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

Abstract

Heavy ion collisions at the BNL Relativistic Heavy Ion Collider produce direct photons at low transverse momentum ${p}_{T}$ from 1--3 $\mathrm{GeV}/c$, 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 $q\ensuremath{-}\overline{q}$ 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 2 for direct photons. ${\ensuremath{\chi}}^{2}$ 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 ${p}_{T}$ less than 3 $\mathrm{GeV}/c$ in both centrality bins. Furthermore, this production mechanism 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 2.