Abstract
Direct photons are a unique probe to study the properties of the medium created in heavy ion collisions. Low transverse momentum ( p T ) direct photons are of special interest since thermal photons are supposed to be dominant, while at high p T direct photons come from initial hard scattering (pQCD). PHENIX has observed a large excess of direct photon yield as well as large azimuthal anisotropy at low p T in Au+Au collisions at the c.m.s energy per nucleon pair s N N = 200 GeV. The mechanism to produce a large direct photon yield with a large elliptic anisotropy ( v 2 ) is not well understood yet. PHENIX has made systematic measurements of direct photons with different collision energies and system configurations. It has been found that direct photon yield d N γ / d η is proportional to charge particle multiplicity ( d N c h / d η ) 1 . 25 . This behavior holds for beam energies measured both at RHIC and at the LHC in large systems. This scaling suggests that there is a transition from p+p to A+A system which could be understood with the analysis of smaller systems like p+Au and d+Au.
Highlights
Due to the fact that photons do not interact strongly in heavy ion collisions, they can carry primordial information about the collision at the time of their production, namely temperature evolution and collective motion of the matter
In large systems and at low p T, it is expected that the dominant source of direct photons is emission from the QGP and from the hadronic gas phase; whereas at high p T, their origin is connected to initial state interactions produced in the hard scattering
A clear direct photon signal excess was found at low p T compared to the pQCD prediction scaled by the number of binary collision (Ncoll)
Summary
Due to the fact that photons do not interact strongly in heavy ion collisions, they can carry primordial information about the collision at the time of their production, namely temperature evolution and collective motion of the matter. Photons are very abundant in heavy ion collisions, most of them come from hadronic decays, mainly π 0 and η to γγ. The so called direct photons are those which do not come from hadronic decays; they can be determined experimentally by subtracting from the total yield the expected fraction coming from hadron decays. In large systems and at low p T , it is expected that the dominant source of direct photons is emission from the QGP and from the hadronic gas phase; whereas at high p T , their origin is connected to initial state interactions produced in the hard scattering
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