10 research outputs found
Bound-State Effects on Light-Element Abundances in Gravitino Dark Matter Scenarios
If the gravitino is the lightest supersymmetric particle and the long-lived
next-to-lightest sparticle (NSP) is the stau, the charged partner of the tau
lepton, it may be metastable and form bound states with several nuclei. These
bound states may affect the cosmological abundances of Li6 and Li7 by enhancing
nuclear rates that would otherwise be strongly suppressed. We consider the
effects of these enhanced rates on the final abundances produced in Big-Bang
nucleosynthesis (BBN), including injections of both electromagnetic and
hadronic energy during and after BBN. We calculate the dominant two- and
three-body decays of both neutralino and stau NSPs, and model the
electromagnetic and hadronic decay products using the PYTHIA event generator
and a cascade equation. Generically, the introduction of bound states drives
light element abundances further from their observed values; however, for small
regions of parameter space bound state effects can bring lithium abundances in
particular in better accord with observations. We show that in regions where
the stau is the NSP with a lifetime longer than 10^3-10^4 s, the abundances of
Li6 and Li7 are far in excess of those allowed by observations. For shorter
lifetimes of order 1000 s, we comment on the possibility in minimal
supersymmetric and supergravity models that stau decays could reduce the Li7
abundance from standard BBN values while at the same time enhancing the Li6
abundance.Comment: 22 pages 6 figure
A Bitter Pill: The Primordial Lithium Problem Worsens
The lithium problem arises from the significant discrepancy between the
primordial 7Li abundance as predicted by BBN theory and the WMAP baryon
density, and the pre-Galactic lithium abundance inferred from observations of
metal-poor (Population II) stars. This problem has loomed for the past decade,
with a persistent discrepancy of a factor of 2--3 in 7Li/H. Recent developments
have sharpened all aspects of the Li problem. Namely: (1) BBN theory
predictions have sharpened due to new nuclear data, particularly the
uncertainty on 3He(alpha,gamma)7Be, has reduced to 7.4%, and with a central
value shift of ~ +0.04 keV barn. (2) The WMAP 5-year data now yields a cosmic
baryon density with an uncertainty reduced to 2.7%. (3) Observations of
metal-poor stars have tested for systematic effects, and have reaped new
lithium isotopic data. With these, we now find that the BBN+WMAP predicts 7Li/H
= (5.24+0.71-0.67) 10^{-10}. The Li problem remains and indeed is exacerbated;
the discrepancy is now a factor 2.4--4.3 or 4.2sigma (from globular cluster
stars) to 5.3sigma (from halo field stars). Possible resolutions to the lithium
problem are briefly reviewed, and key nuclear, particle, and astronomical
measurements highlighted.Comment: 21 pages, 4 figures. Comments welcom
Big Bang Nucleosynthesis and Particle Dark Matter
We review how our current understanding of the light element synthesis during
the Big Bang Nucleosynthesis era may help shed light on the identity of
particle dark matter.Comment: a mini-review for the NJP special issue on dark matte
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
Nuclear Reaction Network for Primordial Nucleosynthesis: a detailed analysis of rates, uncertainties and light nuclei yields
We analyze in details the standard Primordial Nucleosynthesis scenario. In
particular we discuss the key theoretical issues which are involved in a
detailed prediction of light nuclide abundances, as the weak reaction rates,
neutrino decoupling and nuclear rate modeling. We also perform a new analysis
of available data on the main nuclear processes entering the nucleosynthesis
reaction network, with particular stress on their uncertainties as well as on
their role in determining the corresponding uncertainties on light nuclide
theoretical estimates. The current status of theoretical versus experimental
results for 2H, 3He, 4He and 7Li is then discussed using the determination of
the baryon density as obtained from Cosmic Microwave Background anisotropies.Comment: LaTeX, 83 pages, 30 .pdf figures. Some typos in the units of
R-functions in appendix D and relative plots fixe
Gravitational clustering of relic neutrinos and implications for their detection
We study the gravitational clustering of big bang relic neutrinos onto
existing cold dark matter (CDM) and baryonic structures within the flat
CDM model, using both numerical simulations and a semi-analytical
linear technique, with the aim of understanding the neutrinos' clustering
properties for direct detection purposes. In a comparative analysis, we find
that the linear technique systematically underestimates the amount of
clustering for a wide range of CDM halo and neutrino masses. This invalidates
earlier claims of the technique's applicability. We then compute the exact
phase space distribution of relic neutrinos in our neighbourhood at Earth, and
estimate the large scale neutrino density contrasts within the local
Greisen--Zatsepin--Kuzmin zone. With these findings, we discuss the
implications of gravitational neutrino clustering for scattering-based
detection methods, ranging from flux detection via Cavendish-type torsion
balances, to target detection using accelerator beams and cosmic rays. For
emission spectroscopy via resonant annihilation of extremely energetic cosmic
neutrinos on the relic neutrino background, we give new estimates for the
expected enhancement in the event rates in the direction of the Virgo cluster.Comment: 38 pages, 8 embedded figures, iopart.cls; v2: references added, minor
changes in text, to appear in JCA
Dark Matter from Gaugino Mediation.
We study dark matter for gaugino-mediated supersymmetry breaking and compact
dimensions of order the grand unification scale. Higgs fields are bulk fields,
and in general their masses differ from those of squarks and sleptons at the
unification scale. As a consequence, at different points in parameter space,
the gravitino, a neutralino or a scalar lepton can be the lightest (LSP) or
next-to-lightest (NLSP) superparticle. We investigate the constraints from
primordial nucleosynthesis on the different scenarios. While neutralino and
gravitino dark matter with a sneutrino NLSP are consistent for a wide range of
parameters, gravitino dark matter with a stau NLSP is strongly constrained.Comment: 18 pages, updated to published version (minor modifications,
reference added