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

Determining the spectrum of photons emitted by the quark-gluon plasma nonperturbatively remains an open computational challenge. In this paper, we calculate two moments of that spectrum at a tem-perature <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:mi>T</a:mi><a:mo>≈</a:mo><a:mn>254</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mi>MeV</a:mi></a:math>, employing lattice QCD with two flavors of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mi mathvariant="script">O</c:mi><c:mo stretchy="false">(</c:mo><c:mi>a</c:mi><c:mo stretchy="false">)</c:mo></c:math>-improved Wilson fermions, without facing an inverse problem. Our central value for the difference of these two moments, which is sensitive to photon energies <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mi>ω</h:mi><h:mo>≳</h:mo><h:mi>π</h:mi><h:mi>T</h:mi></h:math>, is lower than, but compatible with that obtained by integrating the leading-order weak-coupling photon spectrum. This study informs the “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.