Seminar in Hadronic Physics

Direct photon measurement at RHIC

Takao Sakaguchi

Brookhaven National Laboratory

Direct photons are a powerful penetrating probe to investigate the dynamics of collisions of any species. The probe was firstly measured in p+p collision at the CERN-ISR time. Since then, people have been looking for the direct photon signals in many collision systems. The measurement of direct photons, however, is suffered by huge background from known hadron decays and misidentification of copious hadrons as photons. The history of direct photon measurement can be said as a history of improvement of signal to background ratios.

The PHENIX experiment at RHIC employed conventional methods, hadron-decay photon subtraction and tagging methods, and succeeded to measure direct photons both in p+p and heavy ion collisions over the wider range in photon transverse momentum (pT). In addition, we applied the techniques used for measuring dielectrons into direct photon measurement for the first time and successfully enhanced the signal in the low pT region where the measurement was extremely difficult before.

In this seminar, the history of direct photon measurements is briefly introduced, and the techniques of measuring direct photons at RHIC are explained in some detail. Then, the latest results as well as future prospect of the direct photon measurement will be presented and discussed.