunset diagrams to the usual Hartree-Fock (or Hartree) contributions, I derive a new approximation scheme which extends the standard Hartree-Fock (or Hartree) approximation by the inclusion of nonzero decay widths.
A study of secondary Drell-Yan production in nuclear collisions is presented for SPS energies. In addition to the lepton pairs produced in the initial collisions of the projectile and target nucleons, we consider the potentially high dilepton yield from hard valence antiquarks in produced mesons and antibaryons. We calculate the secondary Drell-Yan contributions taking the collision spectrum of hadrons from the microscopic model URQMD. The con- tributions from meson-baryon interactions, small in hadron-nucleus interac- tions, are found to be substantial in nucleus-nucleus collisions at low dilepton masses. Preresonance collisions of partons may further increase the yields.
A generalized Lagrangian for the description of hadronic matter based on the linear SU(3)L × SU(3)R -model is proposed. Besides the baryon octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken scale invariance is incorporated. The observed values for the vacuum masses of the baryons and mesons are reproduced. In mean-field approximation, vector and scalar interactions yield a saturating nuclear equation of state. We discuss the di culties and possibilities to construct a chiral invariant baryon-meson interaction that leads to a realistic equation of state. It is found that a coupling of the strange condensate to nucleons is needed to describe the hyperon potentials correctly. The effective baryon masses and the appearance of an abnormal phase of nearly massless nucleons at high densities are examined. A nonlinear realization of chiral symmetry is considered, to retain a Yukawa-type baryon-meson interaction and to establish a connection to the Walecka-model.
Abstract. A generalized Lagrangian for the description of hadronic matter based on the linear SU(3)L × SU(3)R -model is proposed. Besides the baryon octet, the spin-0 and spin-1 nonets, a gluon condensate associated with broken scale invariance is incorporated. The observed values for the vacuum masses of the baryons and mesons are reproduced. In mean-field approximation, vector and scalar interactions yield a saturating nuclear equation of state. Finite nuclei can be reasonably described, too. The condensates and the e ective baryon masses at finite baryon density and temperature are discussed.
Untersuchungen von evolutionären Algorithmen zum Training neuronaler Netze in der Sprachverarbeitung
(1997)
Im Rahmen der vorliegenden Diplomarbeit wurde die Leistungsfähigkeit von evolutionären Algorithmen zum Training von RNN untersucht und mit gradientenbasierten Trainingsalgorithmen verglichen. Die Zielsetzung war dabei im besonderen die Prüfung der Verwendbarkeit in der Sprachverarbeitung, speziell der Spracherkennung. Zunächst wurde anhand eines Prädiktionsproblems die prinzipielle Leistungsfähigkeit von EA untersucht, indem ein MLP mit unterschiedlichen evolutionären Algorithmen trainiert wurde. Verschiedene Varianten von GA und ES sind an diesem Beispiel getestet und miteinander verglichen worden. Im Rahmen der Untersuchungen an GA stellte sich heraus, daß eine Mindestgenauigkeit der Quantisierung zur Lösung erforderlich ist. Es zeigt sich, daß die Genauigkeit der Approximation mit abnehmendem Quantisierungsfehler besser wird. Damit ist eine Behandlung dieses Problems mit grob quantisierten Gewichten nachteilig. Demgegenüber profitiert ES sowohl in der Approximationsgenauigkeit, als auch in der Konvergenzgeschwindigkeit von der direkten Darstellung der Objektvariablen als reelle Zahlen. Weiterhin zeigte sich bei ES, daß die Genauigkeit einer Lösung auch von der Populationsgröße abhängig ist, da mit wachsender Populationsgröße der Parameterraum besser abgetastet werden kann. Im Vergleich mit ES benötigten GA längere Konvergenzzeiten und bedingten zudem aufgrund der Codierung und Decodierung einen höheren Rechenaufwand als ES, so daß die Untersuchungen an RNN nur mit ES durchgeführt wurden. Zunächst wurde mit dem Latching-Problem eine, in der Komplexität eng begrenzte, Klassifikationsaufgabe mit Zeitabhängigkeiten untersucht. Die zur Verfügung gestellte Information war bei diesem Beispiel sehr gering, da der Fehler nur am Ende einer Mustersequenz berechnet wurde. Es stellte sich heraus, daß selbst bei dieser sehr einfachen Aufgabenstellung die gradientenbasierten Verfahren nach dem Überschreiten einer maximalen Mustersequenzlänge T keine Lösung finden konnten. Im Gegensatz dazu war ES in der Lage, das Problem für alle gemessenen Variationen des Parameters T zu lösen. Erst wenn während des Trainings dem Gradientenverfahren zusätzliche Informationen durch Fehlereinspeisung zur Verfügung gestellt wurde, hatte der BPTT-Algorithmus die selbe Leistungsfähigkeit. Als weiteres Experiment mit Zeitabhängigkeiten wurde das Automaton-Problem un- tersucht, welches mittels eines RNN gelöst werden sollte. Bei diesem Problem wurde besonderer Wert auf die Untersuchung des Konvergenzverhaltens bei Änderungen der Parameter von ES gelegt. Die Untersuchungen ergaben, daß die einzelnen Parameter in komplexer Weise miteinander interagieren und nur eine gute Abstimmung aller Parameter aufeinander eine befriedigende Leistung in Bezug auf Konvergenzgeschwindigkeit und Klassifikationsergebnis erbringt. Wie bei dem Latching-Problem wurde der Fehler nur am Ende einer Mustersequenz berechnet. Dies bewirkt, daß der BPTT-Algorithmus bereits bei Sequenzlängen von T = 27 nicht mehr in der Lage ist, die Zeitabhängigkeiten in dem Gradienten zu repräsentieren. Mit ES dagegen konnten RNN trainiert werden, die in der Lage sind, Sequenzlängen bis zu T = 41 richtig zu klassifizieren. Die Untersuchungen bestätigen, daß der beschränkende Faktor in erster Linie der Trainingsalgorithmus und nicht das Netzwerksparadigma ist. Die Simulationsexperimente mit zeitnormierten Sprachdaten zeigen, daß mit ES prinzipiell höhere Erkennungsleistungen als mit dem gradientenbasierten Algorithmus des BPTT erzielt werden können. Jedoch nimmt schon bei der Klassifikation der Zahlwörter Zwei und Drei die Klassifikationsleistung mit zunehmender Sequenzlänge ab. Es erfordert eine drastische Vergrößerung der Populationsgröße, um zumindest gleich gute Ergebnisse zu erzielen. Zusätzliche Tests am Automaton-Problem stützen diese Aussage. Jedoch steigt der Rechenaufwand durch Vergrößerung der Populationsgröße so stark an, daß bei nicht zeitnormierten Sprachdaten ES mit adäquater Populationsgröße nicht mehr simulierbar waren. In den Untersuchungen an dem Vokabular mit sechs Wörtern wurde der Fehler für jeden anliegenden Merkmalsvektor berechnet und im Gradienten bzw. zur Bewertung bei ES im Training verwendet. In diesen Messungen erbringen beide Algorithmen nahezu identische Klassifikationsergebnisse. Insgesamt verhindert der drastisch ansteigende Rechenaufwand bei den Sprachdaten die Verarbeitung von größeren Vokabularien und langen Wörtern durch ES. Aus der Beschränkung der Populationsgröße durch die vorhandene Rechnerkapazität resultierte eine nichtoptimale Anpassung von Selektionsdruck, Mutationsrate und Populationsverteilung im Suchraum. Insbesondere erweist sich die globale Anpassung der Strategieparameter bei den vergrößerten Populationen als problematisch. Weitere Untersuchungen an ES mit Strategien zur Selbstadaption dieser Parameter bieten sich daher für zukünftige Forschung an.
The deconfinement transition region between hadronic matter and quark-gluon plasma is studied for finite volumes. Assuming simple model equations of state and a first order phase transition, we find that fluctuations in finite volumes hinder a sharp separation between the two phases around the critical temperature, leading to a rounding of the phase transition. For reaction volumes expected in heavy ion experiments, the softening of the equation of state is reduced considerably. This is especially true when the requirement of exact color-singletness is included in the QGP equation of state.
A self-consistent relativistic integral-di erential equation of the Boltzmann- Uehling-Uhlenbeck-type for the N*(1440) resonance is developed based on an effective Lagrangian of baryons interacting through mesons. The closed time-path Green s function technique and semi-classical, quasi-particle and Born approxima- tions are employed in the derivation. The non-equilibrium RBUU-type equation for the N*(1440) is consistent with that of nucleon s and delta s which we derived before. Thus, we obtain a set of coupled equations for the N,Delta and N*(1440) distribution functions. All the N (1440)-relevant in-medium two-body scattering cross sections within the N,Delta and N*(1440) system are derived from the same effective Lagrangian in addition to the mean field and presented analytically, which can be directly used in the study of relativistic heavy-ion collisions. The theoreticalprediction of the free pp - pp* (1440) cross section is in good agreement with the experimental data. We calculate the in-medium N+N - N+N* , N* +N - N+N and N*+N - N* +N cross sections in cold nuclear matter up to twice the nuclear matter density. The influence of different choices of the N* N* coupling strengths, which can not be obtained through fitting certain experimental data, are discussed. The results show that the density dependence of predicted in-medium cross sections are sensitive to the N* N* coupling strengths used. An evident density dependence will appear when a large scalar coupling strength of g^(sigma) N*N* is assumed. PACS number(s): 24.10.Cn; 25.70.-z; 21.65.+f
Preliminary experimental data for particle number ratios in the collisions of Au+Au at the BNL AGS (11A GeV/c) and Pb+Pb at the CERN SPS (160A GeV/c) are analyzed in a thermodynamically consistent hadron gas model with excluded volume. Large values of temperature, T = 140 185 MeV, and baryonic chemical potential, µb = 590 270 MeV, close to the boundary of the quark-gluon plasma phase are found from fitting the data. This seems to indicate that the energy density at the chemical freezeout is tremendous which would be indeed the case for the point-like hadrons. However, a self-consistent treatment of the van der Waals excluded volume reveals much smaller energy densities which are very far below a lowest limit estimate of the quark-gluon plasma energy density. PACS number(s): 25.75.-q, 24.10.Pa
We study the bremsstrahlung of virtual omega mesons due to the collective deceleration of nuclei at the initial stage of an ultrarelativistic heavy ion collision. It is shown that electromagnetic decays of these mesons may give an important contribution to the observed yields of dileptons. Mass spectra of e+e and µ+µ pairs produced in central Au+Au collisions are calculated under some simplifying assumptions on the space time variation of the baryonic current in a nuclear collision process. Comparison with the CERES data for 160 AGev Pb+Au collisions shows that the proposed mechanism gives a noticeable fraction of the observed e+e pairs in the intermediate region of invariant masses. Sensi tivity of the dilepton yield to the in medium modification of masses and widths of vector mesons is demonstrated.
Collective bremsstrahlung of vector meson fields in relativistic nuclear collisions is studied within the time dependent Walecka model. Mutual deceleration of the colliding nuclei is described by introducing the e ective stopping time and average rapidity loss of baryons. It is shown that electromagnetic decays of virtual ω mesons produced by bremsstrahlung mechanism can provide a substantial contribution to the soft dilepton yield at the SPS bombarding energies. In particular, it may be responsible for the dilepton enhancement observed in 160 AGev central Pb+Au collisions. Suggestions for future experiments to estimate the relative contribution of the collective mechanism are given.
Dielectron mass spectra are examined for various nuclear reactions recently measured by the DLS collaboration. A detailed description is given of all dilepton channels included in the transport model UrQMD 1.0, i.e. Dalitz decays of π, η, ω, ή mesons and of the (1232) resonance, direct decays of vector mesons and pn bremsstrahlung. The microscopic calculations reproduce data for light systems fairly well, but tend to underestimate the data in pp at high energies and in pd at low energies. These conventional sources, however, cannot explain the recently reported enhancement for nucleus-nucleus collisions in the mass region 0.15GeV ≤ Me+e- ≤ 0.6GeV. Chiral scaling and ω meson broadening in the medium are investigated as a source of this mass excess. They also cannot explain the recent DLS data.
The transverse momentum distribution of prompt photons coming from the very early phase of ultrarelativistic heavy ion collisions for the RHIC and LHC energies is calculated by means of perturbative QCD. We calculate the single photon cross section (A + B -> gamma + X) by taking into account the partonic sub processes q + q -> gamma + g and q + g -> gamma + q as well as the Bremsstrahlung corrections to those processes. We choose a lower momentum cut-off k0 = 2 GeV separating the soft physics from perturbative QCD. We compare the results for those primary collisions with the photons produced in reactions of the thermalized secondary particles, which are calculated within scaling hydrodynamics. The QCD processes are taken in leading order. Nuclear shadowing corrections, which alter the involved nuclear structure functions are explicitly taken into account and compared to unshadowed results. Employing the GRV parton distribution parametrizations we find that at RHIC prompt QCD-photons dominate over the thermal radiation down to transverse momenta kT ≈ 2 GeV. At LHC, however, thermal radiation from the QGP dominates for photon transverse momenta kT ≤ 5 GeV, if nuclear shadowing effects on prompt photon production are taken into account.
Direct photon production in central Pb+Pb collisions at CERN-SPS energy is calculated within the relativistic microscopic transport model UrQMD, and within distinctly di erent versions of relativistic hydrodynamics. We find that in UrQMD the local momentum distributions of the secondaries are strongly elongated along the beam axis initially. Therefore, the preequilibrium contribution dominates the photon spectrum at transverse momenta above H 1.5 GeV. The hydrodynamics prediction of a strong correlation between the temperature and radial expansion velocities on the one hand and the slope of the transverse momentum distribution of direct photons on the other hand thus is not recovered in UrQMD. The rapidity distribution of direct photons in UrQMD reveals that the initial conditions for the longitudinal expansion of the photon source (the meson fluid ) resemble rather boostinvariance than Landau-like flow.
A self-consistent relativistic Boltzmann-Uehling-Uhlenbeck equation for the N (1440) resonance is developed based on an effective Lagrangian of baryons interacting through mesons. The equation is consistent with that of nucleon s and delta s which we derived before. Thus, we obtain a set of coupled equations for the N, Delta and N (1440) distribution functions. All the N (1440)-relevant in-medium two-body scattering cross sections within the N, Delta and N (1440) system are derived from the same effective Lagrangian in addition to the mean field and presented analytically. Medium effects on the cross sections are discussed.
We derive the self-consistent relativistic quantum transport equation for the pion distribution function based on an effective Lagrangian of the QHD-II model. The closed time-path Green's function technique, the semi-classical, quasi-particle and Born approximation are employed in the derivation. Both the mean field and collision term are derived from the same Lagrangian and presented analytically. The dynamical equation for the pions is consistent with that for the nucleons and deltas which we developed before. Thus, we obtain a self-consistent relativistic transport model which describes the hadronic matter with N, Delta and pi degrees of freedom simultaneously. Within this approach, we investigate the medium effects on the pion dispersion relation as well as the pion absorption and pion production channels in cold nuclear matter. In contrast to the results of the non-relativistic model, the pion dispersion relation becomes harder at low momenta and softer at high momenta as compared to the free one. The theoretically predicted free pi N to Delta cross section is in agreement with the experimental data. Medium effects on the pi N to Delta cross section and momentum-dependent Delta-decay width are shown to be substantial.
Relativistic quantum transport theory of hadronic matter: the coupled nucleon, delta and pion system
(1998)
We derive the relativistic quantum transport equation for the pion distribution function based on an effective Lagrangian of the QHD-II model. The closed time-path Green s function technique, the semi-classical, quasiparticle and Born approximation are employed in the derivation. Both the mean field and collision term are derived from the same Lagrangian and presented analytically. The dynamical equation for the pions is consistent with that for the nucleons and deltas which we developed before. Thus, we obtain a relativistic transport model which describes the hadronic matter with N,Delta and pi degrees of freedom simultaneously. Within this approach, we investigate the medium e ects on the pion dispersion relation as well as the pion absorption and pion production channels in cold nuclear matter. In contrast to the results of the non-relativistic model, the pion dispersion relation becomes harder at low momenta and softer at high momenta as compared to the free one, which is mainly caused by the relativistic kinetics. The theoretically predicted free pi*N -> Delta cross section is in agreement with the experimental data. Medium e ects on the pi*N -> Delta cross section and momentum-dependent Delta-decay width are shown to be substantial. PACS number(s): 24.10.Cn; 13.75.Cs; 21.65.+f; 25.70.-z
The hard contributions to the heavy quarkonium-nucleon cross sections are calculated based on the QCD factorization theorem and the nonrelativistic quarkonium model. We evaluate the nonperturbative part of these cross sections which dominates at psNN 20 GeV at the Cern Super Proton Synchrotron (SPS) and becomes a correction at psNN 6 TeV at the CERN Large Hadron Collider (LHC). J/psi production at the CERN SPS is well described by hard QCD, when the larger absorption cross sections of the states predicted by QCD are taken into account. We predict an A-dependent polarization of the states. The expansion of small wave packets is discussed.
We perform an event-by-event analysis of the transverse momentum distribution of final state particles in central Pb(160AGeV)+Pb collisions within a microscopic non-equilibrium transport model (UrQMD). Strong influence of rescattering is found. The extracted momentum distributions show less fluctuations in A+A collisions than in p+p reactions. This is in contrast to simplified p+p extrapolations and random walk models.
Die vorliegende Arbeit beschäftigt sich mit der Charakterisierung des ALTRO Chips (ALICE TPC Readout), der ein integraler und wichtiger Bestandteil der Auslesekette des TPC (Time Projection Chamber) Detektors von ALICE (A Large Ion Collider Experiment) ist. ALICE ist ein Experiment am noch im Bau befindlichen LHC (Large Hadron Collider) am CERN mit der zentralen Ausrichtung, Schwerionenkollisionen zu untersuchen. Diese sind von besonderem Interesse, da durch sie ein experimenteller Zugriff zu dem QGP (Quark Gluon Plasma) existiert, dem einzigen vom Standardmodell vorhergesagten Phasenübergang, der unter Laborbedingungen erreichbar ist. Im Jahr 2004 wurden Messungen an einem Teststrahl am CERN PS (Proton Synchrotron) durchgeführt. Der Prototyp wurde voll mit FECs bestückt, was 5400 Kanälen entspricht und einer anderen Gasmixtur (Ne/N2/CO2 90%/5%/5%) befüllt. Für das optimale Leistungsverhalten der ALICE TPC muß der Digitalprozessor im ALTRO, bestehend aus vier Berechnungseinheiten, mit den passenden Werten konfiguriert werden. Der Datenfluss beginnt mit dem BCS1 (Baseline Correction and Subtraction 1) Modul, das systematische Störungen und die Grundlinie entfernt. Da der ALTRO kontinuierlich das anliegende Signal abtastet, entfernt es automatisch langsame Grundlinienveränderungen, die Beispielsweise durch Temperaturänderungen auftreten können. Gefolgt von dem TCF (Tail Cancellation Filter), der den Schweif des langsam fallenden, vom PASA generierten Signals entfernt. Um die nichtsystematischen Störungen der Grundlinie zu entfernen, folgt die BCS2 (Baseline Correction and Subtraction 2), die auf einer gleitenden Mittelwertsberechnung mit Ausschluß von Detektorsignalen über einen doppelten Schwellenwert basiert. Die finale Einheit für die Signalverarbeitung ist die ZSU (Zero Suppression Unit), die Meßpunkte unterhalb eines definierten Schwellwertes entfernt. Hier wird der weg beschrieben die TCF und BCS1 Parameter aus vorhandenen Detektordaten zu extrahieren. Während der Analyse der Daten von kosmischen Teilchen fiel bei Signalen mit hoher Amplitude (>700 ADC) eine zusätzliche Struktur in dem Schweif auf. Der Monitor wurde deswegen mit einem gleitenden Mittelwertfilter erweitert, worauf sich diese Struktur auch in kleineren Signalen (> 200 ADC) zeigte. Dieses Signal wird von Ionen erzeugt, die zur Kathode oder zu den Pads driften, bisher ist jedoch weder die Streuung der Elektronenlawine an der Anode, noch die Variationsbreite in den erzeugten Elektronlawinen verstanden oder gemessen worden. Eine erfolgreiche Messung, sowie Charakterisierung wird in dieser Arbeit beschrieben. Im Jahr 2005 im Sommer beginnt der Einbau der Gaskammern der TPC in ALICE, die Elektronik folgt am Ende dieses Jahres. Parallel hierzu wurde der Prototyp der TPC wieder in Betrieb genommen und im Frühling wird ein kompletter Sektor mit der Detektorelektronik ausgestattet. An diesen zwei Aufbauten wird die ALTRO Charakterisierung fortgeführt, verfeinert und komplettiert.
We reexamine the scenario of homogeneous nucleation of the quark-gluon plasma produced in ultra-relativistic heavy ion collisions. A generalization of the standard nucleation theory to rapidly expanding system is proposed. The nucleation rate is derived via the new scaling parameter Z. It is shown that the size distribution of hadronic clusters plays an important role in the dynamics of the phase transition. The longitudinally expanding system is supercooled to about 3 6%, then it is reheated, and the hadronization is completed within 6 10 fm/c, i.e. 5 10 times faster than it was estimated earlier, in a strongly nonequilibrium way. PACS: 12.38.Mh; 12.39.Ba; 25.75.-q; 64.60.Qb
Nuclei can be described satisfactorily in a nonlinear chiral SU(3)-framework, even with standard potentials of the linearmodel. The condensate value of the strange scalar meson is found to be important for the properties of nuclei even without adding hyperons. By neglecting terms which couple the strange to the nonstrange condensate one can reduce the model to a Walecka model structure embedded in SU(3). We discuss inherent problems with chiral SU(3) models regarding hyperon optical potentials.
A new chiral SU(3) Lagrangian is proposed to describe the properties of kaons and antikaons in the nuclear medium, the ground state of dense matter and the kaon-nuclear interactions consistently. The saturation properties of nuclear matter are reproduced as well as the results of the Dirac-Brückner theory. After taking into account the coupling between the omega meson and the kaon, we obtain similar results for the e ective kaon and antikaon energies as calculated in the one-boson-exchange model while in our model the parameters of the kaon-nuclear interactions are constrained by the SU(3) chiral symmetry. PACS number(s): 14.40.Aq, 12.39.Fe, 21.30.Fe
We study the effects of strict conservation laws and the problem of negative contributions to final momentum distribution during the freeze out through 3-dimensional hypersurfaces with space-like normal. We study some suggested solutions for this problem, and demonstrate it on one example. PACS: 24.10.Nz, 25.75.-q
The extend to which geometrical effects contribute to the production and suppression of the J/psi and qq minijet pairs in general is investigated for high energy heavy ion collisions at SPS, RHIC and LHC energies. For the energy range under investigation, the geometrical e ects referred to are shadowing and anti-shadowing, respectively. Due to those effects, the parton distributions in nuclei deviate from the naive extrapolation from the free nucleon result; fA 6= AfN. The strength of the shadowing/anti-shadowing e ect increases with the mass number. Therefore it is interesting to see the di erence between cross sections for e.g. S+U vs. Pb+Pb at SPS. The recent NA50 results for the survival probability of produced J/psi s has attracted great attention and are often interpreted as a signature of a quark gluon plasma. This publication will present a fresh look on hard QCD e ects for the charmonium production level. It is shown that the apparent suppression of J/psi s must also be linked to the production process. Due to the uncertainty in the shadowing of gluons the suppression of charmonium states might not give reli- able information on a created plasma phase at the collider energies soon available. The consequences of shadowing e ects for the xF distribution of J/psi s at s = 20 GeV, s = 200 GeV and s = 6 TeV are calculated for some relevant combinations of nuclei, as well as the pT distribution of minijets at midrapidity for Nf = 4 in the final state.
In fluid dynamical models the freeze out of particles across a three dimensional space-time hypersurface is discussed. The calculation of final momentum distribution of emitted particles is described for freeze out surfaces, with both space-like and time-like normals, taking into account conservation laws across the freeze out discontinuity.
We estimate the energy density epsilon pile-up at mid-rapidity in central Pb+Pb collisions from 2 200 GeV/nucleon. epsilon is decomposed into hadronic and partonic contributions. A detailed analysis of the collision dynamics in the framework of a microscopic transport model shows the importance of partonic degrees of freedom and rescattering of leading (di)quarks in the early phase of the reaction for Elab 30 GeV/nucleon. In Pb+Pb collisions at 160 GeV/nucleon the energy density reaches up to 4 GeV/fm3, 95% of which are contained in partonic degrees of freedom.
A new chiral SU(3) Lagrangian is proposed to describe the properties of kaons and anti-kaons in the nuclear medium. The saturation properties of nuclear matter are reproduced as well as the results of the Dirac-Brückner theory. After introducing the coupling between the omega meson and the kaon, our results for e ective kaon and anti-kaon energy are quite similar as calculated in the one-boson-exchange model.
The modification of the width of rho mesons due to in-medium decays and collisions is evaluated. The decay width is calculated from the imaginary part of the one-loop selfenergy at finite temperature. The collision width is related to the cross sections of the rho + pion and the rho + nucleon reactions. A calculation based on an e ective Lagrangian shows the importance of including the direct pho pi - > pho pi scattering which is dominated by the a1 exchange. A large broadening of the spectral function is found, accompanied by a strength suppression at the pole. http://www.arxiv.org/abs/nucl-th/9812059
Dissociation rates of J / psi's with comoving mesons : thermal versus nonequilibrium scenario.
(1998)
We study J/psi dissociation processes in hadronic environments. The validity of a thermal meson gas ansatz is tested by confronting it with an alternative, nonequilibrium scenario. Heavy ion collisions are simulated in the frame- work of the microscopic transport model UrQMD, taking into account the production of charmonium states through hard parton-parton interactions and subsequent rescattering with hadrons. The thermal gas and microscopic transport scenarios are shown to be very dissimilar. Estimates of J/psi survival probabilities based on thermal models of comover interactions in heavy ion collisions are therefore not reliable.
We calculate the shadowing of sea quarks and gluons and show that the shadowing of gluons is not simply given by the sea quark shadowing, especially at small x. The calculations are done in the lab frame approach by using the generalized vector meson dominance model. Here the virtual photon turns into a hadronic fluctuation long before the nucleus. The subsequent coherent interaction with more than one nucleon in the nucleus leads to the depletion sigma(gamma* A) < A sigma( gamma*N) known as shadowing. A comparison of the shadowing of quarks to E665 data for 40Ca and 207Pb shows good agreement.
Signatures of quark gluon plasma formation in high-energy heavy ion collisions : a critical review
(1998)
Ultra-relativistic heavy ion collisions offer the unique opportunity to probe highly excited dense nuclear matter under controlled laboratory conditions. The compelling driving force for such studies is the expectation that an entirely new form of matter may be created from such reactions. That form of matter, called the Quark Gluon Plasma (QGP), is the QCD analogue of the plasma phase of ordinary atomic matter. However, unlike such ordinary plasmas, the deconfined quanta of a QGP are not directly observable because of the fundamental confining property of the physical QCD vacuum. What is observable are hadronic and leptonic residues of the transient QGP state. There is a large variety of such individual probes.
We study J/psi suppression in AB collisions assuming that the charmonium states evolve from small, color transparent configurations. Their interaction with nucleons and nonequilibrated, secondary hadrons is simulated us- ing the microscopic model UrQMD. The Drell-Yan lepton pair yield and the J/psi /Drell-Yan ratio are calculated as a function of the neutral transverse en- ergy in Pb+Pb collisions at 160 GeV and found to be in reasonable agreement with existing data.
Abstract: Local thermal and chemical equilibration is studied for central AqA collisions at 10.7 160 AGeV in the Ultrarelativis- . tic Quantum Molecular Dynamics model UrQMD . The UrQMD model exhibits strong deviations from local equilibrium at the high density hadron string phase formed during the early stage of the collision. Equilibration of the hadron resonance matter is established in the central cell of volume Vs125 fm3 at later stages, tG10 fmrc, of the resulting quasi-isentropic expansion. The thermodynamical functions in the cell and their time evolution are presented. Deviations of the UrQMD quasi-equilibrium state from the statistical mechanics equilibrium are found. They increase with energy per baryon and lead to a strong enhancement of the pion number density as compared to statistical mechanics estimates at SPS energies. PACS: 25.75.-q; 24.10.Lx; 24.10.Pa; 64.30.qt