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We investigate the onset of multifragmentation employing an improved version of the N-body ‘‘quantum’’ molecular-dynamics approach. We study in detail the reaction 18O+197Au at 84 MeV/nucleon and find good agreement between the calculated results and the data for the double-differential proton cross section, the mass yield, the multiplicity, the kinetic energy of the fragments, and even for the kinematic correlations between intermediate mass fragments (IMF’s), which have been measured in this experiment for the first time. We observe a strong correlation between the impact parameter and both the size of the target remnant as well as the average proton multiplicity. Hence both observables can be used to determine the impact parameter experimentally. The IMF’s come from the most central collisions. The calculations confirm the experimental result that they are not emitted from an equilibrated system. Although the inclusive energy spectra look thermal, we cannot identify an impact parameter-independent isotropically emitting source. Even in central collisions global equilibrium is not observed. We find that multifragment emission at this bombarding energy is caused by a process very similar to that proposed in the macroscopic cold multifragmentation model. Thus it has a different origin than at beam energies around 1 GeV/nucleon, although the mass yield has an almost identical slope.
We investigate the hydrodynamical flow of nuclear matter in a conical-shock-wave scenario of a central, asymmetric heavy-ion collision. This work is motivated by a suggestion of Chapline and Granik that the creation of a deconfined phase of quarks and gluons behind the shock will appreciably increase the deflection angle of the matter flow. We employ several hadron matter equations of state recently suggested to solve the conical-shock-wave problem and compare the results with a calculation using the bag equation of state. We find that large differences in the deflection angle obtained in the rest frame of the shock vanish in the laboratory system. However, a signature for the deconfinement transition may be the transverse momentum of the matter flow, which is up to a factor of 2 larger for the quark-gluon plasma. Thus, an excitation function of the mean transverse momentum would show an increase at a certain bombarding energy, signaling the onset of the deconfinement transition.
We study the transition from fusion-fission phenomena at about 20 MeV/nucleon multifragmentation at 100–200 MeV/nucleon in the reaction 16O+80Br employing the quantum molecular dynamics model. The time evolution of the density and mass distribution, the charged-particle multiplicity, and spectra as well as angular distributions of light particles are investigated. The results exhibit the transition of the disassembly mechanism, but no sharp change is found. The results are in good agreement with recently measured 4-Pi data.
Inclusive neutron spectra were measured at 0°, 4°, 8°, 15°, 30°, and 42° from Nb-Nb and Au-Au collisions at 800 MeV/nucleon. A peak that originates from neutron evaporation from the projectile appears in the spectra at angles out to 8°. The shapes and magnitudes of the spectra are compared with those calculated from models of nucleus-nucleus collisions. The differential cross sections for Au-Au collisions are about four times those for Nb-Nb collisions. The predictions of the Vlasov-Uehling-Uhlenbeck (VUU) and QMD theories agree with the angular distributions of the differential cross sections except at small angles; the VUU prediction overestimates the angular distributions from a few degrees to about 20°, whereas the QMD prediction underestimates the angular distributions below 8°. The Firestreak model overestimates the angular distribution for Nb-Nb collisions and underestimates it for Au-Au collisions. Also, the VUU and QMD models agree with the measured double-differential cross sections in more angular and energy regions than the Firestreak and intranuclear cascade models; however, none of the models can account for the peaks at small angles (θ≤15°).
Within a relativistic mean-field theory (RMFT) experimental data on the single-particle spectra of lambda hypernuclei are well reproduced. It is shown that the coupling constants cannot be fixed unambiguously from the single-particle spectra. The stability and structure of multi-lambda hypernuclei is explored on the basis of the RMFT using the coupling constants as determined from the observed single lambda hypernuclear levels. It is predicted that multistrange nuclei exhibit an enhanced interaction radius, which further increases in the case of finite temperatures. We suggest that multi-lambda hypernuclei could be produced in high-energy heavy ions and observed in secondary noncharge-changing reactions. The equation of state of lambda matter and the possibility of pure lambda droplets are also discussed.
Shock discontinuities around the confinement-deconfinement transition in baryon-rich dense matter
(1989)
Nuclear transport models are important tools for interpretation of many heavy-ion experiments and are essential in efforts to probe the nuclear equation of state. In order to fulfill these roles, the model predictions should at least agree with observed single-particle-inclusive momentum spectra; however, this agreement has recently been questioned. The present work compares the Vlasov-Uehling-Uhlenbeck model to data for mass-symmetric systems ranging from 12C+12C to 139La+139La, and we find good agreement within experimental uncertainties at 0.4A and 0.8A GeV. For currently available data, these uncertainties are too large to permit effective nucleon-nucleon scattering cross sections in the nuclear medium to be extracted at a useful level of precision.
Nuclear transport models including density- and momentum-dependent mean-field effects are compared to intranuclear-cascade models and tested on recent data on inclusive p-like cross sections for 800A-MeV La+La. We find a remarkable agreement between most model calculations but a systematic disagreement with the measured yield at 20°, possibly indicating a need for modification of nuclear transport properties at high densities.
The quantum molecular dynamic method is used to study multifragmentation and fragment flow and their dependence on in-medium cross sections, momentum dependent interactions, and the nuclear equation of state, for collisions of 197Au+197Au and 93Nb+93Nb in the bombarding energy regime from 100 to 800A MeV. Time and impact parameter dependence of the fragment formation and their implications for the conjectured liquid-vapor phase transition are investigated. We find that the inclusive fragment mass distribution is independent of the equation of state and exhibits a power-law behavior Y(A)∼A-τ with an exponent τ≊-2.3. True multifragmentation events are found in central collisions for energies Elab∼30–200 MeV/nucleon. The associated light fragment (d,t,α) to proton ratios increase with the multiplicity of charged particles and decrease with energy, in agreement with recent experiments. The calculated absolute charged particle multiplicities, the multiplicities of intermediate mass (A>4) fragments, and their respective rapidity distributions do compare well with recent 4π data, but are quite insensitive to the equation of state. On the other hand, these quantities depend sensitively on the nucleon-nucleon scattering cross section, and can be used to determine σ experimentally. The transverse momentum flow of the complex fragments increases with the stiffness of the equation of state. Reduced (in-medium) n-n scattering cross sections reduce the fragment flow. Momentum dependent interactions increase the fragment flow. It is shown that the measured fragment flow at 200A MeV can be reproduced in the model. We find that also the increase of the px/A values with the fragment mass is in agreement with experiments. The calculated fragment flow is too small as compared to the plastic ball data, if a soft equation of state with in-medium corrections (momentum dependent interactions plus reduced cross sections) is employed. An alternative, most intriguing resolution of the puzzle about the stiffness of the equation of state could be an increase of the scattering cross sections due to precritical scattering in the vicinity of a phase transition.
The statistical model is used to illustrate the consequences of a successive binary decay mechanism as the initial nuclear excitation is pushed towards the limits of stability. The partition of the excitation energy between light and heavy fragments is explicitly calculated, as are the consequences of the decay of the primary light fragments to particle-bound residual nuclei which would be observed experimentally. The test nucleus 100 44 Ru is considered at initial excitations of 100, 200, 400, and 800 MeV. Exit channels of n, p, and α; and 100 clusters of 3 ≤ Z ≤ 20 ≤ 4, 6 ≤ A ≤ 48 are considered from all nuclides in the deexcitation cascade. The total primary and final cluster yields are shown versus Z and initial excitation. The primary versus final yields are also shown individually for 12C, 26Mg, and 48Ca. We show how multifragmentation yields will change with the excitation energy due to a successive binary decay mechanism. Measurements that may be prone to misinterpretation are discussed, as are those that should be representative of initial nucleus excitation.
Distillation and survival of strange quark matter droplets in ultrarelativistic heavy ion collisions
(1991)
Recently it has been suggested that rather cold droplets of absolutely stable or metastable strange-quark matter may be distilled in heavy-ion collisions during the phase transition from a baryon-rich quark-gluon plasma (QGP) to hadron matter. Here we present a model describing the hadronization of the QGP through particle emission, which is based solely on thermodynamical arguments. Pions and K+’s and K0’s carry away entropy and antistrangeness from the system, thus facilitating the cooling process and the strangelet formation. Our results are supported by revised more sophisticated rate calculations. Two rather unexpected results are obtained when this model is applied to the investigation of strangelet production. The strangeness separation mechanism and the formation process works well even for higher initial entropies per baryon, tantamount to higher bombarding energies. The surviving strangelets have a rather high strangeness content, fs∼1.2–2 [i.e., Z/A∼(-0.1)–(-0.5)]. Hence droplets of strange-quark matter with a baryon number of ∼10–30 and with a negative charge may be produced. They may serve as a unique signature for the transient formation of a quark-gluon plasma in heavy-ion collisions.
Relativistic heavy ion collisions constitute a prolific source of hyperons: tens of hyperons per event are predicted at energies E≥10 GeV/nucleon, providing a scenario for the formation of metastable exotic multihypernuclear objects. They may exhibit exceptional properties: bound neutral (e.g., 4M2Λ2n, 10M2Λ8n, pure Λ droplets, 8Λ) and even negatively charged composites objects with positive baryon number (e.g., 4M2Σ-2n, 6M2Λ2Ξ-2n) could be formed in rare events. Such negative nuclei can easily be identified in a magnetic spectrometer. They could be considerably more abundant than antinuclei of the same A. We use the relativistic meson-baryon field theory—which gives an excellent description of normal nuclear and single-Λ hypernuclear properties—to calculate the rich spectrum of such exotic objects, their stability, and their structure. We also find solutions for a large variety of bound short-lived nuclei (e.g., 8M2Λ,2Σ-2p2n), which may decay strongly via formation of cascade (Ξ) particles. Multi-Ξ hypernuclei are also evaluated. A variety of potential candidates for such metastable exotic nuclei is presented. It turns out that the properties of such exotic multihypernuclear objects reveal quite similar features as the strangelet proposed as a unique signature for quark-gluon plasma formation in heavy ion collisions.
We present a new type of flow analysis, based on a particle-pair correlation function, in which there is no need for an event-by-event determination of the reaction plane. Consequently, the need to correct for dispersion in an estimated reaction plane does not arise. Our method also offers the option to avoid any influence from particle misidentification. Using this method, streamer chamber data for collisions of Ar+KCl and Ar+BaI2 at 1.2 GeV/nucleon are compared with predictions of a nuclear transport model.
A quasiclassical Pauli potential is used to simulate the Fermi motion of nucleons in a molecular dynamical simulation of heavy ion collisions. The thermostatic properties of a Fermi gas with and without interactions are presented. The inclusion of this Pauli potential into the quantum molecular dynamics (QMD) approach yields a model with well defined fermionic ground states, which is therefore also able to give the excitation energies of the emitted fragments. The deexcitation mechanisms (particle evaporation and multifragmentation) of the new model are investigated. The dynamics of the QMD with Pauli potential is tested by a wide range of comparisons of calculated and experimental double-differential cross sections for inclusive p-induced reactions at incident energies of 80 to 160 MeV. Results at 256 and 800 MeV incident proton energy are presented as predictions for completed experiments which are as yet unpublished.
Stopping power and thermalization in relativistic heavy ion collisions is investigated employing the quantum molecular dynamics approach. For heavy systems stopping of the incoming nuclei is predicted, independent of the energy. The influence of the quantum effects and their increasing importance at low energies, is demonstrated by inspection of the mean free path of the nucleons and the n-n collision number. Classical models, which neglect these effects, overestimate the stopping and the thermalization as well as the collective flow and squeeze out. The sensitivity of the transverse and longitudinal momentum transfer to the in-medium cross section and to the pressure is investigated.
Experimental results are presented on the charge, velocity, and angular distributions of intermediate mass fragments (IMFs) for the reaction Fe+Au at bombarding energies of 50 and 100 MeV/nucleon. Results are compared to the quantum molecular dynamics (QMD) model and a modified QMD which includes a Pauli potential and follows the subsequent statistical decay of excited reaction products. The more complete model gives a good representation of the data and suggests that the major source of IMFs at large angles is due to multifragmentation of the target residue.
We present a RQMD calculation of antiproton yields and their momentum distribution in Ne + NaF collisions at 2 GeV/u. The antiprotons can be produced below threshold due to multi-step excitations for which meson-baryon interactions play a considerable role. In this system the annihilation probability for an initially produced antiproton is predicted to be about 65%.
Strange hadronic matter
(1993)
In an extended mean field theory, we find a large class of bound multistrange objects, formed from combinations of {p,n,Λ,Ξ0,Ξ-} baryons, which are stable against strong decay. We predict a maximal binding energy per baryon of EB/A≊-21 MeV, strangeness per baryon fs≊1.2, charge per baryon fq≊-0.1 to 0, and baryon density 2.5–3 times that of ordinary nuclei. For A≥6, we obtain stable combinations involving only {Λ,Ξ0,Ξ-} hyperons.
Strong correlations between baryon stopping in the projectile rapidity hemisphere and target excitation have been found in the light-ion-induced reactions at the BNL Alternating Gradient Synchrotron (AGS) (E814 group). Results in the framework of the relativistic molecular dynamics approach (RQMD) describe recent E814 data quite well. We discuss the RQMD results together with proton and pion data from the E802 group near midrapidity. They have raised the question of whether partial transparency could be seen in these experiments. The RQMD results indicate strong transverse baryon flow in central Si+Au collisions after the projectile has been stopped in the target.
We present a calculation of antiproton yields in Si+Al and Si+Au collisions at 14.5A GeV in the framework of the relativistic quantum molecular dynamics approach (RQMD). Multistep processes lead to the formation of high-mass flux tubes. Their decay dominates the initial antibaryon yield. However, the subsequent annihilation in the surrounding baryon-rich matter suppresses the antiproton yield considerably: Two-thirds of all antibaryons are annihilated even for the light Si+Al system. Comparisons with preliminary data of the E802 experiment support this analysis.
Accurate impact parameter determination in a heavy-ion collision is crucial for almost all further analysis. We investigate the capabilities of an artificial neural network in that respect. First results show that the neural network is capable of improving the accuracy of the impact parameter determination based on observables such as the flow angle, the average directed inplane transverse momentum and the difference between transverse and longitudinal momenta. However, further investigations are necessary to discover the full potential of the neural network approach.
Azimuthal correlations of pions are studied with the quantum molecular dynamics model. Pions are preferentially emitted perpendicular to the reaction plane. Our analysis shows that this anisotropy is dominated by pion absorption on the spectator matter in the reaction plane. Pions emitted perpendicular to the reaction plane undergo less rescattering than those emitted in the reaction plane and might therefore be more sensitive to the early hot and dense reaction phase.
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.
In order to investigate the role of neuronal synchronization in perceptual grouping, a new method was developed to record selectively from multiple cortical sites of known functional specificity as determined by optical imaging of intrinsic signals. To this end, a matrix of closely spaced guide tubes was developed in cooperation with a company providing the essential manufacturing technique RMPD® (Rapid Micro Product Development). The matrix was embedded into a framework of hard and software that allowed for the mapping of each guide tube onto the cortical site an electrode would be led to if inserted into that guide tube. With these developments, it was possible to determine the functional layout of the cortex by optical imaging and subsequently perform targeted recordings with multiple electrodes in parallel. The method was tested for its accuracy and found to target the electrodes with a precision of 100 µm to the desired cortical locations. Using the developed technique, neuronal activity was recorded from area 18 of anesthetized cats. For stimulation, Gabor-patches in different geometrical configurations were placed over the recorded receptive fields merging into visual objects appropriate for testing the hypothesis of feature binding by synchrony. Synchronization strength was measured by the height of the cross-correlation centre peaks. All pairwise synchronizations were summarized in a correlation index which determined the mean difference of the correlation strengths between conditions in which recording sites should or should not fire in synchrony according to the binding hypothesis. The correlation index deviated significantly from zero for several of these configurations, further supporting the hypothesis that synchronization plays an important role in the process of perceptual grouping. Furthermore, direct evidence was found for the independence of the synchronization strength from the neuronal firing rate and for neurons that change dynamically the ensemble they participate in. In parallel to the experimental approach, mechanisms of oscillatory long range synchronization were studied by network simulations. To this end, a biologically plausible model was implemented using pyramidal and basket cells with Hodgkin-Huxley like conductances. Several columns were built from these cells and intra- and inter-columnar connections were mimicked from physiological data. When activated by independent Poisson spike trains, the columns showed oscillatory activity in the gamma frequency range. Correlation analysis revealed the tendency to locally synchronize the oscillations among the columns, but a rapid phase transition occurred with increasing cortical distance. This finding suggests that the present view of the inter-columnar connectivity does not fully explain oscillatory long range synchronization and predicts that other processes such as top-down influences are necessary for long range synchronization phenomena.
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.