25.75.-q Relativistic heavy-ion collisions (collisions induced by light ions studied to calibrate relativistic heavy-ion collisions should be classified under both 25.75.-q and sections 13 or 25 appropriate to the light ions)
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We investigate the sensitivity of several observables to the density dependence of the symmetry potential within the microscopic transport model UrQMD (ultrarelativistic quantum molecular dynamics model). The same systems are used to probe the symmetry potential at both low and high densities. The influence of the symmetry potentials on the yields of pi-, pi+, the pi-/pi+ ratio, the n/p ratio of free nucleons and the t/3He ratio are studied for neutron-rich heavy ion collisions (208Pb+208Pb, 132Sn+124Sn, 96Zr+96Zr) at E_b=0.4A GeV. We find that these multiple probes provides comprehensive information on the density dependence of the symmetry potential.
Ultrarelativistische Schwerionenstöße werden seit etwa 15 Jahren untersucht, um Kernmaterie unter extremen Bedingungen zu erforschen; in Kollisionen schwerer Atomkerne kann bei hohen Einschußenergien Kernmaterie stark komprimiert und aufgeheizt werden. Die Bedeutung dieser Experimente wird durch Berechnungen der Quanten-Chromo-Dynamik auf raumzeitlichen Gittern hervorgehoben, die bei ausreichend hoher Energiedichte eine Phase voraussagen, in der die Quarks nicht mehr in Hadronen gebunden sind, sondern zusammen mit den Gluonen ein partonisches System ausbilden. Ist das System hinreichend groß und equilibriert, wird es als Quark-Gluon-Plasma bezeichnet. Die als Signatur für das Überschreiten der Phasengrenze vorgeschlagene erhöhte Produktion Seltsamkeit tragender Teilchen wurde in der Gegenüberstellung von elementaren Proton+Proton-Interaktionen und Kern+Kern-Stößen experimentell über einen weiten Energiebereich bestätigt. Eine solche Überhöhung kann aber auch durch rein hadronische Phänomene hervorgerufen werden. So tritt beispielsweise in statistischen Modellen bereits in einem Hadrongas eine Seltsamkeitserhöhung aufgrund des Übergangs von einem kanonischen zu einem großkanonischen Ensemble mit steigender Systemgröße in Kern+Kern-Stößen auf. Das motivierte die Messung der Systemgrößenabhängigkeit der Seltsamkeitsproduktion bei einer Einschußenergie, bei der in den Stößen der größten Kerne die partonische Phase erreicht werden sollte, während Proton-Proton-Interaktionen überlicherweise als hadronische Systeme betrachtet werden. In Kollisionen von Kohlenstoff- und Siliziumkernen bei 158 GeV pro Nukleon, deren Untersuchung Gegenstand dieser Arbeit ist, kann möglicherweise die Umgebung der Phasengrenze abgetastet werden. Besondere Aufmerksamkeit gilt der Frage nach dem Mechanismus der Seltsamkeitsproduktion in diesen Reaktionen. Das Experiment wurde am SPS-Beschleuniger am CERN in Genf durchgeführt, erstmals wurden dort leichte Projektilkerne durch den Aufbruch des primären Bleistrahls an einem Produktionstarget erzeugt. Das Herzstück des NA49-Spektrometers, mit dem die Daten aufgezeichnet wurden, sind die vier großvolumigen Spurendriftkammern, die die große Akzeptanz ermöglichen. Die Lambda- und Antilambda-Hyperonen aus C+C und Si+Si Kollisionen werden anhand ihrer Zerfallstopologie rekonstruiert und ihre Impulsverteilungen über einen weiten Bereich gemessen; mit zusätzlichen Annahmen werden schließlich die totalen Multiplizitäten extrapoliert. Die Produktion der Hyperonen pro Pion ist im Vergleich zu p+p-Daten bereits in C+C-Reaktionen deutlich erhöht, in Si+Si--Kollisionen ist annähernd der Wert aus Pb+Pb-Stößen erreicht. Mehrere Ursachen für diese Beobachtung werden diskutiert und mögliche Interpretationen vorgeschlagen. Der Grad an chemischer Equilibration und die Lage des Ausfrierpunktes im Phasendiagramm und im Vergleich zu anderen Stoßsystemen wird besprochen. Die Rapiditätsspektren der Lambda-Hyperonen entsprechen zunehmendem Stopping mit steigender Anzahl von Stößen pro Nukleon. Dadurch wird die Energie pro Nukleon im Feuerball erhöht, was zunehmende kinetische Energie der Teilchen und eine ansteigende Teilchenproduktion erzeugt. Die Verbreiterung der Transversalimpulsspektren mit der Systemgröße fügt sich in der Tat in das Bild anwachsenden radialen Flußes ein.
Production of J/ψ mesons in heavy ion collisions is considered within the statistical coalescence model. The model is in agreement with the experimental data of the NA50 Collaboration for Pb+Pb collisions at 158 AGeV in a wide centrality range, including the so-called “anomalous” suppression domain. The model description of the J/ψ data requires, however, strong enhancement of the open charm production in central Pb+Pb collisions. This model prediction may be checked in the future SPS runs.
We argue that the shape of the system-size dependence of strangeness production in nucleus-nucleus collisions can be understood in a picture that is based on the formation of clusters of overlapping strings. A string percolation model combined with a statistical description of the hadronization yields a quantitative agreement with the data at sqrt s_NN = 17.3 GeV. The model is also applied to RHIC energies.
We propose that the measurement of the transverse momentum dependence of the double ratio of the nuclear modification factors of charm and bottom jets, RAAc(pT)/RAAb(pT), in central nuclear collisions at the LHC will provide an especially robust observable that can be used to differentiate Standard Model perturbative QCD predictions from recently proposed strong coupling string drag models derived using the AdS/CFT conjecture.
We consider J/ψ production in heavy ion collisions at RHIC energies in the statistical coalescence model with exact (canonical ensemble) charm conservation. Charm quark–antiquark pairs are assumed to be created in primary hard parton collisions, but open and hidden charm particles are formed at the hadronization stage according to the laws of statistical mechanics. The dependence of the J/ψ production on both the number of nucleon participants and the collision energy is studied. The model predicts J/ψ suppression for low energies, whereas at the highest RHIC energy the model reveals J/ψ enhancement.
In the work presented herein the microscopic transport model BAMPS (Boltzmann Approach to Multi-Parton Scatterings) is applied to simulate the time evolution of the hot partonic medium that is created in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) and in Pb+Pb collisions at the recently started Large Hadron Collider (LHC). The study is especially focused on the investigation of the nuclear modification factor R_{AA}, that quantifies the suppression of particle yields at large transverse momentum with respect to a scaled proton+proton reference, and the simultaneous description of the collective properties of the medium in terms of the elliptic flow v_{2} within a common framework.
We calculate the asymptotic high-energy amplitude for electrons scattering at one ion, as well as at two colliding ions, by means of perturbation theory. We show that the interaction with one ion eikonalizes and that the interaction with two ions causally decouples. We are able to put previous results on perturbative grounds and propose further applications for the obtained rules for interactions on the light cone. We discuss the implications of the eikonal amplitude on the pair production probability in ultrarelativistic peripheral heavy-ion collisions. In this context the Weizsäcker-Williams method is shown to be exact in the ultrarelativistic limit, irrespective of the produced particles’ mass. A new equivalent single-photon distribution is derived, which correctly accounts for Coulomb distortions. The impact on single-photon induced processes is discussed.
We solve the coupled Wong Yang–Mills equations for both U(1) and SU(2) gauge groups and anisotropic particle momentum distributions numerically on a lattice. For weak fields with initial energy density much smaller than that of the particles we confirm the existence of plasma instabilities and of exponential growth of the fields which has been discussed previously. Also, the SU(2) case is qualitatively similar to U(1), and we do find significant “abelianization” of the non-Abelian fields during the period of exponential growth. However, the effect nearly disappears when the fields are strong. This is because of the very rapid isotropization of the particle momenta by deflection in a strong field on time scales comparable to that for the development of Yang–Mills instabilities. This mechanism for isotropization may lead to smaller entropy increase than collisions and multiplication of hard gluons, which is interesting for the phenomenology of high-energy heavy-ion collisions.
Nuclear matter, that takes the form of protons and neutrons under normal conditions, is subject to a phase transition at high temperatures and densities, liberating the quarks and gluons that are usually confined in nucleons and creating a medium of free partons: the Quark-Gluon-Plasma. It is generally believed that this state of matter can be created in relativistic collisions of heavy nuclei. The study of the medium created in these collisions is the subject of heavy-ion physics. One topic within this field are particles with high transverse momentum, that are created in initial hard collisions between partons of the incoming nuclei. The energetic partons lose energy due to interactions with the medium before they fragment into a jet of hadrons. Due to momentum conservation, these jets are usually created as back-to-back pairs, or less commonly as three-jet or photon-jet events, where a single jet is balanced by a hard photon. The energy loss can be measured using correlations between particles with high transverse momenta. A trigger particle is selected with very high transversemomentum and the distribution of the azimuthal angle of associated particles in the same event is studied, relative to the azimuth of the trigger particle.These azimuthal correlations show a peak for opening angles around 0 from particles selected from the same jet, and a second peak at opening angles around 180 degrees from back-to-back di-jets. Random combinations with the underlying event generate a flat background, extending over the full range of opening angles. The STAR experiment observed a modification of these correlations in central Au+Au collisions, where trigger particles with 4GeV < pT(trigger) < 6GeV and associated particles with 2GeV < pT(trigger) < 4GeV were selected. A strong suppression has been observed for away-side correlations in central Au+Au collisions, relative to p+p, d+Au and peripheral Au+Au data. This can be explained by assuming two partons going in opposite directions, where at least one has to travel a large distance through the medium, causing energy loss and effectively removing the event from the analysis. For near-side correlations, no significant modification has been observed, which can be explained by surface emission, assuming that the observed jets have travelled only a short distance in themedium, not leaving enough time for interactions with the medium. Both trigger- and associated particles in a correlation analysis with charged hadrons are subject to modifications due to the medium. This can be avoided by using photon-jet events instead of di-jets, because the photon does not interact with the medium and therefore provides the best available measure of the properties of the opposite jet in the presence of the underlying event. This thesis studies azimuthal correlations between regions of high energy deposition in the electro-magnetic calorimeter as trigger- and charged tracks as associated particles. The data sample had been enriched by online event selection, allowing for the selection of trigger particles with a transverse energy of more than 10GeV and associated particles with more than 2,3 or 4 GeV. The away-side yield per trigger particle is strongly suppressed like in correlations between charged particles. The near-side yield is also reduced by about a factor two, clearly different from charged correlations. The trigger particles are a mixture of photon pairs from the decays of neutral pions and single photons, mainly from photon-jet events, with small contributions from other hadron decays and fragmentation photons. Pythia simulations predict a ratio of neutral pions to prompt photons of 3.5:1 in p+p collisions with the same cuts as in the presented analysis. Single particle suppression further reduces this ratio in central Au_Au collisions, down to about 0.8:1, indicating that the majority of trigger particles in central Au+Au collisions are prompt photons. The increasing fraction of prompt photon triggers without an accompanying jet and therefore zero associated yield reduces the average yield per trigger particle. The magnitude of the observed effect agrees well with the expectation from Pythia simulations and the assumption of a single particle suppression by a factor 4-5. An analysis of away-side correlations is more difficult, because both photon-jet and di-jet events contribute. The aim is the separation of these two contributions. As a clear separation is not possible with the available dataset, a comparison with two different scenarios is given, where a surprisingly small suppression by only a factor of about 5 is favoured for both dijet- and photon-jet-correlations. A separate measurement of both contributions will be possible by a shower-shape analysis with the EM calorimeter or a comparison with charged correlations in the same kinematic region.