1,454 research outputs found
A possible origin of superconducting currents in cosmic strings
The scattering and capture of right-handed neutrinos by an Abelian cosmic
string in the SO(10) grand unification model are considered. The scattering
cross-section of neutrinos per unit length due to the interaction with the
gauge and Higgs fields of the string is much larger in its scaling regime than
in the friction one because of the larger infrared cutoff of the former.The
probability of capture in a zero mode of the string accompanied by the emission
of a gauge or Higgs boson shows a resonant peak for neutrino momentum of the
order of its mass. Considering the decrease of number of strings per unit
comoving volume in the scaling epoch the cosmological consequences of the
superconducting strings formed in this regime will be much smaller than those
which could be produced already in the friction one.Comment: 14 pages Latex, 4 figues/ep
WIMP Dark Matter and the QCD Equation of State
Weakly Interacting Massive Particles (WIMPs) of mass m freeze out at a
temperature T_f ~ m/25, i.e. in the range 400 MeV -- 40 GeV for a particle in
the typical mass range 10 -- 1000 GeV. The WIMP relic density, which depends on
the effective number of relativistic degrees of freedom at T_f, may be measured
to better than 1% by Planck, warranting comparable theoretical precision.
Recent theoretical and experimental advances in the understanding of high
temperature QCD show that the quark gluon plasma departs significantly from
ideal behaviour up to temperatures of several GeV, necessitating an improvement
of the cosmological equation of state over those currently used. We discuss how
this increases the relic density by approximately 1.5 -- 3.5% in benchmark
mSUGRA models, with an uncertainly in the QCD corrections of 0.5 -- 1 %. We
point out what further work is required to achieve a theoretical accuracy
comparable with the expected observational precision, and speculate that the
effective number of degrees of freedom at T_f may become measurable in the
foreseeable future.Comment: 4pp, 2figs. More info including Matlab scripts used to generate
equation of state curves at
http://www.pact.cpes.sussex.ac.uk/arXiv/hep-ph/0501232
A New Perspective on Electroweak Strings
The vortex solution (Z-string) of the electroweak interactions can be
interpreted as the 2-dimensional sphaleron at the top of a non-contractible
sphere. The same holds for another type of solution, the W-string.Comment: 13 pages, Latex, NIKHEF-H/94-02 (February 2, 1994), 1 figure
available by fax or mail (send request to [email protected]
Cosmology with Twisted Tori
We consider the cosmological role of the scalar fields generated by the
compactification of 11-dimensional Einstein gravity on a 7D elliptic twisted
torus, which has the attractive features of giving rise to a positive
semi-definite potential, and partially fixing the moduli. This compactification
is therefore relevant for low energy M-theory, 11D supergravity. We find that
slow-roll inflation with the moduli is not possible, but that there is a novel
scaling solution in Friedmann cosmologies in which the massive moduli oscillate
but maintain a constant energy density relative to the background barotropic
fluid
Correlations in Cosmic String Networks
We investigate scaling and correlations of the energy and momentum in an
evolving network of cosmic strings in Minkowski space. These quantities are of
great interest, as they must be understood before accurate predictions for the
power spectra of the perturbations in the matter and radiation in the early
Universe can be made. We argue that Minkowski space provides a reasonable
approximation to a Friedmann background for string dynamics and we use our
results to construct a simple model of the network, in which it is considered
to consist of randomly placed segments moving with random velocities. This
model works well in accounting for features of the two-time correlation
functions, and even better for the power spectra.Comment: 20pp Plain LaTeX, 11 EPS figures, uses epsf.st
Where are the Hedgehogs in Nematics?
In experiments which take a liquid crystal rapidly from the isotropic to the
nematic phase, a dense tangle of defects is formed. In nematics, there are in
principle both line and point defects (``hedgehogs''), but no point defects are
observed until the defect network has coarsened appreciably. In this letter the
expected density of point defects is shown to be extremely low, approximately
per initially correlated domain, as result of the topology
(specifically, the homology) of the order parameter space.Comment: 6 pages, latex, 1 figure (self-unpacking PostScript)
Dual strings and magnetohydrodynamics
We investigate whether dual strings could be solutions of the
magnetohydrodynamics equations in the limit of infinite conductivity. We find
that the induction equation is satisfied, and we discuss the Navier-Stokes
equation (without viscosity) with the Lorentz force included. We argue that the
dual string equations (with a non-universal maximum velocity) should describe
the large scale motion of narrow magnetic flux tubes, because of a large
reparametrization (gauge) invariance of the magnetic and electric string
fields. It is shown that the energy-momentum tensor for the dual string can be
reinterpreted as an energy-momentum tensor for magnetohydrodynamics, provided
certain conditions are satisfied. We also give a brief discussion of the case
when magnetic monopoles are included, and indicate how this can lead to a
non-relativistic "electrohydrodynamics" picture of confinement.Comment: 10 pages. LaTex. A minor correction has been mad
Defect Production in Slow First Order Phase Transitions
We study the formation of vortices in a U(1) gauge theory following a
first-order transition proceeding by bubble nucleation, in particular the
effect of a low velocity of expansion of the bubble walls. To do this, we use a
two-dimensional model in which bubbles are nucleated at random points in a
plane and at random times and then expand at some velocity .
Within each bubble, the phase angle is assigned one of three discrete values.
When bubbles collide, magnetic `fluxons' appear: if the phases are different, a
fluxon--anti-fluxon pair is formed. These fluxons are eventually trapped in
three-bubble collisions when they may annihilate or form quantized vortices. We
study in particular the effect of changing the bubble expansion speed on the
vortex density and the extent of vortex--anti-vortex correlation.Comment: 13 pages, RevTeX, 15 uuencoded postscript figure
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