Graciela Gelmini

Phenomenology of supersymmetry with broken R-parity

Abstract In some phenomenological supersymmetric models R -parity (+1 for particles, −1 for sparticles) is spontaneously broken along with tau-lepton number L τ by a vacuum expectation value υ τ of the tau sneutrino ν τ . To avoid excess stellar energy loss through majorons, there should also be explicit L τ violation through right-handed neutrinos. To have a sufficiently light ν τ , either υ τ is very small which is unnatural and boring, and/or the Higgs mixing parameter ϵ is very small. We find that in the limit ϵ → 0: -both the forward-backward asymmetry in e + e − → τ + τ − and the τ lifetime are unchanged, -Z 0 → ggν⊥ decays are possible where ν τ is an extra neutrino, -squarks and gluinos may decay into τ or ν τ , -the photino γg can decay into ν⊥ff with a detectable secondary vertex, -single production of ( R -odd) sparticles may occur.

Fast invisible neutrino decays

Abstract Any value for the lifetime of neutrinos decaying into invisible modes (another neutrino and a Goldstone boson) is possible in theories in which the lepton number is a spontaneously broken global symmetry with different charges for every family. There are two types of models. Only in one of them neutrinos could, in a natural way, be relevant for cosmology.

GZK photons as ultra-high-energy cosmic rays

We calculate the flux of “GZK photons,” namely, the flux of ultra-high-energy cosmic rays (UHECRs) consisting of photons produced by extragalactic nucleons through the resonant photoproduction of pions, the so-called GZK effect. We show that for primary nucleons, the GZK-photon fraction of the total UHECR flux is between 10−4 and 10–2 above 1019 eV and up to the order of 0.1 above 1020 eV. The GZK-photon flux depends on the assumed UHECR spectrum, the slope of the nucleon flux at the source, and the distribution of sources and intervening backgrounds. Detection of this photon flux would open the way for UHECR gamma-ray astronomy. Detection of a larger photon flux would imply the emission of photons at the source or new physics. We compare the photon fractions expected for GZK photons and the minimal fractions predicted by top-down models. We find that the photon fraction above 1019 eV is a crucial test for top-down models.

MeV sterile neutrinos in low reheating temperature cosmological scenarios

It is commonly assumed that the cosmological and astrophysical bounds on the mixings of sterile with active neutrinos are much more stringent than those obtained from laboratory measurements. We point out that in scenarios with a very low reheating temperature MeV at the end of (the last episode of) inflation or entropy creation, the abundance of sterile neutrinos becomes greatly suppressed with respect to that obtained within the standard framework. Thus, in this case cosmological bounds become much less stringent than usually assumed, allowing sterile neutrinos to be visible in future experiments. Here, we concentrate on massive (mostly sterile) neutrinos with masses ms>1 MeV for TRH≤ms.

Majorons: A Simultaneous Solution to the Large and Small Scale Dark Matter Problems

Abstract It is shown that the existence of majorons, which enable a heavy neutrino, 500 eV ≲ mνH ≲ 25 keV to decay into a light neutrino mνL ≲ 8 eV and a majoron, with lifetime 104 yr ≲ τνH ≲ 108 yr can solve both the large and small scale dark matter problems. For a primordial “Zeldovich” spectrum of fluctuations the limits are m v H ≲ 550 eV and τ v H > 107 to 108 yr (the ranges mνH ≲ eV and τνH ≳ 108 yr are allowed by the model but galaxy formation becomes problematic). The large scale dark matter problem is how to achieve the critical density as implied by inflation, the small scale problems deal with the halos of galaxies and galaxy formation and perturbation growth. The heavy neutrino could provide the solution to the small scale problem by initiating perturbation growth before decoupling. The decay products will be fast and thus not bound to the initial clumps, thus solving the large scale problem. The low mass relic neutrinos that were not decay products would remain bound in the gravitational potentials which grew from the initial perturbations. The resulting universe would be radiation dominated, which is consistent with present observations if H0 ≲ 40 km/s/Mpc. An alternative solution can occur when mνH ≈ 10 eV: the universe can again become matter dominated in the present epoch. This solution still allows H0 ∼ 50 km/s/Mpc. The majoron model parameters which best fit the dark matter considerations are presented.

Squark decays into gauginos at the pp collider

Conventional analyses of missing pT events due to squark production at the CERN pp collider assume q → qγ decays dominate. In principle, the monojet and dijet cross sections could be reduced by competition from q → qW± and q → qZ0 decays. We compute this reduction factor for two mass scenarios: mq > mW > mol and ml ≈ mq > mW. The monojet and dijet cross sections for squarks light enough to be observed in present collider experiments are reduced by no more than 55%, while there may exist an observable cross section for jet(s) + charged lepton(s) + missing pT events. Thus the lower bounds on mq usually derived from q → qγ decays remain valid.

Towards a supersymmetric cosmology

Abstract We seek a supersymmetric GUT which has simultaneously the following desirable cosmological and particle physics properties: cosmological baryosynthesis, inflation, suppression of the five-dimensional operators leading to baryon decay and natural splitting of the GUT and weak interaction scales. We demonstrate the difficulties in reconciling these requirements in most GUT models, in particular in supergravity models with the gravitino mass of order mw, for which a low reheating temperature at the end of the inflationary epoch is required. We then show that it is possible to overcome the above difficulties in models where m 3 2 ⪡ m w or m 3 2 ⪢ m w . We present an example of this strategy with a novel and natural missing partner mechanism, show that the baryon lifetime is sufficiently long and estimate the amount of matter-anitmatter asymmetry in such a model.

Finite temperature effects in primordial inflation

Abstract We present a detailed study of a recently proposed model for primordial inflation based on an N =1 locally supersymmetric potential. For a large class of parameters with which all cosmological constraints are satisfied, the temperature corrections can be neglected during the inflation period. At higher temperatures, the minimum is not at the origin, but very close to it.

Fermion mass hierarchy and global horizontal symmetries

We present a mechanism for quark mass generation in zeroth order using induced representations rather than the minimization of the horizontal potential. Using a simplicity criterion, we derive a realistic mass matrix. We also discuss a possible application of the mechanism to various models.

Primordial inflation with flat supergravity potentials

A new class of primordial inflation models (PRIMO) is presented, based upon the possibility of getting naturally flat potentials in N = 1 supergravity. These models may be considered as the natural cosmological extension of some recently proposed no-scale particle physics models, where all mass scales are derived from the Planck mass (Mp). PRIMO seem to satisfy all presently known cosmological constraints and they are guaranteed (a) to be void of SUSY minima with negative cosmological constant, (b) of a stable (around the origin) potential form against finite temperature effects.