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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 $N_{rm f} = 2$, at $T sim 1.2 T_{rm c}$). 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 $alpha_{rm s}$ 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.
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We present new lattice results on the continuum extrapolation of the vector current correlation function. Lattice calculations have been carried out in the deconfined phase at a temperature of 1.1 Tc, extending our previous results at 1.45 Tc, utilizing quenched non-perturbatively clover-improved Wilson fermions and light quark masses. A systematic analysis on multiple lattice spacings allows to perform the continuum limit of the correlation function and to extract spectral properties in the continuum limit. Our current analysis suggests the results for the electrical conductivity are proportional to the temperature and the thermal dilepton rates in the quark gluon plasma are comparable for both temperatures. Preliminary results of the continuum extrapolated correlation function at finite momenta, which relates to thermal photon rates, are also presented.
Strict next-to-leading order (NLO) results for the dilepton production rate from a QCD plasma at temperatures above a few hundred MeV suffer from a breakdown of the loop expansion in the regime of soft invariant masses M^2 << (pi T)^2. In this regime an LPM resummation is needed for obtaining the correct leading-order result. We show how to construct an interpolation between the hard NLO and the leading-order LPM expression. The final numerical results are presented in a tabulated form, suitable for insertion into hydrodynamical codes.
The production of high invariant mass opposite sign lepton pairs in proton-proton collisions at the LHC is dominated by the Drell-Yan process. In addition to this photon or Z exchange mediated mechanism, gamma-gamma collisions, where photons radiated by the incoming protons collide, can produce lepton pairs. This is an important additional source of background for high mass resonant (like Z) or non-resonant (like contact interactions) searches. In this paper detailed calculations of the Drell-Yan and photon-induced cross sections in the typical acceptance of a multi-purpose LHC detector at center of mass energy 13 TeV are presented. The hint for a diphoton excess at a mass around 750 GeV, reported by the ATLAS and CMS experiments from the analysis of the 2015 data at 13 TeV, raises the possibility that such an excess could be produced through gamma-gamma collisions. A good theoretical understanding and measurements in the dilepton channels at these energies can help to elucidate this production hypothesis.
Thermal screening masses related to the conserved vector current are determined for the case that the current carries a non-zero Matsubara frequency, both in a weak-coupling approach and through lattice QCD. We point out that such screening masses are sensitive to the same infrared physics as light-cone real-time rates. In particular, on the perturbative side, the inhomogeneous Schrodinger equation determining screening correlators is shown to have the same general form as the equation implementing LPM resummation for the soft-dilepton and photon production rates from a hot QCD plasma. The static potential appearing in the equation is identical to that whose soft part has been determined up to NLO and on the lattice in the context of jet quenching. Numerical results based on this potential suggest that screening masses overshoot the free results (multiples of 2piT) more strongly than at zero Matsubara frequency. Four-dimensional lattice simulations in two-flavour QCD at temperatures of 250 and 340 MeV confirm the non-static screening masses at the 10% level. Overall our results lend support to studies of jet quenching based on the same potential at T > 250 MeV.
We report on a recent next-to-leading order perturbative determination of the dilepton rate from a hot QCD plasma for frequency and momentum of the order of the temperature and for much smaller invariant mass $Msim gT$. We briefly review the calculation, which generalizes the previous one for the photon case ($M=0$). We then analyze the consequences of the new calculation for the extraction of the photon rate from the small mass dilepton measurements. We then review a recent NLO determination at large $M$ and we show how to match and merge its results with the low-mass ones, resulting in a single rate which is NLO-accurate over the phenomenologically relevant region.
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