The thermal dilepton rate at NLO at small and large invariant mass

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 $M\sim gT$. 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.Comment: 4 pages, 6 figures. To appear in the Hard Probes 2015 proceeding

GeV-scale hot sterile neutrino oscillations: a numerical solution

The scenario of baryogenesis through GeV-scale sterile neutrino oscillations is governed by non-linear differential equations for the time evolution of a sterile neutrino density matrix and Standard Model lepton and baryon asymmetries. By employing up-to-date rate coefficients and a non-perturbatively estimated Chern-Simons diffusion rate, we present a numerical solution of this system, incorporating the full momentum and helicity dependences of the density matrix. The density matrix deviates significantly from kinetic equilibrium, with the IR modes equilibrating much faster than the UV modes. For equivalent input parameters, our final results differ moderately (~50%) from recent benchmarks in the literature. The possibility of producing an observable baryon asymmetry is nevertheless confirmed. We illustrate the dependence of the baryon asymmetry on the sterile neutrino mass splitting and on the CP-violating phase measurable in active neutrino oscillation experiments.Comment: 26 pages. v2: clarifications and references adde

GeV-scale hot sterile neutrino oscillations: a derivation of evolution equations

Starting from operator equations of motion and making arguments based on a separation of time scales, a set of equations is derived which govern the non-equilibrium time evolution of a GeV-scale sterile neutrino density matrix and active lepton number densities at temperatures T > 130 GeV. The density matrix possesses generation and helicity indices; we demonstrate how helicity permits for a classification of various sources for leptogenesis. The coefficients parametrizing the equations are determined to leading order in Standard Model couplings, accounting for the LPM resummation of 1+n 2+n scatterings and for all 2 2 scatterings. The regime in which sphaleron processes gradually decouple so that baryon plus lepton number becomes a separate non-equilibrium variable is also considered.Comment: 33 pages. v2: clarifications added; published versio

Parton energy loss and momentum broadening at NLO in high temperature QCD plasmas

We present an overview of a perturbative-kinetic approach to jet propagation, energy loss, and momentum broadening in a high temperature quark-gluon plasma. The leading-order kinetic equations describe the interactions between energetic jet-particles and a non-abelian plasma, consisting of on-shell thermal excitations and soft gluonic fields. These interactions include 22 scatterings, collinear bremsstrahlung, and drag and momentum diffusion. We show how the contribution from the soft gluonic fields can be factorized into a set of Wilson line correlators on the light cone. We review recent field-theoretical developments, rooted in the causal properties of these correlators, which simplify the calculation of the appropriate Wilson lines in thermal field theory. With these simplifications lattice measurements of transverse momentum broadening have become possible, and the kinetic equations describing parton transport have been extended to next-to-leading order in the coupling g.Comment: Review to appear in QGP5, 40 pages, 12 figure

Sterile neutrino dark matter via GeV-scale leptogenesis?

It has been proposed that in a part of the parameter space of the Standard Model completed by three generations of keV...GeV right-handed neutrinos, neutrino masses, dark matter, and baryon asymmetry can be accounted for simultaneously. Here we numerically solve the evolution equations describing the cosmology of this scenario in a 1+2 flavour situation at temperatures $T \le 5$ GeV, taking as initial conditions maximal lepton asymmetries produced dynamically at higher temperatures, and accounting for late entropy and lepton asymmetry production as the heavy flavours fall out of equilibrium and decay. For 7 keV dark matter mass and other parameters tuned favourably, $\sim 10\%$ of the observed abundance can be generated. Possibilities for increasing the abundance are enumerated.Comment: 20 page

Precision study of GeV-scale resonant leptogenesis

Low-scale leptogenesis is most efficient in the limit of an extreme mass degeneracy of right-handed neutrino flavours. Two variants of this situation are of particular interest: large neutrino Yukawa couplings, which boost the prospects of experimental scrutiny, and small ones, which may lead to large lepton asymmetries surviving down to T < 5 GeV. We study benchmarks of these cases within a "complete" framework which tracks both helicity states of right-handed neutrinos as well as their kinetic non-equilibrium, and includes a number of effects not accounted for previously. For two right-handed flavours with GeV-scale masses, Yukawa couplings up to $|h| \sim 0.7 \times 10^{-5}$ are found to be viable for baryogenesis, with $\Delta M/M \sim 10^{-8}$ as the optimal degeneracy. Late-time lepton asymmetries are most favourably produced with $\Delta M/M \sim 10^{-11}$. We show that the system reaches a stationary state at T < 15 GeV, in which lepton asymmetries can be more than $10^3$ times larger than the baryon asymmetry, reach flavour equilibrium, and balance against helicity asymmetries.Comment: 43 pages. v2: improvements in presentation, published versio

Neutrino dynamics below the electroweak crossover

We estimate the thermal masses and damping rates of active (m < eV) and sterile (M ~ GeV) neutrinos with thermal momenta k ~ 3T at temperatures below the electroweak crossover (5 GeV < T < 160 GeV). These quantities fix the equilibration or "washout" rates of Standard Model lepton number densities. Sterile neutrinos interact via direct scatterings mediated by Yukawa couplings, and via their overlap with active neutrinos. Including all leading-order reactions we find that the washout rate generally exceeds the Hubble rate for 5 GeV < T < 30 GeV. Therefore it is challenging to generate a large lepton asymmetry facilitating dark matter computations operating at T < 5 GeV, whereas the generation of a baryon asymmetry at T > 130 GeV remains an option. Our differential rates are tabulated in a form suitable for studies of specific scenarios with given neutrino Yukawa matrices.Comment: 42 pages. v2: references and clarifications adde

QCD Shear Viscosity at (almost) NLO

We compute the shear viscosity of QCD with matter, including almost all next-to-leading order corrections -- that is, corrections suppressed by one power of $g$ relative to leading order. We argue that the still missing terms are small. The next-to-leading order corrections are large and bring $\eta/s$ down by more than a factor of 3 at physically relevant couplings. The perturbative expansion is problematic even at $T \simeq 100$ GeV. The largest next-to-leading order correction to $\eta/s$ arises from modifications to the qhat parameter, which determines the rate of transverse momentum diffusion. We also explore quark number diffusion, and shear viscosity in pure-glue QCD and in QED.Comment: 36 pages plus appendices, 11 figures. The main results are summarized in the introduction (Fig. 1