10 research outputs found
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
Towards constraints on the SUSY seesaw from flavour-dependent leptogenesis
We systematically investigate constraints on the parameters of the
supersymmetric type-I seesaw mechanism from the requirement of successful
thermal leptogenesis in the presence of upper bounds on the reheat temperature
of the early Universe. To this end, we solve the
flavour-dependent Boltzmann equations in the MSSM, extended to include
reheating. With conservative bounds on , leading to mildly
constrained scenarios for thermal leptogenesis, compatibility with observation
can be obtained for extensive new regions of the parameter space, due to
flavour-dependent effects. On the other hand, focusing on (normal) hierarchical
light and heavy neutrinos, the hypothesis that there is no CP violation
associated with the right-handed neutrino sector, and that leptogenesis
exclusively arises from the CP-violating phases of the matrix,
is only marginally consistent. Taking into account stricter bounds on
further suggests that (additional) sources of CP violation must
arise from the right-handed neutrino sector, further implying stronger
constraints for the right-handed neutrino parameters.Comment: 42 pages, 12 figures; final version published in JCAP; numerical
results for the efficiency factor can be downloaded from
http://www.newphysics.eu/leptogenesis
The gravitino coupling to broken gauge theories applied to the MSSM
We consider gravitino couplings in theories with broken gauge symmetries. In
particular, we compute the single gravitino production cross section in W+ W-
fusion processes. Despite recent claims to the contrary, we show that this
process is always subdominant to gluon fusion processes in the high energy
limit. The full calculation is performed numerically; however, we give analytic
expressions for the cross section in the supersymmetric and electroweak limits.
We also confirm these results with the use of the effective theory of goldstino
interactions.Comment: 26 pages, 4 figure
Leptogenesis and rescattering in supersymmetric models
The observed baryon asymmetry of the Universe can be due to the
violating decay of heavy right handed (s)neutrinos. The amount of the asymmetry
depends crucially on their number density. If the (s)neutrinos are generated
thermally, in supersymmetric models there is limited parameter space leading to
enough baryons. For this reason, several alternative mechanisms have been
proposed. We discuss the nonperturbative production of sneutrino quanta by a
direct coupling to the inflaton. This production dominates over the
corresponding creation of neutrinos, and it can easily (i.e. even for a rather
small inflaton-sneutrino coupling) lead to a sufficient baryon asymmetry. We
then study the amplification of MSSM degrees of freedom, via their coupling to
the sneutrinos, during the rescattering phase which follows the nonperturbative
production. This process, which mainly influences the (MSSM) flat
directions, is very efficient as long as the sneutrinos quanta are in the
relativistic regime. The rapid amplification of the light degrees of freedom
may potentially lead to a gravitino problem. We estimate the gravitino
production by means of a perturbative calculation, discussing the regime in
which we expect it to be reliable.Comment: (20 pages, 6 figures), references added, typos corrected. Final
version in revte
Classical inflaton field induced creation of superheavy dark matter
We calculate analytically and numerically the production of superheavy dark
matter (X) when it is coupled to the inflaton field \phi within the context of
a slow-roll m_\phi^2 \phi^2/2 inflationary model with coupling g^2 X^2
\phi^2/2. We find that X particles with a mass as large as 1000 H_i, where H_i
is the value of the Hubble expansion rate at the end of inflation, can be
produced in sufficient abundance to be cosmologically significant today. This
means that superheavy dark matter may have a mass of up to 10^{-3} Planck mass.
We also derive a simple formula that can be used to estimate particle
production as a result of a quantum field's interaction with a general class of
homogeneous classical fields. Finally, we note that the combined effect of the
inflaton field and the gravitational field on the X field causes the production
to be a nonmonotonic function of g^2.Comment: 42 page LaTeX file with 8 PostScript figures included with eps
Wall of fundamental constants
We consider the signatures of a domain wall produced in the spontaneous symmetry breaking involving a dilatonlike scalar field coupled to electromagnetism. Domains on either side of the wall exhibit slight differences in their respective values of the fine-structure constant, \u3b1. If such a wall is present within our Hubble volume, absorption spectra at large redshifts may or may not provide a variation in \u3b1 relative to the terrestrial value, depending on our relative position with respect to the wall. This wall could resolve the contradiction between claims of a variation of \u3b1 based on Keck/Hires data and of the constancy of \u3b1 based on Very Large Telescope data. We derive the properties of the wall and the parameters of the underlying microscopic model required to reproduce the possible spatial variation of \u3b1. We discuss the constraints on the existence of the low-energy domain wall and describe its observational implications concerning the variation of the fundamental constants
Post-inflationary gravitino production revisited
We revisit gravitino production following inflation. As a first step, we review the standard calculation of gravitino production in the thermal plasma formed at the end of post-inflationary reheating when the inflaton has completely decayed. Next we consider gravitino production prior to the completion of reheating, assuming that the inflaton decay products thermalize instantaneously while they are still dilute. We then argue that instantaneous thermalization is in general a good approximation, and also show that the contribution of non-thermal gravitino production via the collisions of inflaton decay products prior to thermalization is relatively small. Our final estimate of the gravitino-to-entropy ratio is approximated well by a standard calculation of gravitino production in the post-inflationary thermal plasma assuming total instantaneous decay and thermalization at a time t sime 1.2/\u393phgr. Finally, in light of our calculations, we consider potential implications of upper limits on the gravitino abundance for models of inflation, with particular attention to scenarios for inflaton decays in supersymmetric Starobinsky-like models
Slow and safe gravitinos
It has been argued that supergravity models of inflation with vanishing sound speeds, cs, lead to an unbounded growth in the production rate of gravitinos. We consider several models of inflation to delineate the conditions for which cs=0. In models with unconstrained superfields, we argue that the mixing of the Goldstino and inflatino in a time-varying background prevents the uncontrolled production of the longitudinal modes. This conclusion is unchanged if there is a nilpotent field associated with supersymmetry breaking with constraint S2=0, i.e., (s)Goldstino-less models. Models with a second orthogonal constraint, S(Ï-ÏÂŻ)=0, where Ï is the inflaton superfield, which eliminates the inflatino, may suffer from the overproduction of gravitinos. However, we point out that these models may be problematic if this constraint originates from a UV Lagrangian, as this may require using higher derivative operators. These models may also exhibit other pathologies, such as cs>1, which are absent in theories with the single constraint or unconstrained fields