Highlights from PHENIX - II

This contribution highlights recent results from the PHENIX Collaboration at RHIC with emphasis on those obtained through lepton and photon measurements in PHENIX. Following the discovery of the surprising 'perfect liquid' properties of the dense matter being produced in heavy ion collisions at RHIC (1), effort has intensified to quantify the detailed characteristics of that matter. Photons and leptons are of special interest because they are penetrating probes that do not undergo strong interactions and therefore are unlikely to interact with the dense matter after their production. Thus, they carry information about the system at the time of their production, throughout the entire evolution of the collision. The PHENIX experiment at RHIC was designed with emphasis on the measurement of leptons and photons, with electron and photon identification at mid-rapidity, and muon spectrometers at forward and backward rapidities (see (2) for a description of PHENIX). As a result of their weaker electromagnetic coupling, lepton and photon production are rare processes that require large data samples for precise measurements. As a consequence of their low production rate the measurements are also subject to large backgrounds. In the case of directly radiated photons, the backgrounds are mostly photons from radiative decays of long-lived neutral mesons, predominantly the abundantly produced neutral pions. In the case of the electron measurement the background is mostly the internal or external conversion of these radiative decay photons into electron-position pairs, and in the case of the muon measurements it is the weak decay of charged pions into muons. With the 8th RHIC Run period recently completed, many of the new results reported by PHENIX are improved measurements of previously published results obtained from the smaller data sets of the earlier RHIC runs. This is particularly true of the lepton and photon measurements. As an example, due to the factor of 10 increase in the PHENIX Run 4 data sample compared to Run 2, the measurement of the neutral pion spectra in Au+Au collisions has been extended to nearly 20 GeV/c transverse momentum with decreased statistical and systematic errors (3). This directly translates into improved direct photon and non-photonic electron measurements