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This thesis investigates the jet-medium interactions in a Quark-Gluon Plasma using a hydrodynamical model. Such a Quark-Gluon Plasma represents a very early stage of our universe and is assumed to be created in heavy-ion collisions. Its properties are subject of current research. Since the comparison of measured data to model calculations suggests that the Quark-Gluon Plasma behaves like a nearly perfect liquid, the medium created in a heavy-ion collision can be described applying hydrodynamical simulations. One of the crucial questions in this context is if highly energetic particles (so-called jets), which are produced at the beginning of the collision and traverse the formed medium, may lead to the creation of a Mach cone. Such a Mach cone is always expected to develop if a jet moves with a velocity larger than the speed of sound relative to the medium. In that case, the measured angular particle distributions are supposed to exhibit a characteristic structure allowing for direct conclusions about the Equation of State and in particular about the speed of sound of the medium. Several different scenarios of jet energy loss are examined (the exact form of which is not known from first principles) and different mechanisms of energy and momentum loss are analyzed, ranging from weak interactions (based on calculations from perturbative Quantum Chromodynamics, pQCD) to strong interactions (formulated using the Anti-de-Sitter/Conformal Field Theory Correspondence, AdS/CFT). Though they result in different angular particle correlations which could in principle allow to distinguish the underlying processes (if it becomes possible to analyze single-jet events), it is shown that the characteristic structure observed in experimental data can be obtained due to the different contributions of several possible jet trajectories through an expanding medium. Such a structure cannot directly be connected to the Equation of State. In this context, the impact of a strong flow created behind the jet is examined which is common to almost all jet deposition scenarios. Besides that, the transport equations for dissipative hydrodynamics are discussed which are fundamental for any numerical computation of viscous effects in a Quark-Gluon Plasma.
Kaon and pion production in centrality selected minimum bias Pb+Pb collisions at 40 and 158A GeV
(2009)
Results on charged kaon and negatively charged pion production and spectra for centrality selected Pb+Pb mininimum bias events at 40 and 158A GeV have been presented in this thesis. All analysis are based on data taken by the NA49 experiment at the accelerator Super Proton Synchrotron (SPS) at the European Organization for Nuclear Research (CERN) in Geneva, Switzerland. The kaon results are based on an analysis of the mean energy loss <dE/dx> of the charged particles traversing the detector gas of the time projection chambers (TPCs). The pion results are from an analysis of all negatively charged particles h- corrected for contributions from particle decays and secondary interactions. For the dE/dx analysis of charged kaons, main TPC tracks with a total momentum between 4 and 50 GeV have been analyzed in logarithmic momentum log(p) and transverse momentum pt bins. The resulting dE/dx spectra have been fitted by the sum of 5 Gaussians, one for each main particle type (electrons, pions, kaons, protons, deuterons). The amplitude of the Gaussian used for the kaon part of the spectra has been corrected for efficiency and acceptance and the binning has been transformed to rapidity y and transverse momentum pt bins. The multiplicity dN/dy of the single rapidity bins has been derived by summing the measured range of the transverse momentum spectra and an extrapolation to full coverage with a single exponential function fitted to the measured range. The results have been combined with the mid-rapidity measurements from the time-of-flight detectors and a double Gaussian fit to the dN/dy spectra has been used for extrapolation to rapidity outside of the acceptance of the dE/dx analysis. For the h- analysis of negatively charged pions, all negatively charged tracks have been analyzed. The background from secondary reactions, particle decays, and gamma-conversions has been corrected with the VENUS event generator. The results were also corrected for efficiency and acceptance and the pt spectra were analyzed and extrapolated where necessary to derive the mean yield per rapidity bin dN/dy. The mean multiplicity <pi-> has been derived by summing up the measured dN/dy and extrapolating the rapidity spectrum with a double Gaussian fit to 4pi coverage. The results have been discussed in detail and compared to various model calculations. Microscopical models like URQMD and HSD do not describe the full complexity of Pb+Pb collisions. Especially the production of the positively charged kaons, which carry the major part of strange quarks, cannot be consistently reproduced by the model calculations. Centrality selected minimum bias Pb+Pb collisions can be described as a mixture of a high-density region of multiply colliding nucleons (core) and practically independent nucleon-nucleon collisions (corona). This leads to a smooth evolution from peripheral to central collisions. A more detailed approach derives the ensemble volume from a percolation of elementary clusters. In the percolation model all clusters are formed from coalescing strings that are assumed to decay statistically with the volume dependence of canonical strangeness suppression. The percolation model describes the measured data for top SPS and RHIC energies. At 40A GeV, the system size dependence of the relative strangeness production starts to evolve from the saturation seen at higher energies from peripheral events onwards towards a linear dependence at SIS and AGS. This change of the dependence on system size occurs in the energy region of the observed maximum of the K+ to pi ratio for central Pb+Pb collisions. Future measurements with heavy ion beam energies around this maximum at RHIC and FAIR as well as the upgraded NA49 successor experiment NA61 will further improve our understanding of quark matter and its reflection in modern heavy ion physics and theories.