The weak electroweak phase transition

We present a detailed analysis of the phase transition in the standard model at finite temperature. Using an improved perturbation theory, where plasma masses are determined from a set of one-loop gap equations, we evaluate the effective potential $V_{eff}(\varphi,T)$ in next-to-leading order, i.e., including terms cubic in the gauge coupling $g$, the scalar self-coupling $\lambda^{1/2}$ and the top-quark Yukawa coupling $f_t$. The gap equations yield a non-vanishing magnetic plasma mass for the gauge bosons, originating from the non-abelian self-interactions. We discuss in detail size and origin of higher order effects and conclude that the phase transition is weakly first-order up to Higgs masses of about $70\ GeV$, above which our calculation is no longer self-consistent. For larger Higgs masses even an approximation containing all $g^4$ contributions to $V_{eff}$ is not sufficient, at least a full calculation to order $g^6$ is needed. These results turn out to be rather insensitive to the top-quark mass in the range $m_t=100\ -\ 180\ GeV$. Using Langer's theory of metastability we calculate the nucleation rate of critical droplets and discuss some aspects of the cosmological electroweak phase transition.Comment: LaTeX, 45 pages, 13 figures [not included, can be sent upon request],DESY 93-02

Spectator processes and baryogenesis

Spectator processes which are in thermal equilibrium during the period of baryogenesis influence the final baryon asymmetry. We study this effect quantitatively for thermal leptogenesis where we find a suppression by a factor O(1)

Neutrino Mixing and the Pattern of Supersymmetry Breaking

We study the implications of a large $\nu_\mu - \nu_\tau$ mixing angle on lepton flavour violating radiative transitions in supersymmetric extensions of the standard model. The transition rates are calculated to leading order in $\epsilon$, the parameter which characterizes the flavour mixing. The uncertainty of the predicted rates is discussed in detail. For models with modular invariance the branching ratio $BR(\mu \to e \gamma)$ mostly exceeds the experimental upper limit. In models with radiatively induced flavour mixing the predicted range includes the upper limit, if the Yukawa couplings in the lepton sector are large, as favoured by Yukawa coupling unification.Comment: 12 pages, 2 figures, used axodraw.sty, to be published in Physics Letters

Vacuum stability and supersymmetry at high scales with two Higgs doublets

We investigate the stability of the electroweak vacuum for two-Higgs doublet models with a supersymmetric UV completion. The supersymmetry breaking scale is taken to be of the order of the grand unification scale. We first study the case where all superpartners decouple at this scale. We show that contrary to the Standard Model with one Higgs doublet, matching to the supersymmetric UV completion is possible if the low-scale model contains two Higgs doublets. In this case vacuum stability and experimental constraints point towards low values of tan(beta) < 2 and pseudoscalar masses of at least about a TeV. If the higgsino superpartners of the Higgs fields are also kept light, the conclusions are similar and essentially independent of the higgsino mass. Finally, if all gauginos are also given electroweak-scale masses (split supersymmetry with two Higgs doublets), the model cannot be matched to supersymmetry at very high scales when requiring a 125 GeV Higgs. Light neutral and charged higgsinos therefore emerge as a promising signature of a supersymmetric UV completion of the Standard Model at the grand unification scale.Comment: 27 pages, 4 figures; v2: minor changes in references and text, results unchange

The flavour puzzle from an orbifold GUT perspective

Neutrino masses and mixings are very different from quark masses and mixings. This puzzle is a crucial hint in the search for the mechanism which determines fermion masses in grand unified theories. We study the flavour problem in an SO(10) GUT model in six dimensions compactified on an orbifold. Three sequential families are localized at three branes where SO(10) is broken to its three GUT subgroups. Their mixing with bulk fields leads to large neutrino mixings as well as small mixings among left-handed quarks. The small hierarchy of neutrino masses is due to the mismatch between up-quark and down-quark mass hierarchies.Comment: 13 pages, 3 figures, Talk given at the Fujihara seminar Neutrino Mass and Seesaw Mechanism, KEK, Japan, February, 200

Matter Antimatter Asymmetry and Neutrino Properties

The cosmological baryon asymmetry can be explained as remnant of heavy Majorana neutrino decays in the early universe. We study this mechanism for two models of neutrino masses with a large \nu_\mu-\nu_\tau mixing angle which are based on the symmetries SU(5) x U(1)_F and SU(3)_c x SU(3)_L x SU(3)_R x U(1)_F, respectively. In both cases B-L is broken at the unification scale \Lambda_{GUT}. The models make different predictions for the baryogenesis temperature and the gravitino abundance.Comment: latex2e, 14 pages, 2 figures, Contribution to the Festschrift for L. B. Okun, to appear in a special issue of Physics Reports, eds. V. L. Telegdi and K. Winte

Neutrino Mixing and Flavour Changing Processes

We study the implications of a large nu_mu - nu_tau mixing angle on flavour changing transitions of quarks and leptons in supersymmetric extensions of the standard model. Two patterns of supersymmetry breaking are considered, models with modular invariance and the standard scenario of universal soft breaking terms at the GUT scale. The analysis is performed for two symmetry groups G x U(1)_F, with G=SU(5) and G=SU(3)^3, where U(1)_F is a family symmetry. Models with modular invariance are in agreement with observations only for restricted scalar quark and gaugino masses, (M_squark^2)/(m_gluino^2) \simeq 7/9 and m_bino > 350 GeV. A characteristic feature of models with large tan beta and radiatively induced flavour mixing is a large branching ratio for mu -> e gamma. For both symmetry groups and for the considered range of supersymmetry breaking mass parameters we find BR(mu -> e gamma) > 10^(-14).Comment: 25 pages, 6 figure