5,665 research outputs found
Sterile neutrino Dark Matter production from scalar decay in a thermal bath
We calculate the production rate of singlet fermions from the decay of
neutral or charged scalar fields in a hot plasma. We find that there are
considerable thermal corrections when the temperature of the plasma exceeds the
mass of the decaying scalar. We give analytic expressions for the
temperature-corrected production rates in the regime where the decay products
are relativistic. We also study the regime of non-relativistic decay products
numerically. Our results can be used to determine the abundance and momentum
distribution of Dark Matter particles produced in scalar decays. The inclusion
of thermal corrections helps to improve predictions for the free streaming of
the Dark Matter particles, which is crucial to test the compatibility of a
given model with cosmic structure formation. With some modifications, our
results may be generalised to the production of other Dark Matter candidates in
scalar decays.Comment: This version matches the one published in JHEP. 44 pages, 10 figure
Fast and Accurate Random Walk with Restart on Dynamic Graphs with Guarantees
Given a time-evolving graph, how can we track similarity between nodes in a
fast and accurate way, with theoretical guarantees on the convergence and the
error? Random Walk with Restart (RWR) is a popular measure to estimate the
similarity between nodes and has been exploited in numerous applications. Many
real-world graphs are dynamic with frequent insertion/deletion of edges; thus,
tracking RWR scores on dynamic graphs in an efficient way has aroused much
interest among data mining researchers. Recently, dynamic RWR models based on
the propagation of scores across a given graph have been proposed, and have
succeeded in outperforming previous other approaches to compute RWR
dynamically. However, those models fail to guarantee exactness and convergence
time for updating RWR in a generalized form. In this paper, we propose OSP, a
fast and accurate algorithm for computing dynamic RWR with insertion/deletion
of nodes/edges in a directed/undirected graph. When the graph is updated, OSP
first calculates offset scores around the modified edges, propagates the offset
scores across the updated graph, and then merges them with the current RWR
scores to get updated RWR scores. We prove the exactness of OSP and introduce
OSP-T, a version of OSP which regulates a trade-off between accuracy and
computation time by using error tolerance {\epsilon}. Given restart probability
c, OSP-T guarantees to return RWR scores with O ({\epsilon} /c ) error in O
(log ({\epsilon}/2)/log(1-c)) iterations. Through extensive experiments, we
show that OSP tracks RWR exactly up to 4605x faster than existing static RWR
method on dynamic graphs, and OSP-T requires up to 15x less time with 730x
lower L1 norm error and 3.3x lower rank error than other state-of-the-art
dynamic RWR methods.Comment: 10 pages, 8 figure
Attractor scenarios and superluminal signals in k-essence cosmology
Cosmological scenarios with k-essence are invoked in order to explain the
observed late-time acceleration of the universe. These scenarios avoid the need
for fine-tuned initial conditions (the "coincidence problem") because of the
attractor-like dynamics of the k-essence field \phi. It was recently shown that
all k-essence scenarios with Lagrangians p=L(X)/\phi^2, necessarily involve an
epoch where perturbations of \phi propagate faster than light (the "no-go
theorem"). We carry out a comprehensive study of attractor-like cosmological
solutions ("trackers") involving a k-essence scalar field \phi and another
matter component. The result of this study is a complete classification of
k-essence Lagrangians that admit asymptotically stable tracking solutions,
among all Lagrangians of the form p=K(\phi)L(X) . Using this classification, we
select the class of models that describe the late-time acceleration and avoid
the coincidence problem through the tracking mechanism. An analogous "no-go
theorem" still holds for this class of models, indicating the existence of a
superluminal epoch. In the context of k-essence cosmology, the superluminal
epoch does not lead to causality violations. We discuss the implications of
superluminal signal propagation for possible causality violations in
Lorentz-invariant field theories.Comment: 27 pages, RevTeX4. Minor cosmetic changes, references adde
One-loop Einstein-Hilbert term in minimally supersymmetric type IIB orientifolds
We evaluate string one-loop contributions to the Einstein-Hilbert term in toroidal minimally supersymmetric type IIB orientifolds with D-branes. These have potential applications to the determination of quantum corrections to the moduli Kahler metric in these models
The Kinematics of Cosmic Reheating
We calculate the relaxation rate of a scalar field in a plasma of other
scalars and fermions with gauge interactions using thermal quantum field
theory. It yields the rate of cosmic reheating and thereby determines the
temperature of the "hot big bang" in inflationary cosmology. The total rate
originates from various processes, including decays and inverse decays as well
as Landau damping by scatterings. It involves quantum statistical effects and
off-shell transport. Its temperature dependence can be highly nontrivial,
making it impossible to express the reheating temperature in terms of the model
parameters in a simple way. We pay special attention to the temperature
dependence of the phase space due to the modified dispersion relations in the
plasma. We find that it can have a drastic effect on the efficiency of
perturbative reheating, which depends on the way particles in the primordial
plasma interact. For some interactions thermal masses can effectively close the
phase space for the dominant dissipative processes and thereby impose an upper
bound on the reheating temperature. In other cases they open up new channels of
dissipation, hence increase the reheating temperature. At high temperatures we
find that the universe can even be heated through couplings to fermions, which
are often assumed to be negligible due to Pauli-blocking. These effects may
also be relevant for baryogenesis, dark matter production, the fate of moduli
and in scenarios of warm inflation.Comment: 39 pages, 17 figures. The present version of the article slightly
deviates from the published version. It includes a number of small
corrections, which we summarised in a published Corrigendu
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