2,898 research outputs found

### Sommerfeld enhancement from Goldstone pseudo-scalar exchange

We point out that the exchange of a Goldstone pseudo-scalar can provide an
enhancement in the dark matter annihilation rate capable of explaining the
excess flux seen in high energy cosmic ray data. The mechanism of enhancement
involves the coupling of s and d waves through the tensor force that is very
strong and, in fact, singular at short distances. The results indicate that
large enhancements require some amount of fine tuning. We also discuss the
enhancement due to other singular attractive potentials, such as WIMP models
with a permanent electric dipole.Comment: 14 pages, 4 figures, v2 includes contact informatio

### The twisted open string partition function and Yukawa couplings

We use the operator formalism to derive the bosonic contribution to the
twisted open string partition function in toroidal compactifications. This
amplitude describes, for instance, the planar interaction between g+1
magnetized or intersecting D-branes. We write the result both in the closed and
in the open string channel in terms of Prym differentials on the appropriate
Riemann surface. Then we focus on the g=2 case for a 2-torus. By factorizing
the twisted partition function in the open string channel we obtain an explicit
expression for the 3-twist field correlator, which is the main ingredient in
the computation of Yukawa couplings in D-brane phenomenological models. This
provides an alternative method for computing these couplings that does not rely
on the stress-energy tensor technique.Comment: 32 pages, 5 figures, Latex; v2: typos correcte

### Edge Tunneling of Vortices in Superconducting Thin Films

We investigate the phenomenon of the decay of a supercurrent due to the
zero-temperature quantum tunneling of vortices from the edge in a thin
superconducting film in the absence of an external magnetic field. An explicit
formula is derived for the tunneling rate of vortices, which are subject to the
Magnus force induced by the supercurrent, through the Coulomb-like potential
barrier binding them to the film's edge. Our approach ensues from the
non-relativistic version of a Schwinger-type calculation for the decay of the
2D vacuum previously employed for describing vortex-antivortex pair-nucleation
in the bulk of the sample. In the dissipation-dominated limit, our explicit
edge-tunneling formula yields numerical estimates which are compared with those
obtained for bulk-nucleation to show that both mechanisms are possible for the
decay of a supercurrent.Comment: REVTeX file, 15 pages, 1 Postscript figure; to appear in Phys.Rev.

### Stability of the de Sitter spacetime in Horava-Lifshitz theory

The stability of de Sitter spacetime in Horava-Lifshitz theory of gravity
with projectability but without detailed balance condition is studied. It is
found that, in contrast to the case of the Minkowski background, the spin-0
graviton now is stable for any given $\xi$, and free of ghost for $\xi \le 0$
in the infrared limit, where $\xi$ is the dynamical coupling constant.Comment: Mod. Phys. Lett. A25, 2267-2279 (2010

### Dynamical Vortices in Superfluid Films

The coupling of vortices to phonons in a superfluid is a gauge coupling
dictated by topology. The density and current response to a moving vortex are
computed and contrasted with the standard backflow picture. Exploiting the
analogy to (2+1)-dimensional electrodynamics, we compute the effective vortex
mass $M(\omega)$ and find it to be logarithmically divergent in the low
frequency limit, leading to a super-Ohmic dissipation in response to an
oscillating superflow. Numerical integration of the nonlinear Schroedinger
equation supports these conclusions. Interaction of vortices and impurities is
also discussed.Comment: 13 pages, 6 figure

### Analytic calculations of trial wave functions of the fractional quantum Hall effect on the sphere

We present a framework for the analytic calculations of the hierarchical wave
functions and the composite fermion wave functions in the fractional quantum
Hall effect on the sphere by using projective coordinates. Then we calculate
the overlaps between these two wave functions at various fillings and small
numbers of electrons. We find that the overlaps are all most equal to one. This
gives a further evidence that two theories of the fractional quantum Hall
effect, the hierarchical theory and the composite fermion theory, are
physically equivalent.Comment: 37 pages, revte

### On the scattering amplitude in the Aharonov-Bohm gauge field

A general expression for the scattering amplitude of nonrelativistic spinless
particles in the Aharonov-Bohm gauge potential is obtained within the time
independent formalism. The result is valid also in the backward and forward
directions as well as for any choice of the boundary conditions on the wave
function at the flux tube position.Comment: 18 pages, plain TE

### Nambu-Poisson Bracket and M-Theory Branes Coupled to Antisymmetric Fluxes

By using the recently proposed prescription arXiv:0804.3629 for obtaining the
$M5$ brane action from multiple $M2$ branes action in BLG theory, we examine
such transition when 11 Dimensional background antisymmetric fluxes couple to
the $M2$ brane world volume. Such couplings was suggested in arXiv:0805.3427
where it was used the fact that various fields in BLG theory are valued in a
Lie 3-algebra. We argue that this action and promoting it by Nambu-Poisson
bracket gives the expected coupling of fluxes with $M5$ brane at least at weak
coupling limit. We also study some other aspects of the action for example, the
gauge invariance of the theory.Comment: 14 page

### The phase portrait of a matter bounce in Horava-Lifshitz cosmology

The occurrence of a bounce in FRW cosmology requires modifications of general
relativity. An example of such a modification is the recently proposed
Horava-Lifshitz theory of gravity, which includes a ``dark radiation'' term
with a negative coefficient in the analog of the Friedmann equation. This paper
describes a phase space analysis of models of this sort with the aim of
determining to what extent bouncing solutions can occur. A simplification,
valid in the relevant region, allows a reduction of the dimension of phase
space so that visualization in three dimensions is possible. It is found that a
bounce is possible, but not generic in models under consideration. Apart from
previously known bouncing solutions some new ones are also described. Other
interesting solutions found include ones which describe a novel sort of
oscillating universes.Comment: 14 pages, 8 figure

### Detailed balance condition and ultraviolet stability of scalar field in Horava-Lifshitz gravity

Detailed balance and projectability conditions are two main assumptions when
Horava recently formulated his theory of quantum gravity - the Horava-Lifshitz
(HL) theory. While the latter represents an important ingredient, the former
often believed needs to be abandoned, in order to obtain an ultraviolet stable
scalar field, among other things. In this paper, because of several attractive
features of this condition, we revisit it, and show that the scalar field can
be stabilized, if the detailed balance condition is allowed to be softly
broken. Although this is done explicitly in the non-relativistic general
covariant setup of Horava-Melby-Thompson with an arbitrary coupling constant
$\lambda$, generalized lately by da Silva, it is also true in other versions of
the HL theory. With the detailed balance condition softly breaking, the number
of independent coupling constants can be still significantly reduced. It is
remarkable to note that, unlike other setups, in this da Silva generalization,
there exists a master equation for the linear perturbations of the scalar field
in the flat Friedmann-Robertson-Walker background.Comment: Some typos are corrected. To appear in JCA

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