Luo, Feng2012-11-072012-11-072012-08https://hdl.handle.net/11299/138224University of Minnesota Ph.D. dissertation. August 2012. Major: Physics. Advisor: Keith A. Olive. 1 computer file (PDF); vi, 120 pages, appendices A-B.Many extensions of the Standard Model of particle physics predict the existence of massive unstable or metastable particles. If the Standard Model secondaries induced by the decays of these particles have not been thermalized by the background cosmic plasma before the start of Big-Bang nucleosynthesis (BBN), the decay showers may alter the primordial abundances of the light elements through non-thermal electromagnetic and hadronic interactions with the background nuclei. The concordance of the BBN predictions and observations can thus constrain the abundance, lifetime and decay spectra of the decaying particle. On the other hand, the decays of heavy particles may help drive the concordance in a favorable direction. In particular, depending on the analysis of the observational data adopted, there is a factor of 2 − 4, or 4 − 5σ discrepancy between the predicted and observationally inferred primordial 7Li abundance, and this is known as the ‘7Li problem’. We study the effects on the light-element abundances of the decays of massive gravitinos in neutralino dark matter scenarios within the constrained minimal supersymmetric extension of the Standard Model (CMSSM). When the 7Li constraint is disregarded, for discrete choice of the gravitino mass, we present upper limits on the gravitino abundance for CMSSM parameters along the WMAP strips where the lightest neutralino provides all of the cold dark matter. For some CMSSM benchmark points, we explore the possibility of the effects of the decays of gravitinos as a solution to the 7Li problem, and we find a narrow range for the gravitino mass and abundance where the 7Li problem is alleviated or even marginally solved. We consider the effects of uncertainties in nuclear reaction rates on the cosmological constraints on the decays of massive particles during or after BBN. We identify the nuclear reactions due to non-thermal hadrons that are the most important in perturbing standard BBN, then quantify the uncertainties in these reactions and in the resulting light-element abundances. Applying this analysis to models with unstable gravitinos decaying into neutralinos, we calculate the likelihood function for the light-element abundances measured currently, taking into account the current experimental errors in the determinations of the relevant nuclear reaction rates. We find a region of the gravitino mass and abundance in which the abundances of deuterium, 4He and 7Li may be fit with χ2 = 5.5, compared with χ2 = 31.7 if the effects of gravitino decays are unimportant. The best-fit solution is improved to χ2 ∼ 2.0 when the lithium abundance is taken from globular cluster data. We also study the effects of the residual late-time dark matter particle annihilations during and after BBN on the predicted cosmological abundances of the light elements. Within the CMSSM and its one or two more parameter extensions, with a neutralino lightest supersymmetric particle (LSP), we find negligible effects on the abundances of deuterium, 3He, 4He and 7Li predicted by homogeneous BBN, but potentially a large enhancement in the predicted abundance of 6Li.en-USPhysicsThesis or Dissertation