Probing how bright the quark-gluon plasma glows in lattice QCD

TL;DR

This paper non-perturbatively calculates moments of the photon emission spectrum from the quark-gluon plasma using lattice QCD, providing insights into the direct photon puzzle in heavy-ion collisions.

Contribution

It introduces a lattice QCD method to compute photon spectrum moments without inverse problems, advancing understanding of photon emission in quark-gluon plasma.

Findings

The central value for the spectrum difference is lower but compatible with weak-coupling predictions.

Results inform the direct photon puzzle by constraining photon emission models.

Study employs two-flavor O(a)-improved Wilson fermions at T≈254 MeV.

Abstract

Determining the spectrum of photons emitted by the quark-gluon plasma non-perturbatively remains an open computational challenge. In this letter we calculate two moments of that spectrum at a temperature $T\approx 254\,$MeV, employing lattice QCD with two flavors of $\mathrm{O}(a)$-improved Wilson fermions, without facing an inverse problem. Our central value for the difference of these two moments, which is sensitive to photon energies $\omega\gtrsim \pi T$, is lower than, but compatible with that obtained by integrating the leading-order weak-coupling photon spectrum. This study informs the $\textit{direct photon puzzle}$ in heavy-ion collision phenomenology, where it has proved difficult to simultaneously explain the yield and azimuthal anisotropy of photons not originating from final-state hadronic decays.