SuperWIMP Gravitino Dark Matter from Slepton and Sneutrino Decays

Dark matter may be composed of superWIMPs, superweakly-interacting massive particles produced in the late decays of other particles. We focus on the case of gravitinos produced in the late decays of sleptons or sneutrinos and assume they are produced in sufficient numbers to constitute all of non-baryonic dark matter. At leading order, these late decays are two-body and the accompanying energy is electromagnetic. For natural weak-scale parameters, these decays have been shown to satisfy bounds from Big Bang nucleosynthesis and the cosmic microwave background. However, sleptons and sneutrinos may also decay to three-body final states, producing hadronic energy, which is subject to even more stringent nucleosynthesis bounds. We determine the three-body branching fractions and the resulting hadronic energy release. We find that superWIMP gravitino dark matter is viable and determine the gravitino and slepton/sneutrino masses preferred by this solution to the dark matter problem. In passing, we note that hadronic constraints disfavor the possibility of superWIMPs produced by neutralino decays unless the neutralino is photino-like.Comment: 22 pages, updated figures and minor changes, version to appear in Phys. Rev.

Supergravity with a Gravitino LSP

We investigate supergravity models in which the lightest supersymmetric particle (LSP) is a stable gravitino. We assume that the next-lightest supersymmetric particle (NLSP) freezes out with its thermal relic density before decaying to the gravitino at time t ~ 10^4 s - 10^8 s. In contrast to studies that assume a fixed gravitino relic density, the thermal relic density assumption implies upper, not lower, bounds on superpartner masses, with important implications for particle colliders. We consider slepton, sneutrino, and neutralino NLSPs, and determine what superpartner masses are viable in all of these cases, applying CMB and electromagnetic and hadronic BBN constraints to the leading two- and three-body NLSP decays. Hadronic constraints have been neglected previously, but we find that they provide the most stringent constraints in much of the natural parameter space. We then discuss the collider phenomenology of supergravity with a gravitino LSP. We find that colliders may provide important insights to clarify BBN and the thermal history of the Universe below temperatures around 10 GeV and may even provide precise measurements of the gravitino's mass and couplings.Comment: 24 pages, updated figures and minor changes, version to appear in Phys.Rev.

Thermal production of axino Dark Matter

We reconsider thermal production of axinos in the early universe, adding: a) missed terms in the axino interaction; b) production via gluon decays kinematically allowed by thermal masses; c) a precise modeling of reheating. We find an axino abunance a few times larger than previous computations.Comment: 6 pages, 2 figures. Final version, to appear on JHE

WATER'S GONNA RISE : SEA-LEVEL RISE RISK PERCEPTION, COMMUNICATION AND POLICY-MAKING IN NORTH CAROLINA

Sea level rise is threatening coastal areas around the world with the loss of land, damage to personal and public property, ecological impacts, displacement of populations, and exacerbated risk associated with severe storm events. While the drivers of accelerated sea-level rise are global, it is at the local and regional levels that the most immediate impacts and responses occur. Planning for sea-level rise adaptation is occurring throughout the United States, but significant barriers exist, especially in places where political tensions concerning climate change science prevail. Observation of how people understand and perceive sea-level rise risk, comprehend information about their risk, and enter into processes to manage risk can provide us with better understanding of how risk can be socially amplified or attenuated, and strategies to overcome barriers to adaptation planning. To this end, this three-part dissertation investigates sea-level rise risk at multiple scales with the objective of characterizing the social dimensions of risk production and barriers to adaptation policy in northeastern North Carolina, a region with one of the largest areas of low-lying land threatened by sea-level rise in the United States, and with high social vulnerability to natural hazards among some resident populations. The first part investigates individual risk perception using an audience-driven, document evaluation methodology that assesses reader attention, comprehension, and attitudes. Comprehension difficulties confounded concern about sea-level rise hazard yielding fear, skepticism, and fatalism. The second part examines hegemonic discourses of mistrust and fear that provide insight into barriers to adaptation planning and risk reduction efforts. Fatalistic risk perceptions and risk communication scarcity increase risk in the coastal hazardscape, especially among those with the highest social vulnerability. The lack of risk information and predominant risk perceptions reinforce uneven patterns of risk developed through the marginalization of poor populations and facilitation of land use by those with social and political advantages. The third part is a case study exploration of a public participation process that a local municipality used to confront the barriers to adaptation planning. The study enables an understanding of how mainstreaming can overcome political hurdles, and how bridging organizations help move low-capacity communities past resource limitations. The multi-scalar, risk perception-oriented approach to the examination of sea-level rise risk and policy development may provide further guidance for the study of other complex, politically- charged risks within local contexts.  Ph.D

Hybrid inflation with running inflaton mass

We realize and study a model of hybrid inflation in the context of softly broken supersymmetry. The inflaton is taken to be a flat direction in the superfield space and, due to unsuppressed couplings, its soft supersymmetry breaking mass runs with scale. Both gauge and Yukawa couplings are taken into account and different inflationary scenarios are investigated depending on the relative strenght of the couplings and the mass spectrum.Comment: latex file, 14 pages with 3 eps figures, uses epsf.st

Observational constraints on the spectral index of the cosmological curvature perturbation

We evaluate the observational constraints on the spectral index $n$, in the context of the $\Lambda$CDM hypothesis which represents the simplest viable cosmology. We first take $n$ to be practically scale-independent. Ignoring reionization, we find at a nominal 2-$\sigma$ level $n\simeq 1.0 \pm 0.1$. If we make the more realisitic assumption that reionization occurs when a fraction $f\sim 10^{-5}$ to 1 of the matter has collapsed, the 2-$\sigma$ lower bound is unchanged while the 1-$\sigma$ bound rises slightly. These constraints are compared with the prediction of various inflation models. Then we investigate the two-parameter scale-dependent spectral index, predicted by running-mass inflation models, and find that present data allow significant scale-dependence of $n$, which occurs in a physically reasonable regime of parameter space.Comment: ReVTeX, 15 pages, 5 figures and 3 tables, uses epsf.sty Improved treatment of reionization and small bug fixed in the constant n case; more convenient parameterization and better treatment of the n dependence in the CMB anisotropy for the running mass case; conclusions basically unchanged; references adde

Leptogenesis for Pedestrians

During the process of thermal leptogenesis temperature decreases by about one order of magnitude while the baryon asymmetry is generated. We present an analytical description of this process so that the dependence on the neutrino mass parameters becomes transparent. In the case of maximal CP asymmetry all decay and scattering rates in the plasma are determined by the mass M_1 of the decaying heavy Majorana neutrino, the effective light neutrino mass tilde{m}_1 and the absolute mass scale bar{m} of the light neutrinos. In the mass range suggested by neutrino oscillations, m_{sol} \simeq 8*10^{-3} eV \lesssim \tilde{m}_1 \lesssim m_{atm} \simeq 5*10^{-2} eV, leptogenesis is dominated just by decays and inverse decays. The effect of all other scattering processes lies within the theoretical uncertainty of present calculations. The final baryon asymmetry is dominantly produced at a temperature T_B which can be about one order of magnitude below the heavy neutrino mass M_1. We also derive an analytical expression for the upper bound on the light neutrino masses implied by successful leptogenesis.Comment: 55 pages, 14 figures include

Possible Evidence For Axino Dark Matter In The Galactic Bulge

Recently, the SPI spectrometer on the INTEGRAL satellite observed strong 511 keV line emission from the galactic bulge. Although the angular distribution (spherically symmetric with width of \sim 9 degree) of this emission is difficult to account for with traditional astrophysical scenarios, light dark matter particles could account for the observation. In this letter, we consider the possibility that decaying axinos in an R-parity violating model of supersymmetry may be the source of this emission. We find that \sim 1-300 MeV axinos with R-parity violating couplings can naturally produce the observed emission.Comment: 4 pages, 1 figure. Version accepted by Physical Review

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

Mixed axion/neutralino cold dark matter in supersymmetric models

We consider supersymmetric (SUSY) models wherein the strong CP problem is solved by the Peccei-Quinn (PQ) mechanism with a concommitant axion/axino supermultiplet. We examine R-parity conserving models where the neutralino is the lightest SUSY particle, so that a mixture of neutralinos and axions serve as cold dark matter. The mixed axion/neutralino CDM scenario can match the measured dark matter abundance for SUSY models which typically give too low a value of the usual thermal neutralino abundance, such as models with wino-like or higgsino-like dark matter. The usual thermal neutralino abundance can be greatly enhanced by the decay of thermally-produced axinos to neutralinos, followed by neutralino re-annihilation at temperatures much lower than freeze-out. In this case, the relic density is usually neutralino dominated, and goes as \sim (f_a/N)/m_{axino}^{3/2}. If axino decay occurs before neutralino freeze-out, then instead the neutralino abundance can be augmented by relic axions to match the measured abundance. Entropy production from late-time axino decays can diminish the axion abundance, but ultimately not the neutralino abundance. In mixed axion/neutralino CDM models, it may be possible to detect both a WIMP and an axion as dark matter relics. We also discuss possible modifications of our results due to production and decay of saxions. In the appendices, we present expressions for the Hubble expansion rate and the axion and neutralino relic densities in radiation, matter and decaying-particle dominated universes.Comment: 31 pages including 21 figure