310 research outputs found
Parity Violation in Astrophysics
Core collapse supernovae are gigantic explosions of massive stars that
radiate 99% of their energy in neutrinos. This provides a unique opportunity
for large scale parity or charge conjugation violation. Parity violation in a
strong magnetic field could lead to an asymmetry in the neutrino radiation and
recoil of the newly formed neutron star. Charge conjugation violation in the
neutrino-nucleon interaction reduces the ratio of neutrons to protons in the
neutrino driven wind above the neutron star. This is a problem for r-process
nucleosynthesis in this wind. On earth, parity violation is an excellent probe
of neutrons because the weak charge of a neutron is much larger than that of a
proton. The Parity Radius Experiment (PREX) at Jefferson Laboratory aims to
precisely measure the neutron radius of Pb with parity violating
elastic electron scattering. This has many implications for astrophysics,
including the structure of neutron stars, and for atomic parity nonconservation
experiments.}Comment: 4 pages, 2 figures, proceedings of PAVI04 conference in Grenoble,
Franc
Charge-conjugation violating neutrino interactions in supernovae
The well known charge conjugation violating interactions in the Standard
Model increase neutrino- and decrease anti-neutrino- nucleon cross sections.
This impacts neutrino transport in core collapse supernovae through "recoil"
corrections of order the neutrino energy over the nucleon mass . All
corrections to neutrino transport deep inside a protoneutron star are
calculated from angular integrals of the Boltzmann equation. We find these
corrections significantly modify neutrino currents at high temperatures. This
produces a large mu and tau number for the protoneutron star and can change the
ratio of neutrons to protons. In addition, the relative size of neutrino mean
free paths changes. At high temperatures, the electron anti-neutrino mean free
path becomes {\it longer} than that for mu or tau neutrinos.Comment: 14 pages, 2 included ps figures, subm. to Phys. Rev.
Relativistic nuclear structure effects in quasielastic neutrino scattering
Charged-current cross sections are calculated for quasielastic neutrino and
antineutrino scattering using a relativistic meson-nucleon model. We examine
how nuclear-structure effects, such as relativistic random-phase-approximation
(RPA) corrections and momentum-dependent nucleon self-energies, influence the
extraction of the axial form factor of the nucleon. RPA corrections are
important only at low-momentum transfers. In contrast, the momentum dependence
of the relativistic self-energies changes appreciably the value of the
axial-mass parameter, , extracted from dipole fits to the axial form
factor. Using Brookhaven's experimental neutrino spectrum we estimate the
sensitivity of M to various relativistic nuclear-structure effects.Comment: 26 pages, revtex, 6 postscript figures (available upon request
Realistic Neutrino Opacities for Supernova Simulations With Correlations and Weak Magnetism
Advances in neutrino transport allow realistic neutrino interactions to be
incorporated into supernova simulations. We add tensor couplings to
relativistic RPA calculations of neutrino opacities. Our results reproduce
free-space neutrino-nucleon cross sections at low density, including weak
magnetism and recoil corrections. In addition, our opacities are
thermodynamically consistent with relativistic mean field equations of state.
We find antineutrino mean free paths that are considerably larger then those
for neutrinos. This difference depends little on density. In a supernova, this
difference could lead to an average energy of that is larger than
that for by an amount that is comparable to the energy difference
between and Comment: 15 pages, 10 figures, submitted to PRC, minor changes to figs. (9,10
Macroscopic Parity Violation and Supernova Asymmetries
Core collapse supernovae are dominated by weakly interacting neutrinos. This
provides a unique opportunity for macroscopic parity violation. We speculate
that parity violation in a strong magnetic field can lead to an asymmetry in
the explosion and a recoil of the newly formed neutron star. We estimate the
asymmetry from neutrino-polarized-neutron elastic scattering, polarized
electron capture and neutrino-nucleus elastic scattering in a (partially)
polarized electron gas.Comment: Nine pages Revtex, two postscript figures (included
Comment on "Role of heavy meson exchange in near threshold N N --> d pi"
In a recent paper by C. J. Horowitz (Phys. Rev. C {\bf 48}, 2920 (1993)) a
heavy meson exchange is incorporated into threshold NN --> d pi to enhance the
grossly underestimated cross section. However, that calculation uses an
unjustified assumption on the initial and final momenta, which causes an
overestimate of this effect by a factor of 3--4. I point out that the inclusion
of the Delta(1232) isobar increases the cross section significantly even at
threshold.Comment: 7 pages, figures by fax or mail from [email protected]
The Neutrino Response of Low-Density Neutron Matter from the Virial Expansion
We generalize our virial approach to study spin-polarized neutron matter and
the consistent neutrino response at low densities. In the long-wavelength
limit, the virial expansion makes model-independent predictions for the density
and spin response, based only on nucleon-nucleon scattering data. Our results
for the neutrino response provide constraints for random-phase approximation or
other model calculations, and we compare the virial vector and axial response
to response functions used in supernova simulations. The virial expansion is
suitable to describe matter near the supernova neutrinosphere, and this work
extends the virial equation of state to predict neutrino interactions in
neutron matter.Comment: 8 pages, 5 figures, minor additions, to appear in Phys. Lett.
Self-consistent description of nuclear compressional modes
Isoscalar monopole and dipole compressional modes are computed for a variety
of closed-shell nuclei in a relativistic random-phase approximation to three
different parametrizations of the Walecka model with scalar self-interactions.
Particular emphasis is placed on the role of self-consistency which by itself,
and with little else, guarantees the decoupling of the spurious
isoscalar-dipole strength from the physical response and the conservation of
the vector current. A powerful new relation is introduced to quantify the
violation of the vector current in terms of various ground-state form-factors.
For the isoscalar-dipole mode two distinct regions are clearly identified: (i)
a high-energy component that is sensitive to the size of the nucleus and scales
with the compressibility of the model and (ii) a low-energy component that is
insensitivity to the nuclear compressibility. A fairly good description of both
compressional modes is obtained by using a ``soft'' parametrization having a
compression modulus of K=224 MeV.Comment: 28 pages and 10 figures; submitted to PR
Modeling the strangeness content of hadronic matter
The strangeness content of hadronic matter is studied in a string-flip model
that reproduces various aspects of the QCD-inspired phenomenology, such as
quark clustering at low density and color deconfinement at high density, while
avoiding long range van der Waals forces. Hadronic matter is modeled in terms
of its quark constituents by taking into account its internal flavor (u,d,s)
and color (red, blue, green) degrees of freedom. Variational Monte-Carlo
simulations in three spatial dimensions are performed for the ground-state
energy of the system. The onset of the transition to strange matter is found to
be influenced by weak, yet not negligible, clustering correlations. The phase
diagram of the system displays an interesting structure containing both
continuous and discontinuous phase transitions. Strange matter is found to be
absolutely stable in the model.Comment: 14 pages, 1 table, 8 eps figures, revtex. Submitted to Phys. Rev. C,
Presented at INPC2001 Berkeley, Ca. july 29-Aug
Relativistic mean-field study of neutron-rich nuclei
A relativistic mean-field model is used to study the ground-state properties
of neutron-rich nuclei. Nonlinear isoscalar-isovector terms, unconstrained by
present day phenomenology, are added to the model Lagrangian in order to modify
the poorly known density dependence of the symmetry energy. These new terms
soften the symmetry energy and reshape the theoretical neutron drip line
without compromising the agreement with existing ground-state information. A
strong correlation between the neutron radius of 208Pb and the binding energy
of valence orbitals is found: the smaller the neutron radius of 208Pb, the
weaker the binding energy of the last occupied neutron orbital. Thus, models
with the softest symmetry energy are the first ones to drip neutrons. Further,
in anticipation of the upcoming one-percent measurement of the neutron radius
of 208Pb at the Thomas Jefferson Laboratory, a close relationship between the
neutron radius of 208Pb and neutron radii of elements of relevance to atomic
parity-violating experiments is established.Comment: 14 pages, 5 figure
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