17,189 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
Links Between Heavy Ion and Astrophysics
Heavy ion experiments provide important data to test astrophysical models.
The high density equation of state can be probed in HI collisions and applied
to the hot protoneutron star formed in core collapse supernovae. The Parity
Radius Experiment (PREX) aims to accurately measure the neutron radius of
Pb with parity violating electron scattering. This determines the
pressure of neutron rich matter and the density dependence of the symmetry
energy. Competition between nuclear attraction and coulomb repulsion can form
exotic shapes called nuclear pasta in neutron star crusts and supernovae. This
competition can be probed with multifragmentation HI reactions. We use large
scale semiclassical simulations to study nonuniform neutron rich matter in
supernovae. We find that the coulomb interactions in astrophysical systems
suppress density fluctuations. As a result, there is no first order liquid
vapor phase transition. Finally, the virial expansion for low density matter
shows that the nuclear vapor phase is complex with significant concentrations
of alpha particles and other light nuclei in addition to free nucleons.Comment: 8 pages, 6 figures. To be published in "Dynamics and Thermodynamics
with Nucleon Degrees of Freedom", eds. P. Chomaz, F. Gulminelli, J. Natowitz,
and S. Yennello, http://cyclotron.tamu.edu/wci3/wci_book.htm
Neutron rich nuclei and neutron stars
The PREX experiment at Jefferson Laboratory measures the neutron radius of
208Pb with parity violating electron scattering in a way that is free from most
strong interaction uncertainties. The 208Pb radius has important implications
for neutron rich matter and the structure of neutron stars. We present first
PREX results, describe future plans, and discuss a follow on measurement of the
neutron radius of 48Ca. We review radio and X-ray observations of neutron star
masses and radii. These constrain the equation of state (pressure versus
density) of neutron rich matter. We present a new energy functional that is
simultaneously fit to both nuclear and neutron star properties. In this
approach, neutron star masses and radii constrain the energy of neutron matter.
This avoids having to rely on model dependent microscopic calculations of
neutron matter. The functional is then used to predict the location of the drip
lines and the properties of very neutron rich heavy nuclei.Comment: 8 pages, 4 figures, proceedings of International Conference on
Fission and Neutron Rich Nuclei 5, Sanibel, F
Multi-messenger observations of neutron rich matter
Neutron rich matter is central to many fundamental questions in nuclear
physics and astrophysics. Moreover, this material is being studied with an
extraordinary variety of new tools such as the Facility for Rare Isotope Beams
(FRIB) and the Laser Interferometer Gravitational Wave Observatory (LIGO). We
describe the Lead Radius Experiment (PREX) that uses parity violating electron
scattering to measure the neutron radius in Pb. This has important
implications for neutron stars and their crusts. We discuss X-ray observations
of neutron star radii. These also have important implications for neutron rich
matter. Gravitational waves (GW) open a new window on neutron rich matter. They
come from sources such as neutron star mergers, rotating neutron star
mountains, and collective r-mode oscillations. Using large scale molecular
dynamics simulations, we find neutron star crust to be very strong. It can
support mountains on rotating neutron stars large enough to generate detectable
gravitational waves. Finally, neutrinos from core collapse supernovae (SN)
provide another, qualitatively different probe of neutron rich matter.
Neutrinos escape from the surface of last scattering known as the
neutrino-sphere. This is a low density warm gas of neutron rich matter.
Observations of neutrinos can probe nucleosyntheses in SN. Simulations of SN
depend on the equation of state (EOS) of neutron rich matter. We discuss a new
EOS based on virial and relativistic mean field calculations. We believe that
combing astronomical observations using photos, GW, and neutrinos, with
laboratory experiments on nuclei, heavy ion collisions, and radioactive beams
will fundamentally advance our knowledge of compact objects in the heavens, the
dense phases of QCD, the origin of the elements, and of neutron rich matter.Comment: 13 pages, 4 figures, Added discussion of dipole polarizability, pygmy
resonances, and neutron skin
Semiparametric estimation of a panel data proportional hazards model with fixed effects
This paper considers a panel duration model that has a proportional hazards specification
with fixed effects. The paper shows how to estimate the baseline and integrated
baseline hazard functions without assuming that they belong to known, finitedimensional
families of functions. Existing estimators assume that the baseline hazard
function belongs to a known parametric family. Therefore, the estimators presented here
are more general than existing ones. This paper also presents a method for estimating
the parametric part of the proportional hazards model with dependent right censoring,
under which the partial likelihood estimator is inconsistent. The paper presents some
Monte Carlo evidence on the small sample performance of the new estimators
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