Thermal photons produced in heavy-ion collision experiments are an important observable for understanding quark-gluon plasma (QGP). The thermal photon rate from the QGP at a given temperature can be calculated from the spectral function of the vector current correlator. Extraction of the spectral function from the lattice correlator is known to be an ill-conditioned problem, as there is no unique solution for a spectral function for a given lattice correlator with statistical errors. The vector current correlator, on the other hand, receives a large ultraviolet contribution from the vacuum, which makes the extraction of the thermal photon rate difficult from this channel. We therefore consider the difference between the transverse and longitudinal part of the spectral function, only capturing the thermal contribution to the current correlator, simplifying the reconstruction significantly. The lattice correlator is calculated for light quarks in quenched QCD at <math><mi>T</mi><mo>=</mo><mn>470</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math> (<math><mo>∼</mo><mn>1.5</mn><msub><mi>T</mi><mi>c</mi></msub></math>), as well as in <math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math> flavor QCD at <math><mi>T</mi><mo>=</mo><mn>220</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math> (<math><mo>∼</mo><mn>1.2</mn><msub><mi>T</mi><mrow><mi>p</mi><mi>c</mi></mrow></msub></math>) with <math><msub><mi>m</mi><mi>π</mi></msub><mo>=</mo><mn>320</mn><mtext> </mtext><mtext> </mtext><mi>MeV</mi></math>. In order to quantify the nonperturbative effects, the lattice correlator is compared with the corresponding <math><mi>NLO</mi><mo>+</mo><msup><mi>LPM</mi><mi>LO</mi></msup></math> estimate of correlator. The reconstruction of the spectral function is performed in several different frameworks, ranging from physics-informed models of the spectral function to more general models in the Backus-Gilbert method and Gaussian process regression. We find that the resulting photon rates agree within errors.

Germany

Pakistan

Sajid Ali

Dibyendu Bala

Taiwan

Anthony Francis

United States

France

Greg Jackson

Olaf Kaczmarek

Jonas Turnwald

Tristan Ueding

Nicolas Wink

Lattice QCD estimates of thermal photon production from the QGP

We compute masses of the two lightest glueballs from spectral reconstructions of timelike interaction channels of the four-gluon vertex in Landau gauge Yang-Mills theory. The Euclidean spacelike dressings of the vertex are calculated with the functional renormalization group. For the spectral reconstruction of these Euclidean data, we employ Gaussian process regression. The glueball resonances can be identified straightforwardly and we obtain <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>s</mi><mi>c</mi></mrow></msub><mo>=</mo><mn>1870</mn><mo>(</mo><mn>75</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>MeV</mi></mrow></math> as well as <math><mrow><msub><mrow><mi>m</mi></mrow><mrow><mi>p</mi><mi>s</mi></mrow></msub><mo>=</mo><mn>2700</mn><mo>(</mo><mn>120</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>MeV</mi></mrow></math>, in accordance with functional bound state and lattice calculations.

Jan M. Pawlowski

Coralie S. Schneider

Julian M. Urban

Yang-Mills glueball masses from spectral reconstruction

We compute the strong coupling constant of Landau gauge QCD in the full complex momentum plane, both directly and via spectral reconstruction. In particular, we consider the Taylor coupling given by the product of ghost and gluon dressing functions. Assuming spectral representations for the latter, we first show that also the coupling obeys such a representation. The subsequent spectral reconstruction of the coupling data, obtained from <math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math> flavor lattice QCD results for the ghost and gluon, is based on a probabilistic inversion of this representation using Gaussian process regression with analytically enforced asymptotics. In contradistinction, our direct calculation relies on earlier reconstruction results for the ghost and gluon spectral functions themselves, as well as data obtained in functional QCD. Apart from its relevance for studies of resonances or scattering processes, the calculation also serves as a nontrivial benchmark of our reconstruction approach. The results show remarkable agreement, testifying to the reliability of the method.

Jan Horak

Savvas Zafeiropoulos

Nonperturbative strong coupling at timelike momenta

We reconstruct ghost and gluon spectral functions in <math><mrow><mn>2</mn><mo>+</mo><mn>1</mn></mrow></math> flavor QCD with Gaussian process regression. This framework allows us to largely suppress spurious oscillations and other common reconstruction artifacts by specifying generic magnitude and length scale parameters in the kernel function. The Euclidean propagator data are taken from lattice simulations with domain wall fermions at the physical point. For the infrared and ultraviolet extensions of the lattice propagators as well as the low-frequency asymptotics of the ghost spectral function, we utilize results from functional computations in Yang-Mills theory and QCD. This further reduces the systematic error significantly. Our numerical results are compared against a direct real-time functional computation of the ghost and an earlier reconstruction of the gluon in Yang-Mills theory. The systematic approach presented in this work offers a promising route toward unveiling real-time properties of QCD.

Spain

José Rodríguez-Quintero

Reconstructing QCD spectral functions with Gaussian processes