Testing thermal photon and dilepton rates

Abstract

We confront the thermal NLO vector spectral function (both the transverse and longitudinal channel with respect to spatial momentum, both above and below the light cone) with continuum-extrapolated lattice data (both quenched and with N f = 2, at T ∼ 1.2T c ). The perturbative side incorporates new results, whose main features are summarized. The resolution of the lattice data is good enough to constrain the scale choice of α s on the perturbative side. The comparison supports the previous indication that the true spectral function falls below the resummed NLO one in a substantial frequency domain. Our results may help to scrutinize direct spectral reconstruction attempts from lattice QCD.

Learning about the QCD medium using electromagnetic and weak probes

“…Using these new developments, Ref. [12] shows an unprecedented agreement between NLO pQCD EM spectral function and lattice QCD results of Ref. [13,14], for both quenched and unquenched lattice calculations; thus ushering a new era of precision calculations of EM spectral functions.…”

“…These efforts ultimately culminated in the results found in Refs. [11,12] where using the NLO pQCD EM spectral function was devised combining results both above and below the light cone, as well as employing longitudinal and transverse channels with respect to spatial momentum. Using these new developments, Ref.…”

“…Except for the NLO pQCD EM rates of Ref. [12], the above-mentioned EM production rates have been used in many dynamical models, and compared against experimental data. At top RHIC or LHC energies, these are ranging from early-time dynamics modeled by e.g.…”

Deep inelastic scattering on the quark-gluon plasma

“…Thus it is clear that the structure functions of free quarks in the DIS regime are unstable against 'turning on' their interactions. Clearly, this issue deserves further investigation, which could be carried out with the help of the next-to-leading results [25,26] for the vector spectral functions. We also conclude from these remarks that the content of the moment sum rules is far from trivial, and could be used to test future weak-coupling calculations of the thermal JHEP03(2021)035 spectral functions, including the sophisticated resummations performed in this context; see references [27][28][29] for the case of vanishing virtuality.…”

Lattice QCD estimate of the quark-gluon plasma photon emission rate

“…To set this bound, we have chosen two representative processes with large relaxation time: the diffusion of a charge-density perturbation in strongly-coupled N = 4 SYM, with relaxation rate D strong k 2 = (1/2πT )k 2 , and the dissipation of a static current in weakly-coupled QCD, with leading-order relaxation rate D −1 weak = 0.46 T [4]. We also impose the upper bounds: a(k), b(k) < 2.5k, ω 0 (k) < 3k, motivated by the observation that, in NLO perturbation theory, the onset of the OPE-regime ρ(ω, k; λ = −2) ∼ O(1/ω 4 ) [1] is around ω ∼ 20T [5]. Finally, to enforce spectral-function positivity for ω ≤ k, we require A ≥ 0 and B ≥ −1/k 2 .…”