Jet fragmentation in medium and vacuum with the PHENIX detector

One of the most active areas of investigation in relativistic heavy-ion collisions is the study of the jet quenching phenomenon whereby hard partons lose their energy as they traverse the hot, dense matter created in such collisions. Strong parton energy loss has been observed in central nucleus-nucleus collisions as evidenced by the a large suppression of the yield of high pT hadrons as compared to the expected yield based on measurements in p+p collisions [1]. Moreover, measurements of back-to-back correlations of charged hadrons suggest that jet shapes are strongly modified modified by the medium [2]. The quantitative interpretation of single and di-hadron measurements is, however, complicated by the fact that the initial parton energy is unknown. A more informative measurement would be one in which the initial parton energy is known, allowing the determination of the fragmentation function, which may be effectively modified from its vacuum form by the presence of the medium. Two measurements in which the initial parton energy may be estimated are discussed in these proceedings: jet reconstruction and twoparticle correlations using direct photons. Jet reconstruction in nuclear collisions is challenging due to the large background of soft particles, fluctuations of which give rise to fake jets. Direct photons can be used to estimate the initial parton energy of the recoil jet without recourse to jet reconstruction algorithms. However, such studies suffer from a smaller rate and the direct photon signal must be disentangled from a large background of decay photons. We present jet reconstruction results which use an algorithm suitable for a high multiplicity environment. We also present results of two-particle correlations using direct photons. These results are discussed in the context of medium modification to the fragmentation function.