S. Bertolini

Effects of supergravity-induced electroweak breaking on rare b-decays and mixings

We perform a complete computation of the contributions to b → s + γ, b → s/+/−, b → sνν, b → s + g, Bdo−Bdo and Bso−Bso in minimal spontaneously broken N = 1 supergravity with radiative breaking of the electroweak symmetry group. We compare our results with previous evaluations in non-minimal and minimal supersymmetric models, where the constraint of the SU(2)L × U(1)Y radiative breaking was not imposed. The possibilities of disentangling supersymmetry from the Standard Model in the experimental searches for the abovementioned rare processes are discussed.

New constraints on squark and gluino masses from radiative b decays

Abstract We show on general grounds that radiative mesonic decays, mediated through penguin-like diagrams, are expected to exhibit a significant enhancement from the exchange of supersymmetric particles. An interesting example is the decay b→sγ. An experimental bound BR(B→K ∗ γ) ⩽ 10 −3 is presently available. It is however unclear whether K ∗ γ represents the dominant mode for the inclusive b→sγ decay. We show that an upper bound BR(b→sγ) −3 leads, for m t ⋍ 45–80 GeV, to bounds on squarks and gluino masses comparable to the one coming from the analysis of the monojet events at the CERN pp collider.

Supersymmetric enhancement of non-charmed B-decays

Abstract We study the non-charmed strange decays of the B meson in the context of low-energy minimal N = 1 supergravity. We find that the inclusive channel b → s gluon is greatly enhanced with respect to the standard model prediction. Indeed, for gluino and squark masses of 60–80 GeV and a top mass between 50 and 80 GeV, a branching ratio as high as 10–30% can be reached. This opens a new window for the possibility of testing the presence of new physics in low-energy phenomenology. A comparative review of the standard model results is presented.

The doublet majoron model and solar neutrino oscillations

Abstract We present a minimal extension of the standard electroweak theory which, as a consequence of the spontaneous breaking of lepton number and the radiative origin of the neutrino mass, offers a natural framework for the solution of the solar neutrino problem through matter-enhanced neutrino oscillations (Mikheyev-Smirnov-Wolfenstein mechanism). Indeed, we show that the presently available astrophysical bounds on the lepton-breaking vacuum expectation value naturally lead to neutrino masses in the required regime. The fact that the Majoron belongs to an SU(2)L doublet and not a triplet has relevant phenomenological implications. In particular, the scalar contribution to the Z0 width is four times smaller than in the triplet model and equivalent to 1 2 a neutrino-antineutrino mode. Relevant effects related to the presence of two physical singly charged scalars, both at the quantum and tree level, are studied. As a result we find that the model is tightly constrained by present data. In particular, for a wide range of parameters, the decay μ → eγ is within two orders of magnitude from the present experimental limit.

B0 − B̄0 mixing and related CP violation in supersymmetric models

Abstract We analyze the supersymmetric contributions to B d 0 − B d 0 mixing. For squark and gluino masses which respect the bounds coming from the CERN pp collider, the supersymmetric contribution turns out to be substantially smaller than the corresponding standard model prediction. However, we suggest that the presence of supersymmetry may sizeably contribute to the observability of large CP asymmetries related to the mixing.

Astrophysical bound on the majoron-Higgs-boson coupling

Abstract We show that the coupling of the “standard” Higgs boson to majorons, that could lead to a very fast decay of the neutral Higgs scalar to invisible modes, can be bounded using astrophysical arguments. We discuss the relevance of this bound for low-energy phenomenology related to majoron production. The bound so obtained may also jeopardize the stability of the VEV hierarchy in the doublet and triplet majoron models if the mass of the top quark is less than the W mass. A similar analysis may be applied to other models which exhibit Goldstone-or pseudo-Goldstone-bosons in the spectrum.

Stability of the VEV hierarchy and Higgs boson invisibility in majoron models

Abstract We study the stability of the lepton-number breaking VEV under radiative corrections in the doublet and triplet majoron models, including the effects of a heavy top quark. We find that it is possible to maintain the hierarchy between the VEVs, at the one-loop level, by fine tuning the ratio of two coupling constants in the Higgs potential. For a top quark heavier than 50–60 GeV the required relation depends strongly on the top mass. We use this relation to show that the decay mode of the neutral Higgs boson to majorons may be the dominant one for a wide range of the Higgs mass, thus making its detection, in this class of models, more problematic.

Small Dirac neutrino mass and left-right symmetry

Abstract We analyze the possibility of generating light Dirac neutrinos at the tree level in a left-right symmetric scenario. We present a minimal extension of the standard SU(2) L × SU(2) R × U(1) Y′ model where the above result is achieved through a “see-saw” like mechanism induced by the minimization of the Higgs potential. The Dirac neutrinos thus obtained are naturally light; indeed we show that the scheme is stable under radiative corrections. The neutrino mass is inversely related to the scale of parity breaking, which may naturally be in the TeV range, leading to new phenomenology in an interesting energy domain.

Supersymmetric contributions to b → s + H

Abstract We analyze the conditions for the presence of light neutral Higgs scalars in a class of low-energy N = 1 supergravity models where the SU(2) L × U(1) gauge-symmetry breaking is induced by renormalization effects. We study the implications on the mass of such light scalars arising from their possible emission in the flavor-changing transition b → s + H. Gluino and chargino exchange contributions are considered and shown to be sizeably large. It is argued that a lower bound of about 4 GeV on the scalar mass may be inferred from the present experimental data, barring contrived choices of the relevant supersymmetric parameters.

Majoron effects in rare kaon decays

Abstract We analyze, in the framework of the recently introduced doublet majoron model, the contribution from the emission of a pair of light scalars to the decay K + → π + + nothing . We find that, for reasonable choices of the parameters, the new scalar contribution may be as large as one additional neutrino-antineutrino mode and provide a substantial modification of the pion spectrum. The effect may be a few times larger in the triplet majoron model.