arXiv subjects

Francesco Vissani

Publications and source records attributed to Francesco Vissani.

At least 109 records · Page 6Linked to original sources

arXiv2001

Indirect Bounds on Z-> mu e and Lepton Flavor Violation at Future Colliders

Motivated by the interest in lepton number violating processes, we study the connection between the rate of Z -> mu e decay and those of the low-energy processes mu ->3 e, and mu -> e conversion in nuclear field. We show that if the vector or axial Z form factors are dominant, Br(Z->mu e) is not observable, while if the Z dipole form factors are dominant, the relatively weak indirect bound Br(Z->mu e)<7 10^{-9} does not fully preclude a signal at future colliders, as TESLA. We finally comment on the relation of Z->mu e with Z->tau e and Z->tau mu decays, and suggest a simple scaling law for these three processes.

hep-ph↗

arXiv2001

Expected Properties of Massive Neutrinos for Mass Matrices with a Dominant Block and Random Coefficients Order Unity

We study the class of neutrino mass matrices with a dominant block but unspecified O(1) coefficients, and scan the possible models by the help of random number generators. We discuss which are the most common expectations in dependence of the adjustable parameter of the mass matrices, "epsilon", and emphasise an interesting sub-class of models that have large mixing angles for atmospheric and solar neutrinos. For those models where the lepton mass matrices are subject to Froggatt-Nielsen U(1) selection rules, we show that the neutrino mixing matrix receives important contributions from the rotations operating on charged lepton sector, which increase the predicted value of the angle "theta(13)" and the ee-entry of the neutrino mass.

hep-ph↗

arXiv2001

Charm Production by Cosmic Muons

Narrow muon bundles in underground detectors permit to study muoproduction reactions that take place in the surrounding rock. We analyze the relevance of a QED+QCD reaction, muoproduction of "open charm". The contribution to double muon events is estimated to be 4-8 % of the one due to QED "trident" process, for an ideal detector located under a rock depth of 3 km water equivalent, and an observation threshold of 1 GeV.

hep-ph↗

arXiv2001

Is a Coherent Picture of Massive Neutrinos Emerging?

The hypothesis that neutrinos are massive has a strong experimental support; however, the information we have is quite limited, and many possibilities are open. Theoretical considerations might help fill the gaps (or foresee regularities), as we illustrate with a specific example.

hep-ph↗

arXiv2000

Non-Oscillation Searches of Neutrino Mass in the Age of Oscillations

We focus on the implications of the oscillations for the shape of nuclear beta-spectrum (=direct search for nu mass). This is of interest because of the existing bound, m_(nu_e)<2.2 eV, that could improve by one order of magnitude with future experiments. We stress important connections with the results of Liquid Scintillator Neutrino Detector, nu_e disappearance experiments, supernova neutrinos and neutrinoless double beta decay.

hep-ph↗

arXiv2000

Tevatron Signatures of an R-parity Violating Supersymmetric Theory

We show that an R-parity violating supersymmetric scenario which can account for the atmospheric nu(mu) deficit has testable signals at the Fermilab Tevatron with upgraded energy and luminosity. The explanation of neutrino masses and maximal nu(mu)-nu(tau) oscillation in terms of bilinear R-violating terms in the superpotential associates comparable numbers of muons and tau's resulting from decays of the lightest neutralino. We show that this should lead to like-sign dimuons and ditaus with substantial rates, in a form that separates them from standard model backgrounds and other signals of supersymmetry. One here also has the possibility of completely reconstructing the lightest neutralino.

hep-ph↗

arXiv2000

What is the Standard Model of Elementary Particles and Why We Have to Modify It

We introduce the standard model of elementary particles and discuss the reasons why we have to modify it. Emphasis is put on the indications from the neutrinos and on the role of the Higgs particle; some promising theoretical ideas, like quark-lepton symmetry, existence of super-heavy ``right-handed'' neutrinos, grand unification and supersymmetry at the weak scale, are introduced and shortly discussed.

hep-ph↗

arXiv1999

Proposal to look for an up/down asymmetry in atmospheric neutrinos beyond Multi-GeV region with existing experimental data

We discuss a possible test of neutrino oscillation hypothesis by proposing the combined analysis of high energy atmospheric neutrino induced muon events that have been detected around horizontal direction in the Kolar Gold Field (KGF) underground site and below the horizontal direction by many large detectors such as Super-Kamiokande and MACRO. Up/down asymmetry obtained using contained events recorded by detectors at Kamioka site probes low energy region of atmospheric neutrino whereas, the suggested method probes high energy neutrinos. It mainly depends on the observations and it is free of uncertainties in neutrino flux, interaction cross section etc. In this paper we demonstrate that the method is sensitive to a region of oscillation parameter space that explains all the features of atmospheric neutrino data in the Super-Kamiokande detector; the limiting factor being the statistical strength of the KGF observations. This method provides the only way to study the up/down asymmetry beyond Multi-GeV region which is yet to be measured experimentally.

hep-ph↗

arXiv1999

Neutrino spectrum, oscillation scenarios and neutrinoless double beta decay

We introduce the representation on one unitarity triangle of the constraints resulting (1) from the interpretation of solar and atmospheric neutrino data in terms of oscillations, and (2) from the search for neutrinoless double beta decay. We show its use for the study of a nearly degenerate neutrino spectrum. The representation shows clearly the particular cases when the neutrinoless double beta decay rate can (or cannot) be small, that is: when the connection of the decay rate with the neutrino spectrum is less (or more) direct. These cases turn out to depend crucially on the scenario of oscillation (MSW solutions, vacuum oscillations, averaged oscillations), and in particular on the size of the mixing between the electron neutrino and the neutrino state giving rise to atmospheric neutrino oscillations.

hep-ph↗

arXiv1999

Signal of neutrinoless double beta decay, neutrino spectrum and oscillation scenarios

The lower and upper bounds on the neutrinoless double beta (0nu-2beta) decay rate are obtained, as functions of the parameters of neutrino oscillations and of the lightest neutrino mass. The constraints on these parameters from the search for the 0nu-2beta transition, as well as from the interpretation of solar and atmospheric neutrino data in terms of oscillations, can be conveniently represented in one unitarity triangle. This representation helps to clarify the cases when the 0nu-2beta rate is small; the crucial dependence on the scenarios assumed for solar neutrino oscillations and on the neutrino spectrum is emphasized. We consider hierarchical and non-hierarchical neutrino spectra, and discuss their interest in view of future searches of the 0nu-2beta decay.

hep-ph↗

arXiv1998

Large mixing, family structure, and dominant block in the neutrino mass matrix

A possible connection between the flavour structure of the charged fermions and the large $ν_μ-ν_τ$ mixing motivates an ansatz for the neutrino mass matrix with a dominant block. We distinguish between a general form and the specific forms of the ansatz, and concentrate on the cases of phenomenological interest. The general form can incorporate an observable amount of CP violation in the leptonic sector. Only specific forms can incorporate the Mikheyev-Smirnov-Wolfenstein solutions for solar neutrinos, with small or large mixing angles. Other specific variants explain the Los Alamos neutrino anomaly, or provide a two-neutrino hot dark matter component.

hep-ph↗

arXiv1998

Correlation between neutrino oscillations and collider signals of supersymmetry in an R-parity violating model

Motivated by the recent Super-Kamiokande results on atmospheric neutrinos, we incorporate massive neutrinos, with large angle oscillation between the second and third generations, in a scenario with R-parity violating supersymmetry. We emphasize the testability of such models through the observation of comparable numbers of muons and taus, produced together with the W-boson, in decays of the lightest neutralino. A distinctly measurable decay gap is another remarkable feature of such a scenario.

hep-ph↗

arXiv1998

Neutral-to-charged current events ratio in atmospheric neutrinos and neutrino oscillations

The atmospheric neutrinos produce isolated neutral pions ($π^0$-events) mainly in the neutral current interactions. We propose to study the ratios involving $π^0$-events and the events induced mainly by the charged currents. This minimizes uncertainties related to the original neutrino fluxes, and in certain cases, also to the cross-sections. Experimental study of these ratios will allow one to check the oscillation solution of the atmospheric neutrino problem and to identify the channel of oscillations. Illustrative analysis of existing data is presented.

hep-ph↗

arXiv1997

On the CP asymmetries in Majorana neutrino decays

We study the CP asymmetries in lepton number violating two body scattering processes and show explicitly how they vanish, in agreement with unitarity constraints. We relate these cross section asymmetries to the CP decay rate asymmetries of the intermediate massive neutrinos and show how the inclusion of the Universe expansion via Boltzmann equations is the key ingredient to allow the production of a non-vanishing asymmetry in spite of the unitarity constraint on the cross sections. We then show that the absorptive parts of both the one loop vertex and self energy corrections do contribute to the CP decay asymmetries.

hep-ph↗

arXiv1997

Do experiments suggest a hierarchy problem?

The hierarchy problem of the scalar sector of the standard model is reformulated, emphasizing the role of experimental facts that may suggest the existence of a new physics large mass scale, for instance indications of the instability of the matter, or indications in favor of massive neutrinos. In the see-saw model for the neutrino masses a hierarchy problem arises if the mass of the right-handed neutrinos is larger than approximatively $10^7$ GeV: this problem, and its possible solutions, are discussed.

hep-ph↗

arXiv1997

A study of the scenario with nearly degenerate Majorana neutrinos

Motivated by cosmological considerations, and by the atmospheric and solar neutrino flux deficits, we consider the scenario in which the three Standard Model neutrinos are endowed with a nearly equal Majorana mass in the eV range. Combined constraints coming from (1) direct search for electron neutrino mass, (2) absence of neutrinoless double beta decay, and (3) unsuppressed electron neutrino flux in present reactor experiments, imply a quite well specified pattern of neutrino masses and mixing angles. We discuss the experimental tests of the model and comment on the features of this scenario.

hep-ph↗

arXiv1997

The $β$-spectrum in presence of background potentials for neutrinos

We compute the spectrum of $β$-decay, assuming that (Majorana or Dirac) neutrinos propagate in constant potentials. We study the modifications of the spectrum due to the effect of these potentials. Data on tritium decay and on ${}^3$H--${}^3$He mass difference allow us to infer bounds in the electronvolts range on the potentials.

hep-ph↗

arXiv1997

Finite temperature effects on CP violating asymmetries

We compute the CP violating decay asymmetries relevant for baryogenesis scenarios involving the out of equilibrium decays of heavy particles, including the finite temperature effects arising from the background of light thermal particles which are present during the decay epoch. Thermal effects can modify the size of CP violation by a sizeable fraction in the decay of scalar particles, but we find interesting cancellations in the thermal corrections affecting the asymmetries in the decays of fermions, as well as in the decay of scalars in supersymmetric theories. We also estimate the effects which arise from the motion of the decaying particles with respect to the background plasma.

hep-ph↗