1,775 research outputs found
Effects of a Thermal Bath of Photons on Embedded String Stability
We compute the corrections of thermal photons on the effective potential for
the linear sigma model of QCD. Since we are interested in temperatures lower
than the confinement temperature, we consider the scalar fields to be out of
equilibrium. Two of the scalar field are uncharged while the other two are
charged under the U(1) gauge symmetry of electromagnetism. We find that the
induced thermal terms in the effective potential can stabilize the embedded
pion string, a string configuration which is unstable in the vacuum. Our
results are applicable in a more general context and demonstrate that embedded
string configurations arising in a wider class of field theories can be
stabilized by thermal effects. Another well-known example of an embedded string
which can be stabilized by thermal effects is the electroweak Z-string. We
discuss the general criteria for thermal stabilization of embedded defects.Comment: 6 pages, formatting changed, a few typos correcte
Unconventional cosmology on the (thick) brane
We consider the cosmology of a thick codimension 1 brane. We obtain the
matching conditions leading to the cosmological evolution equations and show
that when one includes matter with a pressure component along the extra
dimension in the brane energy-momentum tensor, the cosmology is of non-standard
type. In particular one can get acceleration when a dust of non-relativistic
matter particles is the only source for the (modified) Friedman equation. Our
equations would seem to violate the conservation of energy-momentum from a 4D
perspective, but in 5D the energy-momentum is conserved. One could write down
an effective conserved 4D energy-momentum tensor attaching a ``dark energy''
component to the energy-momentum tensor of matter that has pressure along the
extra dimension. This extra component could, on a cosmological scale, be
interpreted as matter-coupled quintessence. We comment on the effective 4D
description of this effect in terms of the time evolution of a scalar field
(the 5D radion) coupled to this kind of matter.Comment: 9 pages, v2. eq.(17) corrected, comments on effective theory change
String Breaking in Non-Abelian Gauge Theories with Fundamental Matter Fields
We present clear numerical evidence for string breaking in three-dimensional
SU(2) gauge theory with fundamental bosonic matter through a mixing analysis
between Wilson loops and meson operators representing bound states of a static
source and a dynamical scalar. The breaking scale is calculated in the
continuum limit. In units of the lightest glueball we find . The implications of our results for QCD are discussed.Comment: 4 pages, 2 figures; equations (4)-(6) corrected, numerical results
and conclusions unchange
Using Hubble Space Telescope Imaging of Nuclear Dust Morphology to Rule Out Bars Fueling Seyfert Nuclei
If AGN are powered by the accretion of matter onto massive black holes, how
does the gas in the host galaxy lose the required angular momentum to approach
the black hole? Gas easily transfers angular momentum to stars in strong bars,
making them likely candidates. Although ground-based searches for bars in
active galaxies using both optical and near infrared surface brightness have
not found any excess of bars relative to quiescent galaxies, the searches have
not been able to rule out small-scale nuclear bars. To look for these nuclear
bars we use HST WFPC2-NICMOS color maps to search for the straight dust lane
signature of strong bars. Of the twelve Seyfert galaxies in our sample, only
three have dust lanes consistent with a strong nuclear bar. Therefore, strong
nuclear bars cannot be the primary fueling mechanism for Seyfert nuclei. We do
find that a majority of the galaxies show an spiral morphology in their dust
lanes. These spiral arms may be a possible fueling mechanism.Comment: To be published in the Astronomical Journal, June 1999. 25 pages and
14 figures. Full resolution figures are available at
ftp://www.ciw.edu/pub/mregan/fullfigs.tar.g
Q-ball Formation through Affleck-Dine Mechanism
We present the full nonlinear calculation of the formation of a Q-ball
through the Affleck-Dine (AD) mechanism by numerical simulations. It is shown
that large Q-balls are actually produced by the fragmentation of the condensate
of a scalar field whose potential is very flat. We find that the typical size
of a Q-ball is determined by the most developed mode of linearized
fluctuations, and almost all the initial charges which the AD condensate
carries are absorbed into the formed Q-balls, whose sizes and the charges
depend only on the initial charge densities.Comment: 4 pages, RevTex, 3 postscript figures included, the published versio
Stellar kinematics in double-barred galaxies: the sigma-hollows
We present SAURON integral-field stellar velocity and velocity dispersion
maps for four double-barred early-type galaxies: NGC2859, NGC3941, NGC4725 and
NGC5850. The presence of the inner bar does not produce major changes in the
line-of-sight velocity, but it appears to have an important effect in the
stellar velocity dispersion maps: we find two sigma-hollows of amplitudes
between 10 and 40 km/s on either side of the center, at the ends of the inner
bars. We have performed numerical simulations to explain these features. Ruling
out other possibilities, we conclude that the sigma-hollows are an effect of
the contrast between two kinematically different components: the high velocity
dispersion of the bulge and the more ordered motion (low velocity dispersion)
of the inner bar.Comment: 5 pages, 2 figures. Accepted for publication in ApJ Letter
Large Scale Inhomogeneities from the QCD Phase Transition
We examine the first-order cosmological QCD phase transition for a large
class of parameter values, previously considered unlikely. We find that the
hadron bubbles can nucleate at very large distance scales, they can grow as
detonations as well as deflagrations, and that the phase transition may be
completed without reheating to the critical temperature. For a subset of the
parameter values studied, the inhomogeneities generated at the QCD phase
transition might have a noticeable effect on nucleosynthesis.Comment: 15 LaTeX pages + 6 PostScript figures appended at the end of the
file, HU-TFT-94-1
Q-ball formation in the gravity-mediated SUSY breaking scenario
We study the formation of Q-balls which are made of flat directions that
appear in the supersymmetric extension of the standard model in the context of
gravity-mediated supersymmetry breaking. The full non-linear calculations for
the dynamics of the complex scalar field are made. Since the scalar potential
in this model is flatter than \phi^2, we have found that fluctuations develop
and go non-linear to form non-topological solitons, Q-balls. The size of a
Q-ball is determined by the most amplified mode, which is completely determined
by the model parameters. On the other hand, the charge of Q-balls depends
linearly on the initial charge density of the Affleck-Dine (AD) field. Almost
all the charges are absorbed into Q-balls, and only a tiny fraction of the
charges is carried by a relic AD field. It may lead to some constraints on the
baryogenesis and/or parameters in the particle theory. The peculiarity of
gravity-mediation is the moving Q-balls. This results in collisions between
Q-balls. It may increase the charge of Q-balls, and change its fate.Comment: 9 pages, RevTex, 11 postscript figures included, to appear in Phys.
Rev.
Q-ball formation: Obstacle to Affleck-Dine baryogenesis in the gauge-mediated SUSY breaking ?
We consider the Affleck-Dine baryogenesis comprehensively in the minimal
supersymmetric standard model with gauge-mediated supersymmetry breaking.
Considering the high temperature effects, we see that the Affleck-Dine field is
naturally deformed into the form of the Q ball. In the natural scenario where
the initial amplitude of the field and the A-terms are both determined by the
nonrenormalizable superpotential, we obtain only very a narrow allowed region
in the parameter space in order to explain the baryon number of the universe
for the case that the Q-ball formation occurs just after baryon number
production. Moreover, most of the parameter sets suited have already been
excluded by current experiments. We also find new situations in which the
Q-ball formation takes place rather late compared with baryon number creation.
This situation is more preferable, since it allows a wider parameter region for
naturally consistent scenarios, although it is still difficult to realize in
the actual cosmological scenario.Comment: 27 pages, RevTeX, 21 postscript figures included. The version to be
publishe
Flavour physics constraints in the BMSSM
We study the implications of the presence of the two leading-order,
non-renormalizable operators in the Higgs sector of the MSSM to flavour physics
observables. We identify the constraints of flavour physics on the parameters
of the BMSSM when we: a) focus on a region of parameters for which electroweak
baryogenesis is feasible, b) use a CMSSM-like parametrization, and c) consider
the case of a generic NUHM-type model. We find significant differences as
compared to the standard MSSM case.Comment: 22 pages, 7 figure
- …