765 research outputs found
Diffusion and Debye Screening Near Expanding Domain Walls
We study the effect of Debye screening of hypercharge when a net fermion
number is reflected from a domain wall during a first order phase transition,
which may be relevant for electroweak baryogenesis. We give a simple method for
computing the effect of screening within the diffusion approximation, whose
results are compatible with those of a more elaborate treatment based on the
Boltzmann equation. Our formalism takes into account the differences in
mobility of different particle species. We believe it is conceptually simpler
than other accounts of screening that have appeared in this context. Somewhat
surprisingly, we find that Debye screening can actually {\it enhance}
electroweak baryogenesis by a modest factor ().Comment: 11 pp. latex, uses epsf.tex, 1 uuencoded figur
Evolution of column density distributions within Orion~A
We compare the structure of star-forming molecular clouds in different
regions of Orion A to determine how the column density probability distribution
function (N-PDF) varies with environmental conditions such as the fraction of
young protostars. A correlation between the N-PDF slope and Class 0 protostar
fraction has been previously observed in a low-mass star-formation region
(Perseus) by Sadavoy; here we test if a similar correlation is observed in a
high-mass star-forming region. We use Herschel data to derive a column density
map of Orion A. We use the Herschel Orion Protostar Survey catalog for accurate
identification and classification of the Orion A young stellar object (YSO)
content, including the short-lived Class 0 protostars (with a 0.14 Myr
lifetime). We divide Orion A into eight independent 13.5 pc regions; in
each region we fit the N-PDF distribution with a power-law, and we measure the
fraction of Class 0 protostars. We use a maximum likelihood method to measure
the N-PDF power-law index without binning. We find that the Class 0 fraction is
higher in regions with flatter column density distributions. We test the
effects of incompleteness, YSO misclassification, resolution, and pixel-scale.
We show that these effects cannot account for the observed trend. Our
observations demonstrate an association between the slope of the power-law
N-PDF and the Class 0 fractions within Orion A. Various interpretations are
discussed including timescales based on the Class 0 protostar fraction assuming
a constant star-formation rate. The observed relation suggests that the N-PDF
can be related to an "evolutionary state" of the gas. If universal, such a
relation permits an evaluation of the evolutionary state from the N-PDF
power-law index at much greater distances than those accesible with protostar
counts. (abridged)Comment: A&A Letter, accepte
Supersymmetric Electroweak Phase Transition: Dimensional Reduction versus Effective Potential
We compare two methods of analyzing the finite-temperature electroweak phase
transition in the minimal supersymmetric standard model: the traditional
effective potential (EP) approach, and the more recently advocated procedure of
dimensional reduction (DR). The latter tries to avoid the infrared
instabilities of the former by matching the full theory to an effective theory
that has been studied on the lattice. We point out a limitation of DR that
caused a large apparent disagreement with the effective potential results in
our previous work. We also incorporate wave function renormalization into the
EP, which is shown to decrease the strength of the phase transition. In the
regions of parameter space where both methods are expected to be valid, they
give similar results, except that the EP is significantly more restrictive than
DR for the range of baryogenesis-allowed values of , , the
critical temperature, and the up-squark mass parameter . In contrast, the
DR results are consistent with 2\lsim\tan\beta\lsim 4, GeV, and
sufficiently large to have universality of the squark soft-breaking
masses at the GUT scale, in a small region of parameter space. We suggest that
the differences between DR and EP are due to higher-order perturbative
corrections rather than infrared effects.Comment: 19 pages, Latex, 7 figures, uses epsf.te
Supersymmetric Electroweak Baryogenesis in the WKB approximation
We calculate the baryon asymmetry generated at the electroweak phase
transition in the minimal supersymmetric standard model, treating the particles
in a WKB approximation in the bubble wall background. A set of diffusion
equations for the particle species relevant to baryon generation, including
source terms arising from the CP violation associated with the complex phase
of the parameter, are derived from Boltzmann equations, and
solved. The conclusion is that must be \gsim 0.1 to generate a
baryon asymmetry consistent with nucleosynthesis. We compare our results to
several other recent computations of the effect, arguing that some are
overestimates.Comment: 12 pages, 1 figure, corrected some criticisms of hep-ph/9702409; to
appear in Phys. Lett.
Supersymmetric Electroweak Baryogenesis
We re-examine the generation of the baryon asymmetry in the minimal
supersymmetric standard model (MSSM) during the electroweak phase transition.
We find that the dominant source for baryogenesis arises from the chargino
sector. The CP-violation comes from the complex phase in the mu parameter,
which provides CP-odd contributions to the particle dispersion relations. This
leads to different accelerations for particles and antiparticles in the wall
region which, combined with diffusion, leads to the separation of Higgsinos and
their antiparticles in the front of the wall. These asymmetries get transported
to produce perturbations in the left-handed chiral quarks, which then drive
sphaleron interactions to create the baryon asymmetry. We present a complete
derivation of the semiclassical WKB formalism, including the chargino
dispersion relations and a self-consistent derivation of the diffusion
equations starting from semiclassical Boltzmann equations for WKB-excitations.
We stress the advantages of treating the transport equations in terms of the
manifestly gauge invariant physical energy and kinetic momentum, rather than in
the gauge variant canonical variables used in previous treatments. We show that
a large enough baryon asymmetry can be created for the phase of the complex mu
parameter as small as ~ 0.001, which is consistent with bounds from the neutron
electric dipole moment.Comment: 54 pages, 3 figure
On the stability of spherically symmetric spacetimes in metric f(R) gravity
We consider stability properties of spherically symmetric spacetimes of stars
in metric f(R) gravity. We stress that these not only depend on the particular
model, but also on the specific physical configuration. Typically
configurations giving the desired are strongly
constrained, while those corresponding to are
less affected. Furthermore, even when the former are found strictly stable in
time, the domain of acceptable static spherical solutions typically shrinks to
a point in the phase space. Unless a physical reason to prefer such a
particular configuration can be found, this poses a naturalness problem for the
currently known metric f(R) models for accelerating expansion of the Universe.Comment: Published version, 9 pages, 3 figure
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