95 research outputs found
Cosmological Constraints on Unstable Particles: Numerical Bounds and Analytic Approximations
Many extensions of the Standard Model predict large numbers of additional
unstable particles whose decays in the early universe are tightly constrained
by observational data. For example, the decays of such particles can alter the
ratios of light-element abundances, give rise to distortions in the cosmic
microwave background, alter the ionization history of the universe, and
contribute to the diffuse photon flux. Constraints on new physics from such
considerations are typically derived for a single unstable particle species
with a single well-defined mass and characteristic lifetime. In this paper, by
contrast, we investigate the cosmological constraints on theories involving
entire ensembles of decaying particles --- ensembles which span potentially
broad ranges of masses and lifetimes. In addition to providing a detailed
numerical analysis of these constraints, we also formulate a set of simple
analytic approximations for these constraints which may be applied to generic
ensembles of unstable particles which decay into
electromagnetically-interacting final states. We then illustrate how these
analytic approximations can be used to constrain a variety of toy scenarios for
physics beyond the Standard Model. For ease of reference, we also compile our
results in the form of a table which can be consulted independently of the rest
of the paper. It is thus our hope that this work might serve as a useful
reference for future model-builders concerned with cosmological constraints on
decaying particles, regardless of the particular model under study.Comment: 41 pages, LaTeX, 12 figures, 4 table
The Higgs Boson can delay Reheating after Inflation
The Standard Model Higgs boson, which has previously been shown to develop an
effective vacuum expectation value during inflation, can give rise to large
particle masses during inflation and reheating, leading to temporary blocking
of the reheating process and a lower reheat temperature after inflation. We
study the effects on the multiple stages of reheating: resonant particle
production (preheating) as well as perturbative decays from coherent
oscillations of the inflaton field. Specifically, we study both the cases of
the inflaton coupling to Standard Model fermions through Yukawa interactions as
well as to Abelian gauge fields through a Chern-Simons term. We find that, in
the case of perturbative inflaton decay to SM fermions, reheating can be
delayed due to Higgs blocking and the reheat temperature can decrease by up to
an order of magnitude. In the case of gauge-reheating, Higgs-generated masses
of the gauge fields can suppress preheating even for large inflaton-gauge
couplings. In extreme cases, preheating can be shut down completely and must be
substituted by perturbative decay as the dominant reheating channel. Finally,
we discuss the distribution of reheat temperatures in different Hubble patches,
arising from the stochastic nature of the Higgs VEV during inflation and its
implications for the generation of both adiabatic and isocurvature
fluctuations.Comment: 23 pages, 9 figures. Matches the version published on JCA
A Study of Dark Matter and QCD-Charged Mediators in the Quasi-Degenerate Regime
We study a scenario in which the only light new particles are a Majorana
fermion dark matter candidate and one or more QCD-charged scalars, which couple
to light quarks. This scenario has several interesting phenomenological
features if the new particles are nearly degenerate in mass. In particular, LHC
searches for the light scalars have reduced sensitivity, since the visible and
invisible products tend to be softer. Moreover, dark matter-scalar
co-annihilation can allow even relatively heavy dark matter candidates to be
consistent thermal relics. Finally, the dark matter nucleon scattering cross
section is enhanced in the quasi-degenerate limit, allowing direct detection
experiments to use both spin-independent and spin-dependent scattering to probe
regions of parameter space beyond those probed by the LHC. Although this
scenario has broad application, we phrase this study in terms of the MSSM, in
the limit where the only light sparticles are a bino-like dark matter candidate
and light-flavored squarks.Comment: 24 pages, 5 figures; as published in PRD with significant revision
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