1,080 research outputs found
Large-scale magnetic fields, curvature fluctuations and the thermal history of the Universe
It is shown that gravitating magnetic fields affect the evolution of
curvature perturbations in a way that is reminiscent of a pristine
non-adiabatic pressure fluctuation. The gauge-invariant evolution of curvature
perturbations is used to constrain the magnetic power spectrum. Depending on
the essential features of the thermodynamic history of the Universe, the
explicit derivation of the bound is modified. The theoretical uncertainty in
the constraints on the magnetic energy spectrum is assessed by comparing the
results obtained in the case of the conventional thermal history with the
estimates stemming from less conventional (but phenomenologically allowed)
post-inflationary evolutions.Comment: 21 pages, 6 included figure
Lepton Flavor Violation without Supersymmetry
We study the lepton flavor violating (LFV) processes mu -> e gamma, mu -> 3e,
and mu -> e conversion in nuclei in the left-right symmetric model without
supersymmetry and perform the first complete computation of the LFV branching
ratios B(mu -> f) to leading non-trivial order in the ratio of left- and
right-handed symmetry breaking scales. To this order, B(mu -> e gamma) and B(mu
-> e) are governed by the same combination of LFV violating couplings, and
their ratio is naturally of order unity. We also find B(mu -> 3 e)/B(mu -> e)
\sim 100 under slightly stronger assumptions. Existing limits on the branching
ratios already substantially constrain mass splittings and/or mixings in the
heavy neutrino sector. When combined with future collider studies and precision
electroweak measurements, improved limits on LFV processes will test the
viability of low-scale, non-supersymmetric LFV scenarios.Comment: 24 pages, 7 figures, 2 table
Escape of black holes from the brane
TeV-scale gravity theories allow the possibility of producing small black
holes at energies that soon will be explored at the LHC or at the Auger
observatory. One of the expected signatures is the detection of Hawking
radiation, that might eventually terminate if the black hole, once perturbed,
leaves the brane. Here, we study how the `black hole plus brane' system evolves
once the black hole is given an initial velocity, that mimics, for instance,
the recoil due to the emission of a graviton. The results of our dynamical
analysis show that the brane bends around the black hole, suggesting that the
black hole eventually escapes into the extra dimensions once two portions of
the brane come in contact and reconnect. This gives a dynamical mechanism for
the creation of baby branes.Comment: 4 pages, 6 figure
Pentaquark implications for exotic mesons
If the exotic baryon is a correlated with , then there should exist an exotic meson, GeV with width MeV. The
may be broad members of {\bf 10} \10bar in such a
picture. Vector mesons in the 1.4 - 1.7GeV mass range are also compared with
this picture
Macroscopic amplification of electroweak effects in molecular Bose-Einstein condensates
We investigate the possible use of Bose-Einstein condensates of diatomic
molecules to measure nuclear spin-dependent parity violation effects, outlining
a detection method based on the internal Josephson effect between molecular
states of opposite parity. When applied to molecular condensates, the fine
experimental control achieved in atomic bosonic Josephson junctions could
provide data on anapole moments and neutral weak couplings.Comment: 5 pages. To be published Phys. Rev. A (Rapid Communication) (2012
Cosmological Hysteresis and the Cyclic Universe
A Universe filled with a homogeneous scalar field exhibits `Cosmological
hysteresis'. Cosmological hysteresis is caused by the asymmetry in the equation
of state during expansion and contraction. This asymmetry results in the
formation of a hysteresis loop: , whose value can be non-vanishing
during each oscillatory cycle. For flat potentials, a negative value of the
hysteresis loop leads to the increase in amplitude of consecutive cycles and to
a universe with older and larger successive cycles. Such a universe appears to
possess an arrow of time even though entropy production is absent and all of
the equations respect time-reversal symmetry ! Cosmological hysteresis appears
to be widespread and exists for a large class of scalar field potentials and
mechanisms for making the universe bounce. For steep potentials, the value of
the hysteresis loop can be positive as well as negative. The expansion factor
in this case displays quasi-periodic behaviour in which successive cycles can
be both larger as well as smaller than previous ones. This quasi-regular
pattern resembles the phenomenon of BEATS displayed by acoustic systems.
Remarkably, the expression relating the increase/decrease in oscillatory cycles
to the quantum of hysteresis appears to be model independent. The cyclic
scenario is extended to spatially anisotropic models and it is shown that the
anisotropy density decreases during successive cycles if the hysteresis loop is
negative.Comment: 31 pages, 8 figures. Matches version published in Phys Rev D85,
123542 (2012
Gravitational Uncertainty and Black Hole Remnants
Possible existence of black holes remnants provides a suitable candidates for
dark matter. In this paper we study the possibility of existence for such
remnants. We consider quantum gravitational induced corrections of black hole's
entropy and temperature to investigate the possibility of such relics.
Observational scheme for detection of these remnants and their cosmological
constraints are discussed.Comment: 17 pages, 6 figure
Lyman alpha absorption lines from mini pancakes
[Abridged abstract:] Recent numerical simulations show that many \lyal
absorption lines of column densities \nha \la 10^{15} cm are produced
in transient, mini pancakes. Such pancakes are modeled here, approximating the
initial perturbation leading to the formation of the pancake as a single
sinusoidal wave. The density and temperature profiles of the gas in the pancake
are determined for , where is the collapse redshift. The
\lyal absorption line profiles for a line of sight through the pancake are
then calculated. The absorption lines in general have wings signifying bulk
motions in the gas. It is shown that the deviation from a single Voigt profile
is large for small H I column density lines, in which the effect of bulk
motions is large. For lines with \nha > 10^{13} cm, high temperature
tend to wash out the signatures of bulk motion. The analytical modeling of mini
pancakes associated with \lyal forest lines --- with 10^{13} \la \nha \la
10^{15} cm---gives the corresponding mass scales. It is shown here
that, for typical values of cosmological parameters, absorption lines with
\nha \sim 10^{14} cm correspond to structures with baryonic mass of
M with an overdensity of at .
The value of \nha can change by a factor in the course of evolution
of the pancake in time. It is also shown that there is an upper limit to \nha
from a pancake due to the slow recombination rate and the importance of
collisional ionization at high temperatures. Mini pancakes do not give rise to
\lyal lines with \nha \ga 10^{14.5} cm, for \j21=1 and
.Comment: Latex with aaspp4.sty (25 pages), 6 figures, Accepted for publication
in The Astrophysical Journa
Physical constants and the Gurzadyan-Xue formula for the dark energy
We consider cosmological implications of the formula for the dark energy
density derived by Gurzadyan and Xue which predicts a value fitting the
observational one. Cosmological models with varying by time physical constants,
namely, speed of light and gravitational constant and/or their combinations,
are considered. In one of the models, for example, vacuum energy density
induces effective negative curvature, while another one has an unusual
asymptotic. This analysis also explicitely rises the issue of the meaning and
content of physical units and constants in cosmological context.Comment: version corrected to match the one to appear in Modern Physics
Letters
Toward a Minimum Branching Fraction for Dark Matter Annihilation into Electromagnetic Final States
Observational limits on the high-energy neutrino background have been used to
place general constraints on dark matter that annihilates only into standard
model particles. Dark matter particles that annihilate into neutrinos will also
inevitably branch into electromagnetic final states through higher-order tree
and loop diagrams that give rise to charged leptons, and these charged
particles can transfer their energy into photons via synchrotron radiation or
inverse Compton scattering. In the context of effective field theory, we
calculate the loop-induced branching ratio to charged leptons and show that it
is generally quite large, typically >1%, when the scale of the dark matter mass
exceeds the electroweak scale, M_W. For a branching fraction >3%, the
synchrotron radiation bounds on dark matter annihilation are currently stronger
than the corresponding neutrino bounds in the interesting mass range from 100
GeV to 1 TeV. For dark matter masses below M_W, our work provides a plausible
framework for the construction of a model for "neutrinos only" dark matter
annihilations.Comment: 18 pages, 4 figures, discussion added, matches version in Phys. Rev.
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