Centrality dependence of low-momentum direct-photon production in Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV

TL;DR

This study measures low-momentum direct-photon production in Au+Au collisions at 200 GeV, revealing an excess over scaled p+p yields that increases with collision centrality and exhibits an exponential pT distribution.

Contribution

It provides the first detailed measurement of low-momentum direct photons in heavy-ion collisions, demonstrating their centrality dependence and an exponential pT spectrum with a specific inverse slope.

Findings

Excess direct photons above scaled p+p yield are observed.

Photon yield scales with the number of participants as approximately N_part^1.48.

The pT distribution of excess photons is exponential with a slope of about 240 MeV/c.

Abstract

The PHENIX experiment at RHIC has measured the centrality dependence of the direct photon yield from Au$+$Au collisions at $\sqrt{s_{_{NN}}}=200$ GeV down to $p_T=0.4$ GeV/$c$. Photons are detected via photon conversions to $e^+e^-$ pairs and an improved technique is applied that minimizes the systematic uncertainties that usually limit direct photon measurements, in particular at low $p_T$. We find an excess of direct photons above the $N_{\rm coll}$-scaled yield measured in $p$$+$$p$ collisions. This excess yield is well described by an exponential distribution with an inverse slope of about 240 MeV/$c$ in the $p_T$ range from 0.6--2.0 GeV/$c$. While the shape of the $p_T$ distribution is independent of centrality within the experimental uncertainties, the yield increases rapidly with increasing centrality, scaling approximately with $N_{\rm part}^\alpha$, where $\alpha=1.48{\pm}0.08({\rm stat}){\pm}0.04({\rm syst})$.