Beam-energy and centrality dependence of direct-photon emission from ultra-relativistic heavy-ion collisions

TL;DR

This study measures low-momentum direct-photon yields in Au+Au collisions at different energies, revealing a universal scaling with charged particle multiplicity and providing insights into photon production mechanisms in heavy-ion collisions.

Contribution

First measurements of low-momentum direct-photon yields at 39 and 62.4 GeV, demonstrating a universal scaling behavior across energies and collision systems.

Findings

Photon yields are enhanced compared to prompt process expectations.

Direct-photon yield scales with charged particle multiplicity as (dN_ch/dη)^1.25.

Scaling behavior persists across different energies, centralities, and collision systems.

Abstract

The PHENIX collaboration presents first measurements of low-momentum ($0.4<p_T<3$ GeV/$c$) direct-photon yields from Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=39 and 62.4 GeV. For both beam energies the direct-photon yields are substantially enhanced with respect to expectations from prompt processes, similar to the yields observed in Au$+$Au collisions at $\sqrt{s_{_{NN}}}$=200. Analyzing the photon yield as a function of the experimental observable $dN_{\rm ch}/d\eta$ reveals that the low-momentum ($>$1\,GeV/$c$) direct-photon yield $dN_{\gamma}^{\rm dir}/d\eta$ is a smooth function of $dN_{\rm ch}/d\eta$ and can be well described as proportional to $(dN_{\rm ch}/d\eta)^\alpha$ with $\alpha{\approx}1.25$. This scaling behavior holds for a wide range of beam energies at the Relativistic Heavy Ion Collider and the Large Hadron Collider, for centrality selected samples, as well as for different, $A$$+$$A$ collision systems. At a given beam energy the scaling also holds for high $p_T$ ($>5$\,GeV/$c$) but when results from different collision energies are compared, an additional $\sqrt{s_{_{NN}}}$-dependent multiplicative factor is needed to describe the integrated-direct-photon yield.