arXiv subjects

Paolo Lipari

Publications and source records attributed to Paolo Lipari.

53 records · Page 3Linked to original sources

arXiv2001

CP violation effects and high energy neutrinos

This work discusses critically the prospects of measuring CP and T violation effects in high energy neutrino factories. For this purpose we develop, in the standard framework with three neutrino flavors, simple expressions for the oscillation probabilities in matter that are valid for high E_nu. All CP violating effects vanish as E_nu^(-3) and are very difficult to detect with high energy neutrinos. A significantly easier task is the determination of the absolute value |delta| of the phase in the neutrino mixing matrix that controls the CP and T violation effects, performing precision measurements of the CP and T conserving part of the oscillation probabilities.

hep-ph↗

arXiv2001

The fluxes of sub--cutoff particles detected by AMS, the cosmic ray albedo and atmospheric neutrinos

New measurements of the cosmic ray fluxes (p, e+, e- and Helium) performed by the Alpha Magnetic Spectrometer (AMS) during a ten days flight of space shuttle have revealed the existence of significant fluxes of particles below the geomagnetic cutoff. These fluxes exhibit a number of remarkable properties, such as a He-3/He-4 ratio of order ~10, an e+/e- ratio of order ~4 and production from well defined regions of the Earth that are distinct for positively and negatively charged particles. In this work we show that the natural hypothesis, that these subcutoff particles are generated as secondary products of primary cosmic ray interactions in the atmosphere can reproduce all the observed properties. We also discuss the implications of the subcutoff fluxes for the estimate of the atmospheric neutrino fluxes, and find that they represent a negligibly small correction. On the other hand the AMS results give important confirmations about the assumption of isotropy for the interplanetary cosmic ray fluxes also on large angular scales, and on the validity of the geomagnetic effects that are important elements for the prediction of the atmospheric neutrino fluxes.

astro-ph↗

arXiv2000

The east-west effect for atmospheric neutrinos

The atmospheric neutrino fluxes observed by underground detectors are not symmetric for rotations around the vertical axis. The flux is largest (smallest) for the nu's traveling toward east (west). This asymmetry is the result of the effects of the geomagnetic field on the primary cosmic rays and on their showers. The size of the asymmetry is to a good approximation independent from the existence of neutrino oscillations, and can in principle be predicted without any ambiguity due to existence of unknown new physics. We show that there is an interesting hint of discrepancy between the experimental measurement of the east-west asymmetry by Super-Kamiokande and the existing theoretical predictions. We argue that the discrepancy is a real effect caused by the neglect, in the existing calculations, of the bending of the charged secondary particles (in particular of the muons) in the cosmic ray showers. The inclusion of this effect in the calculation gives results in quantitative agreeement with the data. We comment on the implications of this result for the study of neutrino oscillations.

hep-ph↗

arXiv2000

The geometry of atmospheric neutrino production

The zenith angle distributions of atmospheric neutrinos are determined by the possible presence of neutrino oscillations and the combination of three most important contributions: (1) geomagnetic effects on the primary cosmic rays, that suppress the primary flux in the Earth's magnetic equatorial region, (2) the zenith angle dependence of the neutrino yields, due to the fact that inclined showers produce more neutrinos, and (3) geometrical effects due to the spherical shell geometry of the neutrino production volume. The last effect has been recognized only recently and results in an important enhancement of the flux of sub--GeV neutrinos for horizontal directions. In this work we discuss the geometrical effect and its relevance in the interpretation of the atmospheric neutrino data.

hep-ph↗

arXiv1999

The $ν_μ\leftrightarrow ν_s$ interpretation of the atmospheric neutrino data and cosmological constraints

The data on atmospheric neutrinos can be explained assuming the existence of oscillations between muon neutrinos and light sterile neutrinos with mixing close to maximal, and Delta_m^2 approximately equal to 3 E-3 (eV^2). This interpretation of the data is in potential conflict with the successes of big bang nucleosynthesis (BBN), since oscillations can result in a too large contribution of the sterile state to the energy density of the universe at the epoch of nucleosynthesis. The possibility to evade these cosmological constraints has been recently the object of some controversy. In this work we rediscuss this problem and find that the inclusion of a small mixing of the sterile state with tau neutrino can result in the generation of a large lepton asymmetry that strongly suppress the muon-sterile neutrino oscillations eliminating the possible conflict with BBN bounds. In this scheme the mass of the tau neutrino must be larger than few eV's and is compatible with cosmological bounds. Our calculation is performed using a Pauli-Boltzmann method. In this approach it is also possible to develop analytic calculations that allow physical insight in the processes considered and give support to the numerical results.

hep-ph↗

arXiv1999

The primary protons and the atmospheric neutrino fluxes

The predictions of the atmospheric nu event rates are affected by significant uncertainties, however the evidence for the `disappearance' of nu_mu's and antinu_mu's obtained by SK and other underground detectors is robust and cannot be accounted in the framework of the minimum standard model without assuming very large ad hoc experimental systematic effects. The existence of `new physics' beyond the standard model is therefore close to be established; neutrino oscillations provide a very good fit to all data. The theoretical uncertainties do have an important role in the detailed interpretation of the data, and in the estimate of oscillation parameters.

hep-ph↗

arXiv1999

Phenomenology of atmospheric neutrinos

The relevance of the data concerning upward--going muons for the solution of the atmospheric neutrino problem is stressed. In particular, their inclusion in the analysis confirms the goodness of the neutrino oscillation hypothesis and allows to exclude some alternative, exotic explanations such as neutrino decay, flavour changing neutral currents, violations of the equivalence principle (at least in their simplest forms), and also to discriminate, in principle, between different neutrino oscillation models (nu_mu <-> nu_tau versus nu_mu <-> nu_s), because of the difference in the matter effects.

hep-ph↗

arXiv1999

Atmospheric neutrinos: phenomenological summary and outlook

The predictions of the atmospheric nu event rates are affected by significant uncertainties, however the evidence for the `disappearance' of nu_mu's and nubar_mu's obtained by SK (and other underground detectors) is robust and cannot be accounted in the framework of the minimum standard model without assuming very large ad hoc experimental systematic effects. The existence of `new physics' beyond the standard model is therefore close to be established; neutrino oscillations provide a very good fit to all data. The theoretical uncertainties do have an important role in the detailed interpretation of the data, and in the estimate of oscillation parameters.

hep-ph↗

arXiv1999

Matter effects in long baseline experiments, the flavor content of the heaviest (or lightest) neutrino and the sign of Delta m^2

The neutrinos of long baseline beams travel inside the Earth's crust where the density is approximately rho = 2.8 g cm^-3. If electron neutrinos participate in the oscillations, matter effects will modify the oscillation probabilities with respect to the vacuum case. Depending on the sign of Delta m^2 an MSW resonance will exist for neutrinos or anti-neutrinos with energy approximately E_nu(res) = 4.7 |Δm^2|/(10^-3 eV^2) GeV. For Delta m^2 in the interval indicated by the Super-Kamiokande experiment this energy range is important for the proposed long baseline experiments. For positive Delta m^2 the most important effects of matter are a 9% (25%) enhancement of the transition probability P(nu_mu -> nu_e) for the KEK to Kamioka (Fermilab to Minos and CERN to Gran Sasso) beam(s) in the energy region where the probability has its first maximum, and an approximately equal suppression of P(antinu_mu -> antinu_e). For negative Delta m^2 the effects for neutrinos and anti-neutrinos are interchanged. Producing beams of neutrinos and antineutrinos and measuring the oscillation probabilities for both (nu_mu -> nu_e) and (antinu_mu -> antinu_e) transitions can solve the sign ambiguity in the determination of Delta m^2.

hep-ph↗

arXiv1999

On Exotic Solutions of the Atmospheric Neutrino Problem

The measurements of the fluxes of atmospheric neutrinos give evidence for the disappearance of muon neutrinos. The determination of the dependence of the disappearance probability on the neutrino energy and trajectory allows in principle to establish unambiguosly the existence of neutrino oscillations. Alternative mechanisms for the disappearance of the neutrinos have been proposed, but do not provide a viable description of the data, if one includes both events where the neutrinos interact in the detector and $ν$-induced upward going muons. The proposed mechanisms differ in the energy dependence of the disappearance probability and the upward going muon data that are produced by high energy neutrinos give a crucial constraint.

hep-ph↗

arXiv1998

Atmospheric neutrinos, long-baseline neutrino beams and the precise measurement of the neutrino oscillation parameters

Measurements of atmospheric neutrinos by Super-Kamiokande an other detectors have given evidence for the existence of neutrino oscillations with large mixing and $Δm^2$ in the range 10^{-3}-10^{-2} eV^2. In this work we discuss critically some of the possible experimental strategies to confirm this result and determine more accurately the neutrino oscillation parameters. A possible method is the development of long-baseline accelerator neutrino beams. The accelerator beams can have higher intensity and higher average energy than the atmospheric flux, and if (ν_μ\leftrightarrow ν_τ) oscillations are indeed the cause of the atmospheric neutrino anomaly, they can produce a measurable rate of $τ$ leptons for most (but not all) of the values of the oscillation parameters that are a solution to the atmospheric data. On the other hand measurements of atmospheric neutrinos with large statistics and/or better experimental resolutions, can also provide convincing evidence for oscillations, thanks to unambiguous detectable effects on the energy, zenith angle and L/E_νdistributions of the events. The study of these effects can provide a precise determination of the oscillations parameters. The range of L/E_νavailable for atmospheric neutrinos is much larger than in long-baseline accelerator experiments, and the sensitivity extends to lower values of Δm^2.

hep-ph↗

arXiv1998

Comparison of ν_μ<-> ν_τand and ν_μ<-> ν_s oscillations as solutions of the atmospheric neutrino problem

The simplest explanation for the atmospheric neutrino anomaly is the transition of muon neutrinos and antineutrinos into states of different flavor. Data from the reactor experiment Chooz, and the measurement of the energy and angle distribution of electron--like events in Super--Kamiokande put strong constraints on $ν_μ$--$ν_e$ mixing, but the transitions $ν_μ\to ν_τ$ and $ν_μ\to ν_s$ are viable possibilites. If the muon neutrinos are mixed with a singlet sterile state, the energy and zenith angle dependence of the oscillation probability is modified by matter effects, that are important for $E_ν/|Δm^2| \aprge 10^3$ GeV/eV$^2$. We confront the theoretical predictions with existing data on contained events and neutrino--induced upward going muons.

hep-ph↗

arXiv1998

Geomagnetic Effects on Atmospheric Neutrinos

Geomagnetic effects distort the zenith angle distribution of sub--GeV and few--GeV atmospheric neutrinos, breaking the up--down symmetry that would be present in the absence of neutrino oscillations and without a geomagnetic field. The geomagnetic effects also produce a characteristic azimuthal dependence of the $ν$--fluxes, related to the well known east--west effect, that should be detectable in neutrino experiments of sufficiently large mass. We discuss these effects quantitatively. Because the azimuthal dependence is in first order independent of any oscillation effect, it is a useful diagnostic tool for studying possible systematic effects in the search for neutrino oscillations.

astro-ph↗

arXiv1998

Proton-proton Cross Section at sqrt(s) \sim 30 TeV

There are both theoretical and experimental uncertainties in using data from cosmic-ray air showers to estimate hadronic cross sections above accelerator energies. We outline these problems and compare the physics used to extract σ_{pp}^{tot} from air shower data to the widely used parameterization of the proton-proton cross section of Donnachie and Landshoff and other contemporary models. We conclude that the published cosmic-ray cross section values do not strongly constrain σ_{pp}^{tot} fits from lower energy accelerator data.

hep-ph↗

arXiv1997

The Energy and Zenith Angle Distribution of Upward Going Muons and Neutrino Oscillations

The energy and zenith angle distribution of neutrino induced, upward going muons can give direct information on the presence of $ν$--oscillations in precisely the range of parameters suggested as a solution of the atmospheric neutrino problem. We discuss here the uncertainties in the theoretical prediction. The shape of the zenith angle distribution of the muon flux is quite insensitive to modifications of the theoretical input and is a good probe for the existence of neutrino oscillations. We conclude that the existing data sample on $ν$--induced muons has the statistical power to confirm or refute the $ν$--oscillation solution of the atmospheric neutrino problem.

hep-ph↗

arXiv1995

Atmospheric neutrino flux above 1 GeV

In this paper we extend an earlier calculation of the flux of atmospheric neutrinos to higher energy. The earlier calculation of the neutrino flux below 3 GeV has been used for calculation of the rate of contained neutrino interactions in deep underground detectors. The fluxes are needed up to neutrino energies of 10 TeV to calculate the expected rate of neutrino-induced muons passing into and through large, deep detectors. We compare our results with several other calculations, and we evaluate the uncertainty in the rate of neutrino-induced muons due to uncertainties in the neutrino flux.

hep-ph↗

arXiv1994

The neutrino cross section and upward going muons

The charged current cross section for neutrinos with energy of a few GeV is reanalysed. In this energy range the cross section for the lowest multiplicity exclusive channels is an important fraction of $σ_{CC}$ and the approximation of describing the cross section with deep inelastic scattering formulae may be inaccurate. Possible consequences of our reanalysis of the cross section in the interpretation of the data obtained by deep underground detectors on $ν$--induced upward going muons (both stopping and passing) are discussed.

hep-ph↗