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Proceedings 2019, 10(1), 32;

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.
Published: 17 April 2019
PDF [1208 KB, uploaded 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
This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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

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