The nuMSM, leptonic asymmetries, and properties of singlet fermions

We study in detail the mechanism of baryon and lepton asymmetry generation in the framework of the $\nu$MSM (an extension of the Standard Model by three singlet fermions with masses smaller than the electroweak scale). We elucidate the issue of CP-violation in the model and define the phase relevant for baryogenesis. We clarify the question of quantum-mechanical coherence, essential for the lepton asymmetry generation in singlet fermion oscillations and compute the relevant damping rates. The range of masses and couplings of singlet leptons which can lead to successful baryogenesis is determined. The conditions which ensure survival of primordial (existing above the electroweak temperatures) asymmetries in different leptonic numbers are analysed. We address the question whether CP-violating reactions with lepton number non-conservation can produce leptonic asymmetry {\em below} the sphaleron freeze-out temperature. This asymmetry, if created, leads to resonant production of dark matter sterile neutrinos. We show that the requirement that a significant lepton asymmetry be produced puts stringent constraints on the properties of a pair of nearly degenerate singlet fermions, which can be tested in accelerator experiments. In this region of parameters the $\nu$MSM provides a common mechanism for production of baryonic matter and dark matter in the universe. We analyse different fine-tunings of the model and discuss possible symmetries of the $\nu$MSM Lagrangian that can lead to them.Comment: 56 pages, 16 figures. Many clarifications added, published versio

Some Two-Loop Corrections to the Finite Temperature Effective Potential in the Electroweak Theory

Perturbation theory at finite temperature suffers from well-known infrared problems. In the standard model, as a result, one cannot calculate the effective potential for arbitrarily small values of $\phi$, the Higgs expectation value. Because the Higgs field is now known not to be extremely light, it is necessary to determine whether perturbation theory is a reliable guide to properties of the weak phase transition. In this note, we evaluate the most singular contributions to the potential at two loops as well as the leading strong interaction contributions. Above the critical temperature, the strong interaction corrections are reasonably small, while the weak corrections are about 10\%, even for rather small values of the Higgs field. At the critical temperature, the weak corrections have a more substantial effect, rendering the transition significantly more first order, but not significantly changing the upper bound on the Higgs mass required for baryogenesis.Comment: 13 pages, figures available on request, SCIPP 92/4

On the High TT Phase Transition in the Gauged SU(2)SU(2) Higgs Model

We study the effective field theory of a weakly coupled 3+1d gauged $\phi^4$ type model at high temperature. Our model has $4N$ real scalars ($N$ complex Higgs doublets) and a gauge group $SU(2)$ which is spontaneously broken by a nonzero scalar field $vev$ at zero temperature. We find, for sufficiently large $N$, that the transition from the high temperature symmetry restoring phase to the low temperature phase can be either first order or second order depending on the ratio of the gauge coupling to the scalar self coupling.Comment: 16 pages. 1 Figure, not include

Finite temperature effective theories

Lecture Notes, Summer School on Effective Theories and Fundamental Interactions, Erice, July 1996. I describe the construction of effective field theories for equilibrium high-temperature plasma of elementary particles.Comment: 24 pages, Latex, 5 eps figure

Strong Sphalerons and Electroweak Baryogenesis

We analyze the spontaneous baryogenesis and charge transport mechanisms suggested by Cohen, Kaplan and Nelson for baryon asymmetry generation in extended versions of electroweak theory. We find that accounting for non-perturbative chirality-breaking transitions due to strong sphalerons reduces the baryonic asymmetry by the factor $(m_t/\pi T)^2$ or $\alpha_W$, provided those processes are in thermal equilibrium.Comment: CERN-TH.7080/9

Supersymmetric Q-balls as dark matter

Supersymmetric extensions of the standard model generically contain stable non-topological solitons, Q-balls, which carry baryon or lepton number. We show that large Q-balls can be copiously produced in the early universe, can survive until the present time, and can contribute to dark matter.Comment: 16 pages, 3 figures (epsf); replaced with a final version, to appear in Phys. Lett. B (references added