Gravitino Abundance Research Articles

Gravitino dark matter without R-parity

Cosmological issues are examined when the gravitino is the lightest superparticle (LSP) and R-parity is broken. Decays of the next lightest superparticles occur rapidly via R-parity violating interaction, and thus they do not upset the big-bang nucleosynthesis, unlike the R-parity conserving case. The gravitino LSP becomes unstable, but its lifetime is typically much longer than the age of the Universe. It turns out that observations of the diffuse photon background coming from radiative decays of the gravitino do not severely constrain the gravitino abundance, and thus a gravitino weighing less than around 1 GeV can be the dark matter of the Universe when bilinear R-parity violation generates a neutrino mass which accounts for the atmospheric neutrino anomaly.

Leptogenesis in an inflationary universe

We investigate the leptogenesis via decays of heavy Majorana neutrinos which are produced non-thermally in inflaton decays. We make a comprehensive study on the leptogenesis assuming various supersymmetric (SUSY) models for hybrid, new and topological inflations. For an estimation of the lepton asymmetry we adopt the Froggatt-Nielsen mechanism for mass matrices of quarks and leptons. We find that all of these models are successful to produce the lepton asymmetry enough to explain the baryon number in the present universe. Here we impose low reheating temperatures such as $T_R \lesssim 10^8$ GeV in order to suppress the abundance of gravitinos not to conflict with the big-bang nucleosynthesis. Furthermore, we find that the leptogenesis works very well even with $T_R \simeq 10^{6}$ GeV in the SUSY hybrid or new inflation model. It is known that such a reheating temperature is low enough to suppress the abundance of gravitinos of mass $m_{3/2} \simeq 100$ GeV--1 TeV. Thus, the leptogenesis is fully consistent with the big-bang nucleosynthesis in a wide region of the gravitino mass.

The abundance of moduli, modulini and gravitinos produced by the vacuum fluctuation

Moduli, modulini and the gravitino have gravitational-strength interactions, and thermal collisions after reheating create all of them with roughly the same abundance. With their mass of order 100 GeV , corresponding to gravity-mediated supersymmetry breaking, this leads to the well-known bound γT R ≲10 9 GeV on the reheat temperature, where γ≤1 is the entropy dilution factor. The vacuum fluctuation also creates these particles, with abundance determined by the solution of the equation for the mode function. Taking the equation in each case to be the one corresponding to a free field, we consider carefully the behaviour of the effective mass during the crucial era after inflation. It may have a rapid oscillation, which does not however affect the particle abundance. Existing estimates are confirmed; the abundance of modulini and (probably) of moduli created from the vacuum is less than from thermal collisions, but the abundance of gravitinos may be much bigger, leading to a tighter bound on T R if supersymmetry breaking is gravity-mediated.

Matter–antimatter asymmetry and neutrino properties

The cosmological baryon asymmetry can be explained as remnant of heavy Majorana neutrino decays in the early universe. We study this mechanism for two models of neutrino masses with a large ν μ − ν τ mixing angle which are based on the symmetries SU(5)× U(1) F and SU(3) c × SU(3) L × SU(3) R × U(1) F , respectively. In both cases B− L is broken at the unification scale Λ GUT. The models make different predictions for the baryogenesis temperature and the gravitino abundance.

Preheating of fermions

In inflationary cosmology, the particles constituting the Universe are created after inflation in the process of reheating due to their interaction with the oscillating inflaton field. In the bosonic sector, the leading channel of particle production is the non-perturbative regime of parametric resonance, preheating, during which bosons are created exponentially fast. Pauli blocking prohibits the unbounded creation of fermions. For this reason, it has been silently assumed that the creation of fermions can be treated with perturbation theory for the decay of individual inflatons. We consider the production of fermions interacting with the coherently oscillating inflatons. We find that the actual particle production occurs in a regime of the parametric excitation of fermions, leading to preheating of fermions. Fermion preheating differs significantly from the perturbative expectation. It turns out that the number density of fermions varies periodically with time. The total number of fermions quickly saturates to an average value within a broad range of momenta ∝ q 1/4, where q is the usual resonance parameter. The resonant excitation of fermions may affect the transfer of inflaton energy, estimations of the reheating temperature, and the abundance of superheavy fermions and gravitinos. Back in the bosonic sector, outside of the parametric resonance bands there is an additional effect of parametric excitation of bosons with bounded occupation number in the momentum range ∝ q 1/4.

Particle production and gravitino abundance after inflation.

Thermal history after inflation is studied in a chaotic inflation model with supersymmetric couplings of the inflaton to matter fields. Time evolution equation is solved in a formalism that incorporates both the back reaction of particle production and the cosmological expansion. The effect of the parametric resonance gives rise to a rapid initial phase of the inflaton decay followed by a slow stage of the Born term decay. Thermalization takes place immediately after the first explosive stage for a medium strength of the coupling among created particles. As an application we calculate time evolution of the gravitino abundance that is produced by ordinary particles directly created from the inflaton decay, which typically results in much more enhanced yield than what a naive estimate based on the Born term would suggest.

The heavy polonyi field as a solution of the polonyi problem

We study properties of the Polonyi field in the renormalizable hidden sector in supergravity models. It is shown that radiative corrections induce a sizeable coupling of the Polonyi field to gravitinos as well as raise its mass to an intermediate scale. We find that it provides a solution of the Polonyi problem and show that the gravitino abundance produced by the Polonyi decay is not large. It is also pointed out that soft supersymmetry breaking terms induced by this hidden sector are very restricted.

The R-axion from dynamical supersymmetry breaking

All generic, calculable models of dynamical supersymmetry breaking have a spontaneously broken R-symmetry and therefore contain an R-axion. We showthat the axion is massive in any model in which the cosmological constant is fine-tuned to zero through an explicit R-symmetry-breaking c constant. In visible-sector models, the axion mass is in the 100 MeV range and thus evades astrophysical bounds. In nonrenormalizable hidden-sector models, the mass is of order of the weak scale and can have dangerous cosmological consequences similar to those already present from other fields. In renormalizable hidden-sector models, the axion mass is generally quite large, of order 10 7 GeV. Typically, these axions are cosmologically safe. However, if the dominant decay mode is to gravitinos, the potentially large gravitino abundance that arises from axion decay after inflation might affect the successful predictions of big-bang nucleosynthesis. We show that the upper bound on the reheat temperature after standard inflation can be competitive with or stronger than bounds from thermal gravitino production, depending on the model and the gravitino mass.

Antinucleons from decaying gravitinos in supersymmetric cosmology

The influence of antinucleons from hadronically decaying gravitinos of mass 300 GeV<m G <30 TeV on cosmological nucleo-synthesis processes is analyzed by including annihilation reactions into the balance equations of light element abundances. Requiring to stay inside the atrophysically allowed regions yields under conservative assumptions an upper boundary on the gravitino abundance corresponding in supersymmetric inflationary cosmology to a minimum reheating temperature of the universe of 10 9−11 GeV depending on m G , which improves earlier limits by five to three orders of magnitude in this mass range. Other bounds on the gravitino abundance like photofission processes are shortly rediscussed for the hadronic decay mode, and 3He, d overproduction is shown to set the limit for 20 GeV<m G <300 GeV at temperatures of 10 8−10 GeV. Such low temperatures values are hardly to reconcile within minimal N=1 supergravity models.

Diffuse cosmic gamma-ray background as a probe of cosmological gravitino regeneration and decay.

We predict the presence of a spectral feature in the isotropic cosmic gamma-ray background associated with gravitino decays at high red shifts. With a gravitino abundance that falls in the relatively narrow range expected for thermally regenerated gravitinos following an inflationary epoch in the very early universe, gravitinos of mass several gigaelectronvolts are found to yield an appreciable flux of 1---10-MeV diffuse gamma rays.

Baryogenesis in supergravity inflationary models

Realistic N=1 supergravity theories with a gravitino mass of order 1 TeV require a period of inflation to dilute the gravitino abundance. Moreover, if the gravitino is unstable the reheat temperature is bounded to be no greater than O(10 8 GeV). We show that such models may still have acceptable rates of baryosynthesis and discuss possible mechanisms.

Constraints on cosmologically regenerated gravitinos

Cosmological constraints on the regenerated abundance of gravitinos as a function of the maximum reheating temperature T h are examined. We combine the bound that arises in order for stable photinos produced by gravitino decays to not overdominate the mass density of the universe together with that inferred from the distortions produced in the cosmic microwave background radiation as well as constraints from the observed deuterium and 3He abundances. This suffices to constrain T h to be below between 10 17 and 10 10 GeV for gravitions in the mass below ∼ 10 TeV. More massive gravitinos, however, could be produced at T h<10 14 GeV.

Cosmological gravitino regeneration and decay

We present a detailed study of gravitino production and decay subsequent to cosmological inflation. We calculate the cross sections for gravitino production in collisions of particles in the supersymmetric standard model, and use them to calculate the regenerated abundance of gravitinos as a function of the maximum reheating temperature T max. An upper limit on the gravitino mass density during cosmological nucleosynthesis requires T max <0.90 × 10 16 GeV and considerations of the entropy released when gravitinos decay require T max <2.2 × 10 13 GeV, while more careful analyses of their decay products' disruptive effects on light nuclei and on the microwave background radiation suggest T max < 10 9 − 10 10 GeV.

Supersymmetric inflationary cosmology

We discuss a supersymmetric inflationary cosmological scenario in which inflation takes place after GUT breaking but before SUSY breaking. The framework is within N = 1 supergravity with the inflation and SUSY breaking sectors hidden from one another. We present a specific one-parameter inflation sector coupled with F-type SUSY breaking, with more than sufficient entropy release. Adequate baryon asymmetry is produced by an out-of-equilibrium decay mechanism in spite of a low reheat temperature. The condition that the gravitino abundance should not affect nucleosynthesis fixes the single parameter and the resultant density flucctuation to be of O(10 −4). If gravitinos are stable their resulting abundance is sufficient to account for the dark matter.

Is it easy to save the gravitino?

New constraints on the cosmological gravitino abundance are discussed. These constraints lead to some rather stringent constraints on the structure of the theories in which the primordial gravitino problem can be solved.