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In order to fully understand the new state of matter formed in heavy ion collisions, it is vital to isolate the always present final state hadronic contributions within the primary Quark-Gluon Plasma (QGP) experimental signatures. Previously, the hadronic contributions were determined using the properties of the known mesons and baryons. However, according to Hagedorn, hadrons should follow an exponential mass spectrum, which the known hadrons follow only up to masses of M = 2 GeV. Beyond this point the mass spectrum is flat, which indicates that there are "missing" hadrons, that could potentially contribute significantly to experimental observables. In this thesis I investigate the influence of these "missing" Hagedorn states on various experimental signatures of QGP. Strangeness enhancement is considered a signal for QGP because hadronic interactions (even including multi-mesonic reactions) underpredict the hadronic yields (especially for strange particles) at the Relativistic Heavy Ion Collider, RHIC. One can conclude that the time scales to produce the required amount of hadronic yields are too long to allow for the hadrons to reach chemical equilibrium within the lifetime of a cooling hadronic fireball. Because gluon fusion can quickly produce strange quarks, it has been suggested that the hadrons are born into chemical equilibrium following the Quantum Chromodynamics (QCD) phase transition. However, we show here that the missing Hagedorn states provide extra degrees of freedom that can contribute to fast chemical equilibration times for a hadron gas. We develop a dynamical scheme in which possible Hagedorn states contribute to fast chemical equilibration times of X X pairs (where X = p, K, Lambda, or Omega) inside a hadron gas and just below the critical temperature. Within this scheme, we use master equations and derive various analytical estimates for the chemical equilibration times. Applying a Bjorken picture to the expanding fireball, the hadrons can, indeed, quickly chemically equilibrate for both an initial overpopulation or underpopulation of Hagedorn resonances. We compare the thermodynamic properties of our model to recent lattice results and find that for both critical temperatures, Tc = 176 MeV and Tc = 196 MeV, the hadrons can reach chemical equilibrium on very short time scales. Furthermore the ratios p/pi, K/pi , Lambda/pi, and Omega/pi match experimental values well in our dynamical scenario. The effects of the "missing" Hagedorn states are not limited to the chemical equilibration time. Many believe that the new state of matter formed at RHIC is the closet to a perfect fluid found in nature, which implies that it has a small shear viscosity to entropy density ratio close to the bound derived using the uncertainty principle. Our hadron resonance gas model, including the additional Hagedorn states, is used to obtain an upper bound on the shear viscosity to entropy density ratio, eta/s, of hadronic matter near Tc that is close to 1/(4pi). Furthermore, the large trace anomaly and the small speed of sound near Tc computed within this model agree well with recent lattice calculations. We also comment on the behavior of the bulk viscosity to entropy density ratio of hadronic matter close to the phase transition, which qualitatively has a different behavior close to Tc than a hadron gas model with only the known resonances. We show how the measured particle ratios can be used to provide non-trivial information about Tc of the QCD phase transition. This is obtained by including the effects of highly massive Hagedorn resonances on statistical models, which are generally used to describe hadronic yields. The inclusion of the "missing" Hagedorn states creates a dependence of the thermal fits on the Hagedorn temperature, TH , and leads to a slight overall improvement of thermal fits. We find that for Au+Au collisions at RHIC at sqrt{sN N} = 200 GeV the best square fit measure, chi^2 , occurs at TH = Tc = 176 MeV and produces a chemical freeze-out temperature of 172.6 MeV and a baryon chemical potential of 39.7 MeV.
For this thesis photon and pi0 spectra in Gold-Gold-collisions at an energy of sqrt(s_NN) = 62 GeV were measured using the STAR-experiment at RHIC. Heavy ion collisions allow to study strongly interacting matter under extreme condiditons in the laborartory. Nuclear matter is strongly compressed and heated. Theories predict in a system of strongy interacting matter at high temperature and pressure a phase transition from hadronic matter, in which quarks are bound into hadrons, to a plasma of free quarks and gluons (QGP). To study the properties of this created medium, a number of different observables is available. One possibility to determine the temperature of such a system, is to measure the photon emission from the medium. The experimental difficulty is that there are more mechanisms producing photons than just the thermal production. Photons are produced in hard scattering processes or can be the result of the interaction of hard partons with the medium. According to theoretical calculations the photon yield from hard processes exceeds the thermal production for transverse momenta above 3 GeV/c. Photons from hard processes and thermal photons are referred to as direct photons, because they are produced inside of the medium. The largest part of the photons below pt=3GeV/c, however, comes from electromagnetic decays of hadrons in the final state of the collision. The largest fraction comes from the pi0- and the eta-mesons. Their contribution to the photon spectra can be determined by measuring the spectra of these decaying particles and calculating the resulting, corresponding photon spectra. The experimental difficulty is to measure these spectra to an accuracy of a few percent because the decay photons make up about 90% of all photons in the relevant phase space region. The STAR-experiment provides different detectors to measure photons and pi0-mesons. The primary detector for this kind of measurement are the electromagnetic calorimeters. However, the analysis described in this thesis uses the time projection chamber (TPC). Because photons don't carry electric charge and the TPC is only sensitive to charged particles, a conversion of the photon into an electron-positron-pair is required. This happens inside the electromagnetic fields of the nuclei and the electrons in the atomic shell of the detector material in the experimental setup of STAR. The resulting electron and positron tracks are measrued in the TPC. In chapter 3 the reconstruction of conversions from the measured tracks is described. Chapter 4 discusses the efficiency of the measurement, which is determined with a Monte-Carlo-Method, and the uncertainties of the correction. Chapter 5 presents the results of the analysis. The data set, on which the analysis is based, consists of Gold-Gold-Collisions an a center of mass energy of sqrt(s_NN)=62GeV. The selection criteria for individual events during data taking and during the analysis are explained. The data set is divided into four centrality selection classes. The first result are the transverse momentum and rapidity spectra of inclusive photons for all four centralities and the whole data set. Pi0-spectra versus transverse momentum for the four centralities and the whole data set are also shown. The pi0-spectra are compared to the spectra of pi0-mesons measured by the PHENIX-Collaboration at the same energy and with pi0-spectra measured by STAR at full RHIC energy. In addition a comparison to charged pi+- and pi--spectra is shown, which were also measured by the STAR collaboration. It is attempted to extract the fraction of direct photons by dividing the spectra of inclusive photons by the spectra of simulated decay photons. In these simulations pi0- and eta-spectra are modeled based on the pi+- and pi--spectra. Studying the uncertainties of this procedure shows that the size of the uncertainties is of the same magnitude as the signal of direct photons. Also the systematic uncertainties of the pi+- and pi--spectra are similar. Therefore the measurement of direct photon spectra is not possible. In chapter 6 possibilities are described to reduce the large systematic uncertainties. In addition it is discussed, what could be done with an already existing data set at full RHIC energy and how the addition of a dedicated converter during a future data taking period could reduce the systematic errors. The result of this thesis are inklusive photon and pi0 spectra. The systematic uncertainties were extensively studied. It is described, which enhancements are necessary to provide the perspective for measuring direct photons in the area of 1 to 3 GeV/c transverse momentum.