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

• Main Author: Veronica Canoa Roman
• Format: Article
• Language: English
• Published: MDPI AG 2019-04-01
• Series: Proceedings
• Subjects: direct photons; electromagnetic probes; photon scaling
• Online Access: https://www.mdpi.com/2504-3900/10/1/32
• ISSN: 2504-3900

Direct photons are a unique probe to study the properties of the medium created in heavy ion collisions. Low transverse momentum (<inline-formula> <math display="inline"> <semantics> <msub> <mi>p</mi> <mi>T</mi> </msub> </semantics> </math> </inline-formula>) direct photons are of special interest since thermal photons are supposed to be dominant, while at high <inline-formula> <math display="inline"> <semantics> <msub> <mi>p</mi> <mi>T</mi> </msub> </semantics> </math> </inline-formula> 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 <inline-formula> <math display="inline"> <semantics> <msub> <mi>p</mi> <mi>T</mi> </msub> </semantics> </math> </inline-formula> in Au+Au collisions at the c.m.s energy per nucleon pair <inline-formula> <math display="inline"> <semantics> <mrow> <msqrt> <msub> <mi>s</mi> <mrow> <mi>N</mi> <mi>N</mi> </mrow> </msub> </msqrt> <mo>=</mo> <mn>200</mn> </mrow> </semantics> </math> </inline-formula> GeV. The mechanism to produce a large direct photon yield with a large elliptic anisotropy (<inline-formula> <math display="inline"> <semantics> <msub> <mi>v</mi> <mn>2</mn> </msub> </semantics> </math> </inline-formula>) 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 <inline-formula> <math display="inline"> <semantics> <mrow> <mi>d</mi> <msub> <mi>N</mi> <mi>γ</mi> </msub> <mo>/</mo> <mi>d</mi> <mi>η</mi> </mrow> </semantics> </math> </inline-formula> is proportional to charge particle multiplicity <inline-formula> <math display="inline"> <semantics> <msup> <mrow> <mo>(</mo> <mi>d</mi> <msub> <mi>N</mi> <mrow> <mi>c</mi> <mi>h</mi> </mrow> </msub> <mo>/</mo> <mi>d</mi> <mi>η</mi> <mo>)</mo> </mrow> <mrow> <mn>1</mn> <mo>.</mo> <mn>25</mn> </mrow> </msup> </semantics> </math> </inline-formula>. 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.