Thermal right-handed neutrino production rate in the relativistic regime

The production rate of right-handed neutrinos from a Standard Model plasma at a temperature above a hundred GeV is evaluated up to NLO in Standard Model couplings. The results apply in the so-called relativistic regime, referring parametrically to a mass M ~ pi T, generalizing thereby previous NLO results which only apply in the non-relativistic regime M >> pi T. The non-relativistic expansion is observed to converge for M > 15 T, but the smallness of any loop corrections allows it to be used in practice already for M > 4 T. In the latter regime any non-covariant dependence of the differential rate on the spatial momentum is shown to be mild. The loop expansion breaks down in the ultrarelativistic regime M << pi T, but after a simple mass resummation it nevertheless extrapolates reasonably well towards a result obtained previously through complete LPM resummation, apparently confirming a strong enhancement of the rate at high temperatures (which facilitates chemical equilibration). When combined with other ingredients the results may help to improve upon the accuracy of leptogenesis computations operating above the electroweak scale.Comment: 37 pages. v2: clarification added; published versio

Thermal phase transitions in cosmology

We review briefly the current status of thermal phase transitions within the Standard Model and its simplest extensions. We start with an update on QCD thermodynamics, then discuss the electroweak phase transition, particularly in supersymmetric extensions of the Standard Model, and end with a few remarks on the cosmological constraints that thermal phase transitions might impose on even higher scale particle physics.Comment: 10 pages. Plenary talk at Cosmo-01, Rovaniemi, Finland, 200

Hot QCD and warm dark matter

One of the possible explanations for the dark matter needed in the standard cosmological model is so-called warm dark matter, in the form of right-handed ("sterile") neutrinos with a mass in the keV range. I describe how various properties of QCD at temperatures of a few hundred MeV play an important role in the theoretical computations that are needed for consolidating or falsifying this scenario. In particular the points where lattice QCD could help are underlined.Comment: 15 pages. Updated version of plenary talk at International Symposium on Lattice Field Theory, July 23-28, 2006, Tucson, Arizona, US

What is the simplest effective approach to hot QCD thermodynamics?

The dimensionally reduced action is believed to provide for a theoretically consistent and numerically precise effective description of the thermodynamics of the quark-gluon plasma, once the temperature is above a few hundred MeV. Although dramatically simpler than the original QCD it is, however, still a strongly interacting, confining theory. In this talk I speculate on whether there could exist a further simplified recipe within that theory, for physically relevant temperatures, which would already lead to a phenomenologically satisfactory description of the free energy and various correlation lengths of hot QCD, but with only a minimal amount of numerical non-perturbative input needed.Comment: 10 pages. Talk at SEWM 2002, Heidelberg, Germany, 2-5 Oct 2002. Some references update

Vortex phases in condensed matter and cosmology

Placing a high-Tc superconductor in an increasing external magnetic field, the flux first penetrates the sample through an Abrikosov vortex lattice, and then a first order transition is observed by which the system goes to the normal phase. We discuss the cosmological motivation for considering the electroweak phase transition in the presence of an external magnetic field, the analogies this system might have with the superconductor behaviour described above, and in particular whether at large physical Higgs masses, corresponding to the high-Tc regime, an analogue of the vortex phase and an associated first order phase transition could be generated.Comment: 7 pages; plenary talk at COSMO-99, Trieste, Italy, Sep 27 - Oct 2, 199

Electroweak phase transition beyond the Standard Model

Standard theories of electroweak interactions are based on the concept of a gauge symmetry broken by the Higgs mechanism. If they are placed in an environment with a sufficiently high temperature, the symmetry gets restored. It turns out that the characteristics of the symmetry restoring phase transition, such as its order, are important for cosmological applications, such as baryon asymmetry generation. We first briefly review how, by a combination of analytic and numerical methods, the properties of the phase transition can be systematically resolved for any given type of a (weakly interacting) Higgs sector. We then summarise the numerical results available for the Standard Model, and present a generic model independent statement as to how the Higgs sector should differ from the Standard Model for the properties of the transition to be very different. As an explicit example, we discuss the possibilities available for a strong transition in the experimentally allowed parameter region of the Minimal Supersymmetric Standard Model.Comment: 12 pages. Invited talk at Strong and Electroweak Matter, Marseille, France, June 13-17, 200