Neutral Pion and Direct Photon Production in the SPS Energy Regime

The search for the quark-gluon plasma (QGP) has been the major driving force behind research activities in the field of ultra-relativisic heavy-ion physics in the last decades. Since the start of the experimental program at the BNL-AGS, pursuit of such extreme states of matter have been the focus of many experiments. The predicted deconfinement of quarks and gluons is not only of interest with respect to the Standard Model of Particle Physics, it is also relevant for cosmology. Current models assume that the universe was filled with a quark-gluon plasma shortly after the Big Bang. Experimental results from heavy-ion experiments can help to refine the understanding of the early phase of the cosmos. A first milestone in the search for the QGP was the CERN press release in 2000, announcing the discovery of a hot and dense state of matter bearing many properties of the predicted QGP. In 2005, the four major RHIC experiments jointly announced the creation of an extreme state of matter, similar to the predicted QGP, although some key properties, e.g. the similarity to a perfect fluid, were unexpected. The start of the LHC research program scheduled for the end of this year, evidence from lattice QCD calculations for the existence of a critical point in the phase diagram of hadronic matter, and the beginning of the commissioning for the new FAIR facility have fueled interest in understanding the energy dependence of the properties of the extreme state of matter created in ultra-relativistic heavy-ion collisions. In this thesis, the analyses of experimental data from two heavy-ion experiments are described: the PHENIX experiment has obtained data of Cu+Cu collisions at a center-of-mass energy of 22.4 GeV, close to the top energy of the SPS accelerator for heavy ions. WA98 has recorded data sets from p+Pb and p+C collisions at √ sNN = 17.3 GeV. Two possible signatures of a QGP are the focus of these analyses: the search for experimental evidence of jet quenching and the search for a thermal direct photon signal. While the discovery of jet quenching is clearly established at RHIC energies. a possible influence of this effect in the SPS energy regime has remained unclear. Especially the lack of a reliable, measured reference with high pT coverage has posed a problem. The measurement of the neutral pion production in p+Pb and p+C col- lisions can be used to address this issue by serving as a baseline for the previously published Pb+Pb results. The measurement of the neutral pion production in the PHENIX Cu+Cu data can also help to quantify jet quenching in the SPS energy regime and allows controlling the consistency of RHIC results with SPS measure- 1 2 Chapter 1: Introduction ments experimentally. The analysis of the direct photon production in the p+C and p+Pb data can help to set limits on the contribution of prompt direct photons at SPS energies to the inclusive photon spectrum. With such limits it may be possible to quantify a thermal contribution in the published direct photon measurements from Pb+Pb collisions.