422 research outputs found
Tracking Quintessence and Cold Dark Matter Candidates
We study the generation of a kination-dominated phase in the context of a
quintessential model with an inverse-power-law potential and a Hubble-induced
mass term for the quintessence field. The presence of kination is associated
with an oscillating evolution of the quintessence field and the barotropic
index. We find that, in sizeable regions of the parameter space, a tracker
scaling solution can be reached sufficiently early to alleviate the coincidence
problem. Other observational constraints originating from nucleosynthesis, the
inflationary scale, the present acceleration of the universe and the
dark-energy-density parameter can be also met. The impact of this modified
kination-dominated phase on the thermal abundance of cold dark matter
candidates is investigated too. We find that: (i) the enhancement of the relic
abundance of the WIMPs with respect to the standard paradigm, crucially depends
on the hierarchy between the freeze-out temperature and the temperature at
which the extrema in the evolution of the quintessence field are encountered,
and (ii) the relic abundance of e-WIMPs takes its present value close to the
temperature at which the earliest extremum of the evolution of the quintessence
field occurs and, as a consequence, both gravitinos and axinos arise as natural
cold dark matter candidates. In the case of unstable gravitinos, the gravitino
constraint can be satisfied for values of the initial temperature well above
those required in the standard cosmology.Comment: Final versio
Quintessential Kination and Cold Dark Matter Abundance
The generation of a kination-dominated phase by a quintessential exponential
model is investigated and the parameters of the model are restricted so that a
number of observational constraints (originating from nucleosynthesis, the
present acceleration of the universe and the dark-energy-density parameter) are
satisfied. The decoupling of a thermal cold dark matter particle during the
period of kination is analyzed, the relic density is calculated both
numerically and semi-analytically and the results are compared with each other.
It is argued that the enhancement, with respect to the standard paradigm, of
the cold dark matter abundance can be expressed as a function of the
quintessential density parameter at the onset of nucleosynthesis. We find that
values of the latter quantity close to its upper bound require the
thermal-averaged cross section times the velocity of the cold relic to be
almost three orders of magnitude larger than this needed in the standard
scenario so as compatibility with the cold dark matter constraint is achieved.Comment: Published versio
Colliders as a simultaneous probe of supersymmetric dark matter and Terascale cosmology
Terascale supersymmetry has the potential to provide a natural explanation of
the dominant dark matter component of the standard lambda-CDM cosmology.
However once we impose the constraints on minimal supersymmetry parameters from
current particle physics data, a satisfactory dark matter abundance is no
longer prima facie natural. This Neutralino Tuning Problem could be a hint of
nonstandard cosmology during and/or after the Terascale era. To quantify this
possibility, we introduce an alternative cosmological benchmark based upon a
simple model of quintessential inflation. This benchmark has no free
parameters, so for a given supersymmetry model it allows an unambiguous
prediction of the dark matter relic density. As a example, we scan over the
parameter space of the CMSSM, comparing the neutralino relic density
predictions with the bounds from WMAP. We find that the WMAP--allowed regions
of the CMSSM are an order of magnitude larger if we use the alternative
cosmological benchmark, as opposed to lambda-CDM. Initial results from the CERN
Large Hadron Collider will distinguish between the two allowed regions.Comment: 14 pages, 8 figure
Determining Reheating Temperature at Colliders with Axino or Gravitino Dark Matter
After a period of inflationary expansion, the
Universe reheated and reached full thermal equilibrium at the reheating
temperature T_R. In this work we point out that, in the context of effective
low-energy supersymmetric models, LHC measurements may allow one to determine
T_R as a function of the mass of the dark matter particle assumed to be either
an axino or a gravitino. An upper bound on their mass may also be derived.Comment: 19 pages, some improvements, JHEP versio
Dirac gaugino as leptophilic dark matter
We investigate the leptophilic properties of Dirac gauginos in an
R--symmetric N=2 supersymmetric model with extended gauge and Higgs sectors.
The annihilation of Dirac gauginos to leptons requires no chirality flip in the
final states so that it is not suppressed as in the Majorana case. This implies
that it can be sizable enough to explain the positron excess observed by the
PAMELA experiment with moderate or no boost factors. When squark masses are
heavy, the annihilation of Dirac gauginos to hadrons is controlled by their
Higgsino fraction and is driven by the and final states.
Moreover, at variance with the Majorana case, Dirac gauginos with a
non-vanishing higgsino fraction can also have a vector coupling with the
gauge boson leading to a sizable spin--independent scattering cross section off
nuclei. Saturating the current antiproton limit, we show that Dirac gauginos
can leave a signal in direct detection experiments at the level of the
sensitivity of dark matter searches at present and in the near future.Comment: 24 pages, 10 figures, typos corrected, final version published on
JCA
Connecting LHC, ILC, and Quintessence
If the cold dark matter consists of weakly interacting massive particles
(WIMPs), anticipated measurements of the WIMP properties at the Large Hadron
Collider (LHC) and the International Linear Collider (ILC) will provide an
unprecedented experimental probe of cosmology at temperatures of order 1 GeV.
It is worth emphasizing that the expected outcome of these tests may or may not
be consistent with the picture of standard cosmology. For example, in
kination-dominated quintessence models of dark energy, the dark matter relic
abundance can be significantly enhanced compared to that obtained from freeze
out in a radiation-dominated universe. Collider measurements then will
simultaneously probe both dark matter and dark energy. In this article, we
investigate the precision to which the LHC and ILC can determine the dark
matter and dark energy parameters under those circumstances. We use an
illustrative set of four benchmark points in minimal supergravity in analogy
with the four LCC benchmark points. The precision achievable together at the
LHC and ILC is sufficient to discover kination-dominated quintessence, under
the assumption that the WIMPs are the only dark matter component. The LHC and
ILC can thus play important roles as alternative probes of both dark matter and
dark energy.Comment: 38 pages, 9 figure
Neutralino-Nucleon Cross Section and Charge and Colour Breaking Constraints
We compute the neutralino-nucleon cross section in several supersymmetric
scenarios, taking into account all kind of constraints. In particular, the
constraints that the absence of dangerous charge and colour breaking minima
imposes on the parameter space are studied in detail. In addition, the most
recent experimental constraints, such as the lower bound on the Higgs mass, the
branching ratio, and the muon are considered. The
astrophysical bounds on the dark matter density are also imposed on the
theoretical computation of the relic neutralino density, assuming thermal
production. This computation is relevant for the theoretical analysis of the
direct detection of dark matter in current experiments. We consider first the
supergravity scenario with universal soft terms and GUT scale. In this scenario
the charge and colour breaking constraints turn out to be quite important, and
\tan\beta\lsim 20 is forbidden. Larger values of can also be
forbidden, depending on the value of the trilinear parameter . Finally, we
study supergravity scenarios with an intermediate scale, and also with
non-universal scalar and gaugino masses where the cross section can be very
large.Comment: Final version to appear in JHE
SUSY Constraints, Relic Density, and Very Early Universe
The sensitivity of the lightest supersymmetric particle relic density
calculation to different cosmological scenarios is discussed. In particular, we
investigate the effects of modifications of the expansion rate and of the
entropy content in the Early Universe. These effects, even with no
observational consequences, can still drastically modify the relic density
constraints on the SUSY parameter space. We suggest general parametrizations to
evaluate such effects, and derive also constraints from Big-Bang
nucleosynthesis. We show that using the relic density in the context of
supersymmetric constraints requires a clear statement of the underlying
cosmological model assumptions to avoid misinterpretations. On the other hand,
we note that combining the relic density calculation with the eventual future
discoveries at the LHC will hopefully shed light on the Very Early Universe
properties.Comment: 11 pages, 5 figures. v2: new figures adde
Observational constraints on Horava-Lifshitz cosmology
We use observational data from Type Ia Supernovae (SNIa), Baryon Acoustic
Oscillations (BAO), and Cosmic Microwave Background (CMB), along with
requirements of Big Bang Nucleosynthesis (BBN), to constrain the cosmological
scenarios governed by Horava-Lifshitz gravity. We consider both the detailed
and non-detailed balance versions of the gravitational sector, and we include
the matter and radiation sectors. We conclude that the detailed-balance
scenario cannot be ruled out from the observational point of view, however the
corresponding likelihood contours impose tight constraints on the involved
parameters. The scenario beyond detailed balance is compatible with
observational data, and we present the corresponding stringent constraints and
contour-plots of the parameters. Although this analysis indicates that
Horava-Lifshitz cosmology can be compatible with observations, it does not
enlighten the discussion about its possible conceptual and theoretical
problems.Comment: 11 pages, 6 figures, version published in JCA
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