83 research outputs found
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 in next-to-leading order, i.e.,
including terms cubic in the gauge coupling , the scalar self-coupling
and the top-quark Yukawa coupling . 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 , above which our calculation
is no longer self-consistent. For larger Higgs masses even an approximation
containing all contributions to is not sufficient, at least a
full calculation to order is needed. These results turn out to be rather
insensitive to the top-quark mass in the range . 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 mixing angle on
lepton flavour violating radiative transitions in supersymmetric extensions of
the standard model. The transition rates are calculated to leading order in
, 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 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
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