Nuclear Physics with Electroweak Probes

In recent years, the italian theoretical Nuclear Physics community has played a leading role in the development of a unified approach, allowing for a consistent and fully quantitative description of the nuclear response to electromagnetic and weak probes. In this paper I review the main achievements in both fields, point out some of the open problems, and outline the most promising prospects.Comment: Invited Talk at the XII Workshop on Theoretical Nuclear Physics in Italy, Cortona, October 8-10, 200

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 |\Delta m^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.Comment: Latex, 28 pages, 12 postscript figure

Atmospheric neutrino flux from 3-dimensional simulation

The atmospheric muon and neutrino flux have been simulated using the same approach which successfully accounted for the recent secondary proton, electron and positron flux measurements in orbit by the AMS experiment. For the muon flux, a good agreement is obtained with the CAPRICE and HEAT data for altitudes ranging from sea level up to about 38 km. The general features of the calculated atmospheric neutrino flux are reported and discussed. The flux obtained at the Super-Kamiokande experiment location are reported and compared with other calculations. For low neutrino energies the flux obtained is significantly smaller than that used in the data analysis of underground experiment. The simulation results for the SOUDAN experiment site are also reported.Comment: 33 pages, 27 figures, 12 tables, final version for Phys. Rev.

The Unified Model & Evolution of Active Galaxies: Implications from a Spectropolarimetric Study

We extend the analysis presented in Tran (2001) of a spectropolarimetric survey of the CfA and 12micron samples of Seyfert 2 galaxies (S2s). We confirm that S2s with hidden broad line regions (HBLRs) tend to have hotter circumnuclear dust temperatures, show mid-IR spectra more characteristic of S1 galaxies, and are intrinsically more luminous than non-HBLR S2s. The level of obscuration and circumnuclear star formation, however, appear to be similar between HBLR and non-HBLR S2 galaxies, based on an examination of various observational indicators. HBLR S2s, on average, share many similar large-scale, presumably isotropic, characteristics with Seyfert 1 galaxies (S1s), as would be expected if the unified model is correct, while non-HBLR S2s generally do not. The active nuclear engines of non-HBLR S2s then, appear to be truly weaker than HBLR S2s, which in turn, are fully consistent with being S1s viewed from another direction. There is also evidence that the fraction of detected HBLR increases with radio power of the AGN. Thus, not all Seyfert 2 galaxies may be intrinsically similar in nature, and we speculate that evolutionary processes may be at work.Comment: 15 pages with embedded figs, ApJ in press, vol. 583, 2003 Feb. 1. v2: minor corrections to text, some typos removed; updated reference list: some added, some remove

Gamma rays and neutrinos from the Crab Nebula produced by pulsar accelerated nuclei

We investigate the consequences of the acceleration of heavy nuclei (e.g. iron nuclei) by the Crab pulsar. Accelerated nuclei can photodisintegrate in collisions with soft photons produced in the pulsar's outer gap, injecting energetic neutrons which decay either inside or outside the Crab Nebula. The protons from neutron decay inside the nebula are trapped by the Crab Nebula magnetic field, and accumulate inside the nebula producing gamma-rays and neutrinos in collisions with the matter in the nebula. Neutrons decaying outside the Crab Nebula contribute to the Galactic cosmic rays. We compute the expected fluxes of gamma-rays and neutrinos, and find that our model could account for the observed emission at high energies and may be tested by searching for high energy neutrinos with future neutrino telescopes currently in the design stage.Comment: 8 pages, 4 figures, LaTeX uses revtex.sty, submitted to Phys. Rev. Let

Highly anisotropic g-factor of two-dimensional hole systems

Coupling the spin degree of freedom to the anisotropic orbital motion of two-dimensional (2D) hole systems gives rise to a highly anisotropic Zeeman splitting with respect to different orientations of an in-plane magnetic field B relative to the crystal axes. This mechanism has no analogue in the bulk band structure. We obtain good, qualitative agreement between theory and experimental data, taken in GaAs 2D hole systems grown on (113) substrates, showing the anisotropic depopulation of the upper spin subband as a function of in-plane B.Comment: 4 pages, 3 figure

Global Analysis of the post-SNO Solar Neutrino Data for Standard and Non-Standard Oscillation Mechanisms

What can we learn from solar neutrino observations? Is there any solution to the solar neutrino anomaly which is favored by the present experimental panorama? After SNO results, is it possible to affirm that neutrinos have mass? In order to answer such questions we analyze the current available data from the solar neutrino experiments, including the recent SNO result, in view of many acceptable solutions to the solar neutrino problem based on different conversion mechanisms, for the first time, using the same statistical procedure. This allows us to do a direct comparison of the goodness of the fit among different solutions, from which we can discuss and conclude on the current status of each proposed dynamical mechanism. These solutions are based on different assumptions: (a) neutrino mass and mixing, (b) non-vanishing neutrino magnetic moment, (c) the existence of non-standard flavor-changing and non-universal neutrino interactions and (d) the tiny violation of the equivalence principle. We investigate the quality of the fit provided by each one of these solutions not only to the total rate measured by all the solar neutrino experiments but also to the recoil electron energy spectrum measured at different zenith angles by the Super-Kamiokande collaboration. We conclude that several non-standard neutrino flavor conversion mechanisms provide a very good fit to the experimental data which is comparable with (or even slightly better than) the most famous solution to the solar neutrino anomaly based on the neutrino oscillation induced by mass.Comment: Minor changes in the solar magnetic field profile used, and some refferences added. Final version to appear in PR

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 $\sigma_{CC}$ 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 $\nu$--induced upward going muons (both stopping and passing) are discussed.Comment: 9 pages, REVTEX, 1 uu-encoded compressed file with 3 figure

The Lake Baikal neutrino experiment: selected results

We review the present status of the lake Baikal Neutrino Experiment and present selected physical results gained with the consequetive stages of the stepwise increasing detector: from NT-36 to NT-96. Results cover atmospheric muons, neutrino events, very high energy neutrinos, search for neutrino events from WIMP annihilation, search for magnetic monopoles and environmental studies. We also describe an air Cherenkov array developed for the study of angular resolution of NT-200.Comment: 25 pages, 12 figures. To appear in the Procrrdings of International Conference on Non-Accelerator New Physics, June 28 - July 3, 1999, Dubna, Russi

Can lepton flavor violating interactions explain the atmospheric neutrino problem?

We investigate whether flavor changing neutrino interactions (FCNIs) can be sufficiently large to provide a viable solution to the atmospheric neutrino problem. Effective operators induced by heavy boson exchange that allow for flavor changing neutrino scattering off quarks or electrons are related by an $SU(2)_L$ rotation to operators that induce anomalous tau decays. Since $SU(2)_L$ violation is small for New Physics at or above the weak scale, one can use the upper bounds on lepton flavor violating tau decays or on lepton universality violation to put severe, model-independent bounds on the relevant non-standard neutrino interactions. Also $Z$-induced flavor changing neutral currents, due to heavy singlet neutrinos, are too small to be relevant for the atmospheric neutrino anomaly. We conclude that the FCNI solution to the atmospheric neutrino problem is ruled out.Comment: 16 pages, no figures, Late