1,265 research outputs found
Thermal Relics in Modified Cosmologies: Bounds on Evolution Histories of the Early Universe and Cosmological Boosts for PAMELA
Alternative cosmologies, based on extensions of General Relativity, predict
modified thermal histories in the Early Universe during the pre Big Bang
Nucleosynthesis (BBN) era, epoch which is not directly constrained by
cosmological observations. When the expansion rate is enhanced with respect to
the standard case, thermal relics typically decouple with larger relic
abundances. The correct value of the relic abundance is therefore obtained for
larger annihilation cross--sections, as compared to standard cosmology. A
direct consequence is that indirect detection rates are enhanced. Extending
previous analyses of ours, we derive updated astrophysical bounds on the dark
matter annihilation cross sections and use them to constrain alternative
cosmologies in the pre--BBN era. We also determine the characteristics of these
alternative cosmologies in order to provide the correct value of relic
abundance for a thermal relic for the (large) annihilation cross--section
required to explain the PAMELA results on the positron fraction, therefore
providing a "cosmological boost" solution to the dark matter interpretation of
the PAMELA data.Comment: 19 pages, 27 figures, matches published versio
Constraining pre Big-Bang-Nucleosynthesis Expansion using Cosmic Antiprotons
A host of dark energy models and non-standard cosmologies predict an enhanced
Hubble rate in the early Universe: perfectly viable models, which satisfy Big
Bang Nucleosynthesis (BBN), cosmic microwave background and general relativity
tests, may nevertheless lead to enhancements of the Hubble rate up to many
orders of magnitude. In this paper we show that strong bounds on the pre-BBN
evolution of the Universe may be derived, under the assumption that dark matter
is a thermal relic, by combining the dark matter relic density bound with
constraints coming from the production of cosmic-ray antiprotons by dark matter
annihilation in the Galaxy. The limits we derive can be sizable and apply to
the Hubble rate around the temperature of dark matter decoupling. For dark
matter masses lighter than 100 GeV, the bound on the Hubble-rate enhancement
ranges from a factor of a few to a factor of 30, depending on the actual
cosmological model, while for a mass of 500 GeV the bound falls in the range
50-500. Uncertainties in the derivation of the bounds and situations where the
bounds become looser are discussed. We finally discuss how these limits apply
to some specific realizations of non-standard cosmologies: a scalar-tensor
gravity model, kination models and a Randall-Sundrum D-brane model.Comment: 19 pages, 15 figures, LaTex, uses revtex
Spin-1 Thermal Targets for Dark Matter Searches at Beam Dump and Fixed Target Experiments
The current framework for dark matter searches at beam dump and fixed target
experiments relies on four benchmark models, the complex scalar, inelastic
scalar, pseudo-Dirac and finally, Majorana DM models. While this approach has
so far been successful in the interpretation of the available data, it a priori
excludes the possibility that DM is made of spin-1 particles -- a restriction
which is neither theoretically nor experimentally justified. In this work we
extend the current landscape of sub-GeV DM models to a set of models for spin-1
DM, including a family of simplified models (involving one DM candidate and one
mediator -- the dark photon) and an ultraviolet complete model based on a
non-abelian gauge group where DM is a spin-1 Strongly Interacting Massive
Particle. For each of these models, we calculate the DM relic density, the
expected number of signal events at beam dump experiments, the rate of energy
injection in the early universe thermal bath and in the Intergalactic Medium,
as well as the helicity amplitudes for forward processes subject to the unitary
bound. We then compare these predictions with experimental results from Planck,
CMB surveys, IGM temperature observations, LSND, MiniBooNE, NA64, and BaBar and
with available projections from LDMX and Belle II. Through this comparison, we
identify the regions in the parameter space of the models considered in this
work where DM is simultaneously thermally produced, compatible with present
observations, and within reach at Belle II and LDMX. We find that the
simplified models are strongly constrained by current beam dump experiments and
the unitarity bound, and will thus be conclusively probed in the first stage of
LDMX data taking. We also find that the SIMP model explored in this work
predicts the observed DM abundance, is compatible with current observations and
within reach at LDMX in a wide region of the parameter space
Axion-Dilaton Cosmology and Dark Energy
We discuss a class of flat FRW cosmological models based on D=4 axion-dilaton
gravity universally coupled to cosmological background fluids. In particular,
we investigate the possibility of recurrent acceleration, which was recently
shown to be generically realized in a wide class of axion-dilaton models, but
in absence of cosmological background fluids. We observe that, once we impose
the existence of radiation -and matter- dominated earlier stages of cosmic
evolution, the axion-dilaton dynamics is altered significantly with respect to
the case of pure axion-dilaton gravity. During the matter dominated epoch the
scalar fields remain either frozen, due to the large expansion rate, or enter a
cosmological scaling regime. In both cases, oscillations of the effective
equation of state around the acceleration boundary value are impossible. Models
which enter an oscillatory stage in the low redshift regime, on the other hand,
are disfavored by observations. We also comment on the viability of the
axion-dilaton system as a candidate for dynamical dark energy. In a certain
subclass of models, an intermediate scaling regime is succeeded by eternal
acceleration. We also briefly discuss the issue of dependence on initial
conditions.Comment: 28 pages, 11 figure
Einstein and Jordan frames reconciled: a frame-invariant approach to scalar-tensor cosmology
Scalar-Tensor theories of gravity can be formulated in different frames, most
notably, the Einstein and the Jordan one. While some debate still persists in
the literature on the physical status of the different frames, a frame
transformation in Scalar-Tensor theories amounts to a local redefinition of the
metric, and then should not affect physical results. We analyze the issue in a
cosmological context. In particular, we define all the relevant observables
(redshift, distances, cross-sections, ...) in terms of frame-independent
quantities. Then, we give a frame-independent formulation of the Boltzmann
equation, and outline its use in relevant examples such as particle freeze-out
and the evolution of the CMB photon distribution function. Finally, we derive
the gravitational equations for the frame-independent quantities at first order
in perturbation theory. From a practical point of view, the present approach
allows the simultaneous implementation of the good aspects of the two frames in
a clear and straightforward way.Comment: 15 pages, matches version to be published on Phys. Rev.
Dynamical Relaxation of the Dark Matter to Baryon Ratio
A scalar field interacting differently with dark matter and baryons may
explain why their ratio is of order unity today. We provide three working
examples, checking them against the observations of CMB, Large Scale Structure,
supernovae Ia, and post-newtonian tests of gravity. Such a scenario could make
life much easier for supersymmetric dark matter candidates.Comment: 7 pages, 5 .eps figures. Discussion of the approach of the field to
the fixed point added. Figures modified accordingly. Conclusions unchanged.
Version to be published on Phys Rev.
SUSY dark matter(s)
We review here the status of different dark matter candidates in the context
of supersymmetric models, in particular the neutralino as a realization of the
WIMP-mechanism and the gravitino. We give a summary of the recent bounds in
direct and indirect detection and also of the LHC searches relevant for the
dark matter question. We discuss also the implications of the Higgs discovery
for the supersymmetric dark matter models and give the prospects for the future
years.Comment: 16 pages, 3 figure
Enlarging mSUGRA parameter space by decreasing pre-BBN Hubble rate in Scalar-Tensor Cosmologies
We determine under what conditions Scalar Tensor cosmologies predict an
expansion rate which is reduced as compared to the standard General Relativity
case. We show that ST theories with a single matter sector typically predict an
enchanced Hubble rate in the past, as a consequence of the requirement of an
attractive fixed point towards General Relativity at late times. Instead, when
additional matter sectors with different conformal factors are added, the late
time convergence to General Relativity is mantained and at the same time a
reduced expansion rate in the past can be driven. For suitable choices of the
parameters which govern the scalar field evolution, a sizeable reduction (up to
about 2 orders of magnitude) of the Hubble rate prior to Big Bang
Nucleosynthesis can be obtained. We then discuss the impact of these
cosmological models on the relic abundance of dark matter is minimal
Supergravity models: we show that the cosmologically allowed regions in
parameter space are significantly enlarged, implying a change in the potential
reach of LHC on the neutralino phenomenology.Comment: 10 pages, 7 figure
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