Photon production from gluon mediated quark-anti-quark annihilation at confinement

Abstract

Heavy ion collisions at RHIC produce direct photons at low transverse momentum, $p_T$ from 1-3 GeV/c, in excess of the $p$$+$$p$ spectra scaled by the nuclear overlap factor, $T_{AA}$. These low $p_T$ photons have a large azimuthal anisotropy, $v_2$. Theoretical models, including hydrodynamic models, struggle to quantitatively reproduce the large low $p_T$ direct photon excess and $v_2$ in a self-consistent manner. This paper presents a description of the low $p_T$ photon flow as the result of increased photon production from soft-gluon mediated $q$-$\bar{q}$ interactions as the system becomes color-neutral. This production mechanism will generate photons that follow constituent quark number, $n_q$, scaling of $v_2$ with an $n_q$ value of two for direct photons. ${\chi }^2$ comparisons of the published PHENIX direct photon and identified particle $v_2$ measurements finds that $n_q$-scaling applied to the direct photon $v_2$ data prefers the value $n_q=1.8$ and agrees with $n_q=2$ within errors in most cases. The 0-20% and 20-40% Au$+$Au direct photon data are compared to a coalescence-like Monte Carlo simulation that calculates the direct photon $v_2$ while describing the shape of the direct photon $p_T$ spectra in a consistent manner. The simulation, while systematically low compared to the data, is in agreement with the Au$+$Au measurement at $p_T<3$ GeV/c in both centrality bins. Furthermore, this model predicts that higher order flow harmonics, $v_n$, in direct photons will follow the modified $n_q$-scaling laws seen in identified hadron $v_n$ with an $n_q$ value of two.

11 pages, 10 figures, 3 tables, accepted for publication into Physical Review C

References in corpus (4)

• Scaling properties of azimuthal anisotropy in Au+Au and Cu+Cu collisions at sqrt(s_NN) = 200 GeV
• Measurement of direct photon production in p + p collisions at sqrt(s) = 200 GeV
• Electromagnetic radiation by quark-gluon plasma in magnetic field
• Elliptic flow of thermal photons and formation time of quark gluon plasma at RHIC

Cited by in corpus (13)

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• Electromagnetic Radiation from QCD Matter: Theory Overview
• Direct photon spectrum and elliptic flow produced from Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at the CERN Large Hadron Collider within an integrated hydrokinetic model
• Photon and dilepton observables: An experimental overview
• Photon spectra and anisotropic flow in heavy ion collisions at the top RHIC energy within the integrated hydrokinetic model with photon hadronization emission
• Impact of residual contamination on inclusive and direct photon flow
• The next-to-leading order of the differential cross-section of the subprocess of Compton scattering of quark-gluon of prompt photon production in proton-proton collisions at NICA energies
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