arXiv subjects

Francesco Vissani

Publications and source records attributed to Francesco Vissani.

117 records · Page 7Linked to original sources

arXiv1996

The supersymmetric prediction for the muon transverse polarization in the $K^+ \leftarrow π^0 μ^+ ν_μ$ decay

The muon transverse polarization in the $K^+ \leftarrow π^0 μ^+ ν_μ$ decay will be measured at the $10^{-4}$ level in forthcoming experiments. We compare the phenomenological perspectives with the theoretical predictions in supersymmetric extensions of the standard model. In the minimal extension, CP-violating phases lead to a non-zero transverse polarization, that however is too small to account for a positive experimental signal. The problems that one encounters when departing from minimal assumptions are discussed. An observable effect is possible if the hypothesis of R-parity conservation is relaxed, but only at the price of assuming a very special pattern for the R-parity breaking couplings.

hep-ph↗

arXiv1996

R-parity breaking and the grand unification program

We present the study of the possibility to have R-parity breaking interactions in minimal SU(5). An interesting scenario emerges in which the R-parity breaking coupling $λ'_{333}$ is large, and its size is related to the value of the tau neutrino mass, assuming that supersymmetry is broken according to low energy supergravity framework. This scenario may also have implications on the issue of $b-τ$ unification.

hep-ph↗

arXiv1996

Long range neutrino forces and the lower bound on neutrino mass

Stellar objects (including our Sun, other stars of main sequence, white dwarfs, neutron stars etc.) contain strongly degenerate low energy sea of neutrinos (in neutron stars) or antineutrinos. The presence of this sea leads due to Pauli principle and thermal effects to effective blocking of the long-range neutrino forces. This blocking can resolve the problem of the unphysically large value of the self-energy of stars stipulated by many body long-range neutrino interactions. As a consequence no lower bound on the neutrino mass is inferred, in contrast with the statement by Fischbach.

hep-ph↗

arXiv1996

Large $R$-parity Violating Couplings and Grand Unification

We consider a possibility that $R$-parity violating interactions of particles which do not involve the first generation have large (up to 1) coupling constants, $Λ$. Such couplings, if exist, could have a number of phenomenological consequences: renormalization of $b-τ$ mass ratio, generation of $ν_τ$ mass in MeV region, {\em etc.}. In Grand Unified models, where $B$- and $L$-violating couplings appear simultaneously, the proton decay can be forbidden in virtue of hierarchical flavor structure of $Λ.$ However, due to Cabibbo-Kobayashi-Maskawa mixing this decay is induced already in one-loop. Present experimental data give the upper bound $Λ\ltap 10^{-8}$ (or $|λ' λ''| \ltap 7\cdot 10^{-16},$ on products of certain $L$- and $B$-violating coupling constants, in more general context). The bound can be avoided, if there is an asymmetry between the $L$- and $B$-violating couplings of usual matter fields. In the $SU(5)$ model the asymmetry can be related to the doublet-triplet splitting.

hep-ph↗

arXiv1996

$R$-Parity Breaking Phenomenology

We review various features of the $R$-parity breaking phenomenology, with particular attention to the low energy observables, and to the patterns of the $R$-parity breaking interactions that arise in Grand Unified models.

hep-ph↗

arXiv1996

Upper bound on all products of R-parity violating couplings $λ'$ and $λ''$ from proton decay

We prove that {\em any} product of R-parity violating couplings $λ'$ (L-violating) and $λ''$ (B-violating) can be strongly restricted by proton decay data. For any pair $λ'$ and $λ''$ the decay exists at least at one loop level. For squark masses below 1 TeV we find the conservative bounds $|λ' \cdot λ''| < 10^{-9}$ in absence of squark flavor mixing, and $|λ' \cdot λ''| < 10^{-11}$ when this mixing is taken into account. We study the dependence of the bounds on the flavor basis in which R-parity breaking couplings are determined.

hep-ph↗

arXiv1995

Low Energy Implications of Minimal Superstring Unification

We study the phenomenological implications of effective supergravities based on string vacua with spontaneously broken N=1 supersymmetry by dilaton and moduli $F$-terms. We further require Minimal String Unification, namely that large string threshold corrections ensure the correct unification of the gauge couplings at the grand unification scale. The whole supersymmetric mass spectrum turns out to be determined in terms of only two independent parameters, the dilaton-moduli mixing angle and the gravitino mass. In particular we discuss the region of the parameter space where at least one superpartner is ``visible" at LEP2. We find that the most likely candidates are the scalar partner of the right-handed electron and the lightest chargino, with interesting correlations between their masses and with the mass of the lightest higgs. We show how discovering SUSY particles at LEP2 might rather sharply discriminate between scenarios with pure dilaton SUSY breaking and mixed dilaton-moduli breaking.

hep-ph↗

arXiv1995

$(B+L)$-conserving Nucleon Decays in Supersymmetric Models

The presence of the $(B+L)$-conserving decay modes $n \to K^+ e^-,$ $n \to K^+ μ^-,$ $p \to K^+ e^- π^+$ and $p \to K^+ μ^- π^+$ is shown to be a characteristic feature of a class of models with explicit breaking of $R$-parity. These modes dominate over the $(B-L)$-conserving ones in certain regions of the parameter space; the impact of this scenario for nucleon decay search at the Super-Kamiokande is discussed.

hep-ph↗

arXiv1994

Supersymmetric predictions for the inclusive $b\to sγ$ decay

We study the penguin induced transition $b\to s\ γ$ in the minimal N=1 supersymmetric extension of the Standard Model with radiative breaking of the electroweak group. We include the effects of one-loop corrections to the Higgs potential and scalar masses. We show that the present upper and lower experimental limits on the inclusive decay sharply constrain the parameter space of the model in a wide range of $\tanβ$ values. The implications of the recently advocated relation $|B|\ge 2$ for the bilinear SUSY soft breaking parameter in grand unified theories are also analyzed.

hep-ph↗