Cross Section for Prompt Photon Production in Proton-Proton Collisions at $\sqrt{s}$ = 62.4 GeV

Measurements of cross section for prompt photon production in protonproton collisions at √ s = 62.4 GeV in PHENIX are reported. The prompt photon production is expected to be useful to test the applicability of perturbative Quantum ChoromoDynamics (pQCD), to determine gluon spin contribution ∆g to the proton spin through double helicity asymmetries ALL in polarized proton-proton collisions, and to investigate of Quark-Gluon Plasma (QGP). Once the applicability of pQCD is confirmed, the framework of pQCD can be used to predict other quantities of interest such as ALL, cross section for other processes etc. Proton is composed of quarks and gluons. Proton spin, 1/2, comes from the spin of quarks ∆q, the spin of gluons ∆g and angular momenta of quarks and gluons. In 1980’s, the quark spin contribution to the proton spin, ∆q, is measured by European Muon Collaboration (EMC) at CERN, and is found to be very small. Now, measurements of ∆g and the angular momentum contribution of quarks and gluons to the proton spin are going on in various experiments. For the measurement of prompt photon ALL, the study of the cross section for prompt photon production is important as the first step. In this thesis work, the prompt photons are measured down to pT ∼ 2 GeV/c for the first time. The prompt photons of low pT correspond to low x gluons inside of the proton. Here x is Bjorken x, the fraction of the parton momentum to the total proton momentum. The prompt photon is mainly produced via quark-gluon scattering (qg → γq) in proton-proton collisions. The prompt photon carries information on the quark-gluon scattering. Therefore, the prompt photon production is one of the tools to extract the ∆g(x). Quarks and gluons, which have a degree of freedom of color, are ordinarily confined in hadron as a color-singlet state. On the other hand, as the temperature or density of many-body system of hadrons increases, a normal nuclear state is expected to transform into another state of matter where quarks and gluons become color de-confined. This state is called QGP. For investigations of QGP which is produced in nucleus-nucleus collisions, the cross section for prompt photon production in proton-proton collisions is important as a reference. PHENIX, which stands for Pioneering High Energy Nuclear Interaction eXperiment, is an experiment at Brookhaven National Laboratory (BNL) using the Relativistic Heavy Ion Collider (RHIC). My work at PHENIX is, as the first step, to operate and optimize the modules of ERT (ElectroMagnetic Calorimeter (EMCal)-RICH). The fire status of the ERT trigger module and related electronics are carefully checked. The ERT trigger plays an important role in the measurement of photon, electron, etc. As the second step, I measured the cross section for prompt photon production in protonproton collisions at √ s = 62.4 GeV. This √ s is achieved with the colliding proton beam of 31.2 GeV. In this measurement, the polarization of the proton beam is averaged out. The data are taken in 2006. The data are accumulated with PHENIX central arm spectrometer with the ERT trigger (≤ 0.6 GeV). The integrated luminosity is about 0.065 pb−1. The PHENIX central arm spectrometer consists of two arms which are placed almost back-to-back on their azimuth. Each arm covers pseudo-rapidity |η| < 0.35 and 90◦ azimuthal angle. The cross section for prompt photon production as a function of pT is measured over 4 orders of magnitude from 2 to 7 GeV/c. In particular, the cross section in the pT region from 2 to 4 GeV/c is measured for the first time. Here, pT is the transverse momentum. The cross section shows a steep decrease as pT increases. The cross section is consistent with the one measured earlier at high pT at CERN-ISR. It is found that the cross section predicted by pQCD is lower than the measured cross section at low pT region. The relation between the prompt photon pT and gluon x is evaluated using PYTHIA simulation. The gluon x proved in this measurement is 0.025 ∼ 0.3 corresponding to pT = 2 ∼ 7 GeV/c. The gluons are localized at small x, so the study of gluons in this low x region is important. The present result of prompt photon production at low pT provides a basis for further theoretical studies of QCD, gluon spin contribution measurement and the study of QGP.