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Direct Photons at the PHENIX Experiment: From Large to Small Systems

Deparment of Physics and Astronomy, Stony Brook University, NY 11794, USA
Presented at Hot Quarks 2018—Workshop for Young Scientists on the Physics of Ultrarelativistic Nucleus-Nucleus Collisions, Texel, The Netherlands, 7–14 September 2018.
Proceedings 2019, 10(1), 32; https://doi.org/10.3390/proceedings2019010032
Published: 17 April 2019
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.
Keywords: direct photons; electromagnetic probes; photon scaling direct photons; electromagnetic probes; photon scaling
MDPI and ACS Style

Roman, V.C. Direct Photons at the PHENIX Experiment: From Large to Small Systems. Proceedings 2019, 10, 32.

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