857 research outputs found
More on nucleon-nucleon cross sections in symmetric and asymmetric matter
Following a recent work, we present numerical results for total two-nucleon
effective cross sections in isospin symmetric and asymmetric matter. The
present calculations include the additional effect of Pauli blocking of the
final states.Comment: 9 pages, no figures, 5 table
Predicting the single-proton/neutron potentials in asymmetric nuclear matter
We discuss the one-body potentials for protons and neutrons obtained from
Dirac-Brueckner-Hartree-Fock calculations of neutron-rich matter, in particular
their dependence upon the degree of proton/neutron asymmetry. The closely
related symmetry potential is compared with empirical information from the
isovector component of the nuclear optical potential.Comment: 9 pages, 6 figures. Minor revisions, added comments, reference
Non-adiabatic level crossing in (non-) resonant neutrino oscillations
We study neutrino oscillations and the level-crossing probability
P_{LZ}=\exp(-\gamma_n\F_n\pi/2) in power-law like potential profiles
. After showing that the resonance point coincides only for a
linear profile with the point of maximal violation of adiabaticity, we point
out that the ``adiabaticity'' parameter can be calculated at an
arbitrary point if the correction function \F_n is rescaled appropriately. We
present a new representation for the level-crossing probability,
P_{LZ}=\exp(-\kappa_n\G_n), which allows a simple numerical evaluation of
in both the resonant and non-resonant cases and where \G_n contains
the full dependence of on the mixing angle . As an application
we consider the case important for oscillations of supernova neutrinos.Comment: 4 pages, revtex, 3 eps figure
Spin polarized neutron matter within the Dirac-Brueckner-Hartree-Fock approach
The relation between energy and density (known as the nuclear equation of
state) plays a major role in a variety of nuclear and astrophysical systems.
Spin and isospin asymmetries can have a dramatic impact on the equation of
state and possibly alter its stability conditions. An example is the possible
manifestation of ferromagnetic instabilities, which would indicate the
existence, at a certain density, of a spin-polarized state with lower energy
than the unpolarized one. This issue is being discussed extensively in the
literature and the conclusions are presently very model dependent. We will
report and discuss our recent progress in the study of spin-polarized neutron
matter. The approach we take is microscopic and relativistic. The calculated
neutron matter properties are derived from realistic nucleon-nucleon
interactions. This makes it possible to understand the nature of the EOS
properties in terms of specific features of the nuclear force model.Comment: 6 pages, 11 figures, revised/extended calculation
A Study of the Day - Night Effect for the Super - Kamiokande Detector: I. Time Averaged Solar Neutrino Survival Probability
This is the first of two articles aimed at providing comprehensive
predictions for the day-night (D-N) effect for the Super-Kamiokande detector in
the case of the MSW \nu_e \to \numt transition solution of the solar neutrino
problem. The one-year averaged probability of survival of the solar \nue
crossing the Earth mantle, the core, the inner 2/3 of the core, and the (core +
mantle) is calculated with high precision (better than 1%) using the elliptical
orbit approximation (EOA) to describe the Earth motion around the Sun. Results
for the survival probability in the indicated cases are obtained for a large
set of values of the MSW transition parameters and
from the ``conservative'' regions of the MSW solution,
derived by taking into account possible relatively large uncertainties in the
values of the B and Be neutrino fluxes. Our results show that the
one-year averaged D-N asymmetry in the survival probability for
neutrinos crossing the Earth core can be, in the case of , larger than the asymmetry in the probability for (only mantle
crossing + core crossing) neutrinos by a factor of up to six. The enhancement
is larger in the case of neutrinos crossing the inner 2/3 of the core. This
indicates that the Super-Kamiokande experiment might be able to test the
region of the MSW solution of the solar neutrino
problem by performing selective D-N asymmetry measurements.Comment: LaTeX2e - 18 Text Pages + 21 figures = 39 Pages. - Figures in PS +
text file sk1b14.tex requires two auxiliary files (included
New Constraints on Neutrino Oscillations in Vacuum as a Possible Solution of the Solar Neutrino Problem
Two-neutrino oscillations in vacuum are studied as a possible solution of the
solar neutrino problem. New constraints on the parameter sn2, characterizing
the mixing of the electron neutrino with another active or sterile neutrino, as
well as on the mass--squared difference, dm2, of their massive neutrino
components, are derived using the latest results from the four solar neutrino
experiments. Oscillations into a sterile neutrino are ruled out at 99 % C.L. by
the observed mean event rates even if one includes the uncertainties of the
standard solar model predictions in the analysis.Comment: 10 pages + 3 figures attached as postscript files, IFP-480-UNC and
Ref. SISSA 177/93/EP (Updated Version which takes into account the latest
GALLEX results from 30 runs
How Well Do We (and Will We) Know Solar Neutrino Fluxes and Oscillation Parameters?
Assuming neutrino oscillations occur, the pp electron neutrino flux is
uncertain by at least a factor of two, the flux by a factor of
five, and the flux by a factor of forty-five. Calculations of the
expected results of future solar neutrino experiments (SuperKamiokande, SNO,
BOREXINO, ICARUS, HELLAZ, and HERON) are used to illustrate the extent to which
these experiments will restrict the range of the allowed neutrino mixing
parameters. We present an improved formulation of the ``luminosity constraint''
and show that at 95\% confidence limit this constraint establishes the best
available limits on the rate of creation of pp neutrinos in the solar interior
and provides the best upper limit to the neutrino flux.Comment: 37 pages, uuencoded Z-compressed postscript file (with figures);
Submitted to Physical Review
Three-Neutrino Mixing and Combined Vacuum Oscillations and MSW Transitions of Solar Neutrinos
Assuming three flavour neutrino mixing takes place in vacuum, we investigate
the possibility that the solar nu_e take part in MSW transitions in the Sun due
to Delta m^2_{31} from 10^{-7} eV^2 to 10^{-4} eV^2, followed by long wave
length vacuum oscillations on the way to the Earth, triggered by Delta m^2_{21}
(or Delta m^2_{32}) from 10^{-12} eV^2 to 10^{-10} eV^2, Delta m^2_{31} and
Delta m^2_{21} (Delta m^2_{32}) being the corresponding neutrino mass squared
differences. The solar nu_e survival probability is shown to be described in
this case by a simple analytic expression. Depending on whether the vacuum
oscillations are due to Delta m^2_{21} or Delta m^2_{32} there are two very
different types of interplay between the MSW transitions and the vacuum
oscillations of the solar nu_e. Performing an analysis of the most recently
published solar neutrino data we have found several qualitatively new solutions
of the solar neutrino problem of the hybrid MSW transitions + vacuum
oscillations type. The solutions differ in the way the pp, 7Be and 8B neutrino
fluxes are affected by the transitions in the Sun and the oscillations in
vacuum. The specific features of the new solutions are discussed.Comment: 37 pages Latex, 16 Postscript Figure
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