QCD analysis and effective temperature of direct photons in lead-lead collisions at the LHC

TL;DR

This paper provides a detailed QCD-based analysis of low-$p_T$ direct photon production in lead-lead collisions at the LHC, revealing a thermal photon spectrum with an effective temperature around 300 MeV and a high-$p_T$ power-law tail.

Contribution

It offers the first systematic theoretical study combining NLO QCD calculations with experimental data to extract the effective temperature of photons from the quark-gluon plasma.

Findings

Thermal photon spectrum with slope ~300 MeV identified.

Power-law behavior ($p_T^{-4}$) observed for high-$p_T$ photons.

Quantified uncertainties from various theoretical sources.

Abstract

We present a systematic theoretical analysis of the ALICE measurement of low-$p_T$ direct-photon production in central lead-lead collisions at the LHC with a centre-of-mass energy of $\sqrt{s_{NN}}=2.76$ TeV. Using next-to-leading order of perturbative QCD, we compute the relative contributions to prompt-photon production from different initial and final states and the theoretical uncertainties coming from independent variations of the renormalisation and factorisation scales, the nuclear parton densities and the fragmentation functions. Based on different fits to the unsubtracted and prompt-photon subtracted ALICE data, we consistently find an exponential, possibly thermal, photon spectrum from the quark-gluon plasma (or hot medium) with slope $T=304\pm 58$ MeV and $309\pm64$ MeV at $p_T\in[0.8;2.2]$ GeV and $p_T\in[1.5;3.5]$ GeV as well as a power-law ($p_T^{-4}$) behavior for $p_T>4$ GeV as predicted by QCD hard scattering.