arXiv subjects
Publications and source records attributed to Klaus Reygers.
A brief overview of direct-photon measurements in ultra-relativistic nucleus-nucleus collisions is given. The results for Pb+Pb collisions at sqrt{s_NN} = 17.3 GeV and for Au+Au collisions at sqrt{s_NN} = 200 GeV are compared to estimates of the direct-photon yield from hard scattering. Both results leave room for a significant thermal photon component. A description purely based on hard scattering processes, however, is not ruled out so far.
Results from the PHENIX experiment on direct-photon production in Au+Au collisions at sqrt{s_{NN}} = 200 GeV for transverse momenta 1 < p_T < 13 GeV/c are presented. Direct-photon yields at high p_T scale as expected for particle production in hard processes. This supports jet-quenching models which attribute the suppression of high-p_T hadrons to the energy loss of fast partons in the quark-gluon plasma. The low-p_T direct-photon spectra, measured via e+e- pairs with small invariant masses, are possibly related to the production of thermal direct photons in Au+Au collisions at RHIC.
Direct-photon measurements in p+p and Au+Au collisions at sqrt{s_{NN}} = 200 GeV from the PHENIX experiment are presented. The p+p results are found to be in good agreement with next-to-leading-order (NLO) perturbative QCD calculations. Direct-photon yields in Au+Au collisions scale with the number of inelastic nucleon-nucleon collisions and don't exhibit the strong suppression observed for charged hadrons and neutral pions. This observation is consistent with models which attribute the suppression of high-pT hadrons to energy loss of quarks and gluons in the hot and dense medium produced in Au+Au collisions at RHIC.
Direct photons are an important tool for the detection of the quark-gluon plasma in ultra-relativistic nucleus-nucleus collisions. Direct-photon measurements were made in Pb+Pb collisions at sqrt{s_NN}=17.2 GeV and in Au+Au collisions at sqrt{s_NN}=200 GeV. These results are reviewed and compared with model calculations.
Transverse momentum (p_T) spectra of neutral pions and charged hadrons measured in Au+Au and d+Au collisions at sqrt{s_NN}=200 GeV by the PHENIX experiment at RHIC are compared to p+p reference spectra at the same sqrt{s_NN}. In central Au+Au collisions a factor 4-5 suppression for neutral pions and charged hadrons with p_T > 5 GeV/c is found relative to the p+p reference scaled by the nuclear overlap function . In contrast, such a suppression of high p_T particles is absent in d+Au collisions.
The status of the search for direct photons in Au+Au collisions at sqrt{s_NN} = 130 GeV and sqrt{s_NN} = 200 GeV with the PHENIX experiment is presented. Within errors, no excess of direct photons was found in a first analysis pass done on a limited data set. Significantly reduced systematic and statistical uncertainties are expected in future analyses.