Electromagnetic Cascade in the Early Universe and its Application to the Big-Bang Nucleosynthesis

We investigate the electromagnetic cascade initiated by injection of very high energy photons in the early Universe and calculate the cascade spectrum by solving a set of Boltzmann equations numerically. In the calculation we take account of Compton scattering off the background electrons and pair creation off the background nucleons as well as photon-photon processes and inverse Compton scattering. We also apply our cascade spectrum to the big bang nucleosynthesis with photo-dissociation processes due to heavy unstable particles and obtain the constraint on their lifetime and abundance.Comment: 21pages (compressed and uuencoded postscript file including 6 figures

Gravitino Production in the Inflationary Universe and the Effects on Big-Bang Nucleosynthesis

Gravitino production and decay in the inflationary universe are reexamined. Assuming that the gravitino mainly decays into a photon and a photino, we calculate the upperbound on the reheating temperature. Compared to previous works, we have essentially improved the following two points: (i) the helicity $\pm\frac{3}{2}$ gravitino production cross sections are calculated by using the full relevant terms in the supergravity lagrangian, and (ii) the high energy photon spectrum is obtained by solving the Boltzmann equations numerically. Photo-dissociation of the light elements (D, T, $^3$He, $^4$He) leads to the most stringent upperbound on the reheating temperature, which is given by ($10^{6}$--$10^{9}$)GeV for the gravitino mass 100GeV--1TeV. On the other hand, requiring that the present mass density of photino should be smaller than the critical density, we find that the reheating temperature have to be smaller than ($10^{11}$--$10^{12}$)GeV for the photino mass (10--100)GeV, irrespectively of the gravitino mass. The effect of other decay channels is also considered

Footprints of Supersymmetry on Higgs Decay

Motivated by future collider proposals that aim to measure the Higgs properties precisely, we study the partial decay widths of the lightest Higgs boson in the minimal supersymmetric standard model with an emphasis on the parameter region where all superparticles and heavy Higgs bosons are not accessible at the LHC. Taking account of phenomenological constraints such as the Higgs mass, flavor constraints, vacuum stability, and perturbativity of coupling constants up to the grand unification scale, we discuss how large the deviations of the partial decay widths from the standard model predictions can be. These constraints exclude large fraction of the parameter region where the Higgs widths show significant deviation from the standard model predictions. Nevertheless, even if superparticles and the heavy Higgses are out of the reach of 14TeV LHC, the deviation may be large enough to be observed at future $e^+e^-$ collider experiments.Comment: 24 pages, 8 figures, version accepted in JHE

Supersymmetric Heavy Higgses at e^+e^- Linear Collider and Dark-Matter Physics

We consider the capability of the e^+e^- linear collider (which is recently called as the International Linear Collider, or ILC) for studying the properties of the heavy Higgs bosons in the supersymmetric standard model. We pay special attention to the large \tan\beta region which is motivated, in particular, by explaining the dark-matter density of the universe (i.e., so-called ``rapid-annihilation funnels''). We perform a systematic analysis to estimate expected uncertainties in the masses and widths of the heavy Higgs bosons assuming an energy and integrated luminosity of \sqrt{s}=1 TeV and L=1 ab^{-1}. We also discuss its implication to the reconstruction of the dark-matter density of the universe.Comment: 28 pages, 13 figures, version to appear in PR

Gravitino Dark Matter with Weak-Scale Right-Handed Sneutrino

We consider cosmological implications of supersymmetric models with right-handed (s)neutrinos where the neutrino masses are purely Dirac-type. We pay particular attention to the case where gravitino is the lightest superparticle while one of the right-handed sneutrinos is next-to-the-lightest superparticle. We study constraints from big-bang nuleosynthesis and show that the constraints could be relaxed compared to the case without right-handed sneutrinos. As a result, the gravitino-dark-matter scenario becomes viable with relatively large value of the gravitino mass. We also discuss constraints from the structure formation; in our model, the free-streaming length of the gravitino dark matter may be as long as O(1 Mpc), which is comparable to the present observational upper bound on the scale of free-streaming.Comment: 18 pages, 6 figure

Exact Event Rates of Lepton Flavor Violating Processes in Supersymmetric SU(5) Model

Event rates of various lepton flavor violating processes in the minimal supersymmetric SU(5) model are calculated, using exact formulas which include Yukawa vertices of lepton-slepton-Higgsino. We find subtlety in evaluating event rates due to partial cancellation between diagrams. This cancellation typically reduces the event rates significantly, and the size of the reduction strongly depends on superparticle mass spectrum.Comment: 11pages, 8 figures. Fig.5 where the mu-e conversion rates in nuclei was shown was incorrect due to an error in our numerical computation.It is replaced in this corrected version. All conclusions remain unchange

MSSM curvaton in the gauge-mediated SUSY breaking

We study the curvaton scenario using the MSSM flat directions in the gauge-mediated SUSY breaking model. We find that the fluctuations in the both radial and phase directions can be responsible for the density perturbations in the universe through the curvaton mechanism. Although it has been considered difficult to have a successful curvaton scenario with the use of those flat directions, it is overcome by taking account of the finite temperature effects, which induce a negative thermal logarithmic term in the effective potential of the flat direction.Comment: 12 page

Renormalization-Scale Uncertainty in the Decay Rate of False Vacuum

We study radiative corrections to the decay rate of false vacua, paying particular attention to the renormalization-scale dependence of the decay rate. The decay rate exponentially depends on the bounce action. The bounce action itself is renormalization scale dependent. To make the decay rate scale-independent, radiative corrections, which are due to the field fluctuations around the bounce, have to be included. We show quantitatively that the inclusion of the fluctuations suppresses the scale dependence, and hence is important for the precise calculation of the decay rate. We also apply our analysis to a supersymmetric model and show that the radiative corrections are important for the Higgs-stau system with charge breaking minima.Comment: 15 pages, 2 figures; added reference

On the Gauge Invariance of the Decay Rate of False Vacuum

We study the gauge invariance of the decay rate of the false vacuum for the model in which the scalar field responsible for the false vacuum decay has gauge quantum number. In order to calculate the decay rate, one should integrate out the field fluctuations around the classical path connecting the false and true vacua (i.e., so-called bounce). Concentrating on the case where the gauge symmetry is broken in the false vacuum, we show a systematic way to perform such an integration and present a manifestly gauge-invariant formula of the decay rate of the false vacuum.Comment: 17 pages, published versio

Dilaton Destabilization at High Temperature

Many compactifications of higher-dimensional supersymmetric theories have approximate vacuum degeneracy. The associated moduli fields are stabilized by non-perturbative effects which break supersymmetry. We show that at finite temperature the effective potential of the dilaton acquires a negative linear term. This destabilizes all moduli fields at sufficiently high temperature. We compute the corresponding critical temperature which is determined by the scale of supersymmetry breaking, the beta-function associated with gaugino condensation and the curvature of the K"ahler potential, T_crit ~ (m_3/2 M_P)^(1/2) (3/\beta)^(3/4) (K'')^(-1/4). For realistic models we find T_crit ~ 10^11-10^12 GeV, which provides an upper bound on the temperature of the early universe. In contrast to other cosmological constraints, this upper bound cannot be circumvented by late-time entropy production.Comment: 19 pages, 9 figure