A background for thermal photons in heavy ion collisions

TL;DR

This paper analyzes the transverse momentum spectra of photons in heavy ion collisions at various energies, revealing insights into quark-gluon plasma formation and photon production mechanisms.

Contribution

It presents a detailed model of photon production in heavy ion collisions, incorporating multiple sources and comparing with experimental data across energies.

Findings

Good match with experimental data at high p_T

Evidence of quark-gluon plasma formation at 200 GeV

Photon yield scaling with charged particle multiplicity

Abstract

In this work, we present the transverse momentum spectra of prompt and decay photons in Au-Au collisions for $\sqrt{s_{NN}}=$ 200 GeV, 62.4 GeV, 39 GeV, and 27 GeV. The major sources of the photons in Angantyr include hard processes, Parton showers, and resonance decay. The multiparton interactions and hadronic rescatterings significantly increase the photon yield. The model shows a good match with the available experimental data at high $p_T$. The difference in yield at low $p_T $ suggests that Quark Gluon Plasma of $T_{eff}$ = 0.167 GeV/c in central Au-Au collision at 200 GeV is formed, the new effective temperature is less than the ones extracted without removing background photons. At low $p_T$ the decay photon spectra scales with $(\frac{dN_{ch}}{d\eta})^{1.25}$, the scaling is independent of collision energy and system size. The scaling no longer holds at high $p_T$ and the spectra become beam energy dependent. The scaled $p_T$ spectra of p-p and d-Au collisions show an opposite trend at high $p_T$, their scaled yield is greater than the Au-Au collision at the same energy.