Direct Photon Tagged Jets in 200 GeV Au+Au Collisions at PHENIX

Abstract

A hot dense medium called the quark gluon plasma (QGP) has been created at the Relativistic Heavy Ion Collider (RHIC). Quarks and gluons are deconfined in the QGP state but many of its properties are still under investigation. One interesting observation is that high momentum partons (quarks and gluons), which result from hard scatterings in the initial collision, lose energy as they travel through the medium. These partons fragment into the particles observed in the detector. Since fully reconstructing all the "jet" particles associated with the initial parton is complicated by the high multiplicity background produced in heavy-ion collisions, two particle correlations which trigger on a high momentum, pT, particle and measure the yield of associated particles in the event as a function of the azimuthal angle, ? ??, are used instead. Di-hadron correlations are useful for observing suppression of the away-side (? ??>??/2) jet yield and some features potentially due to the medium's response to the lost energy. However, the hadron triggers, since they are fragments of a modified jet themselves, are biased to be near the surface of the medium and the jet energy is unknown. Since photons do not interact via the strong force, they are unmodified by the medium and provide an unbiased trigger. Direct photons result directly from the hard scattering. They balance the energy of the opposing parton and provide knowledge of the opposing jet momentum. Therefore, by measuring the hadron yield on the away-side, opposite the direct photon trigger, the jet fragmentation function, which describes how partons fragment into hadrons, can be measured as a function of zT= pTh/pT? . By comparing the spectra in Au+Au collisions to that in p+p collisions, the effective modifications of fragmentation function can be quantified. Using the data collected by PHENIX during the 2007 RHIC Run, suppression of the away-side yield and the modified fragmentation function is measured via direct photon-hadron correlations. By including lower pT hadrons in the measurement, the altered shape of modified fragmentation function is studied. Possible enhancement of the lowest zT particles suggests that the energy lost at high pT is redistributed to low pT particle production.