25,377 research outputs found
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
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
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
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
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