4,585 research outputs found
The Non-Mesonic Weak Decay of Double-Lambda Hypernuclei: A Microscopic Approach
The non--mesonic weak decay of double-- hypernuclei is studied
within a microscopic diagrammatic approach. Besides the nucleon--induced
mechanism, , widely studied in single-- hypernuclei,
additional hyperon--induced mechanisms, ,
and , are
accessible in double-- hypernuclei and are investigated here. As in
previous works on single-- hypernuclei, we adopt a nuclear matter
formalism extended to finite nuclei via the local density approximation and a
one--meson exchange weak transition potential (including the ground state
pseudoscalar and vector octets mesons) supplemented by correlated and
uncorrelated two--pion--exchange contributions. The weak decay rates are
evaluated for hypernuclei in the region of the experimentally accessible light
hypernuclei Be and B. Our
predictions are compared with a few previous evaluations. The rate for the
decay is dominated by --, -- and
--exchange and turns out to be about 2.5\% of the free decay
rate, , while the total rate for the and decays, dominated by
--exchange, amounts to about 0.25\% of . The
experimental measurement of these decays would be essential for the beginning
of a systematic study of the non--mesonic decay of strangeness
hypernuclei. This field of research could also shed light on the possible
existence and nature of the --dibaryon.Comment: 17 pages, 2 figure
Nucleon and gamma N -> Delta lattice form factors in a constituent quark model
A covariant quark model, based both on the spectator formalism and on vector
meson dominance, and previously calibrated by the physical data, is here
extended to the unphysical region of the lattice data by means of one single
extra adjustable parameter - the constituent quark mass in the chiral limit. We
calculated the Nucleon (N) and the gamma N -> Delta form factors in the
universe of values for that parameter described by quenched lattice QCD. A
qualitative description of the Nucleon and gamma N -> Delta form factors
lattice data is achieved for light pions.Comment: To appear in J.Phys.
A pure S-wave covariant model for the nucleon
Using the manifestly covariant spectator theory, and modeling the nucleon as
a system of three constituent quarks with their own electromagnetic structure,
we show that all four nucleon electromagnetic form factors can be very well
described by a manifestly covariant nucleon wave function with zero orbital
angular momentum. Since the concept of wave function depends on the formalism,
the conclusions of light-cone theory requiring nonzero angular momentum
components are not inconsistent with our results. We also show that our model
gives a qualitatively correct description of deep inelastic scattering,
unifying the phenomenology at high and low momentum transfer. Finally we review
two different definitions of nuclear shape and show that the nucleon is
spherical in this model, regardless of how shape is defined.Comment: 20 pages and 10 figures; greatly expanded version with new fits and
discussion of DIS; similar to published versio
Phase transitions with finite atom number in the Dicke Model
Two-level atoms interacting with a one mode cavity field at zero temperature
have order parameters which reflect the presence of a quantum phase transition
at a critical value of the atom-cavity coupling strength. Two popular examples
are the number of photons inside the cavity and the number of excited atoms.
Coherent states provide a mean field description, which becomes exact in the
thermodynamic limit. Employing symmetry adapted (SA) SU(2) coherent states
(SACS) the critical behavior can be described for a finite number of atoms. A
variation after projection treatment, involving a numerical minimization of the
SA energy surface, associates the finite number phase transition with a
discontinuity in the order parameters, which originates from a competition
between two local minima in the SA energy surface.Comment: 8 pages, 10 figures, Conference Proceedings of CEWQO-2012, to be
published as a Topical Issue of the journal Physica Script
Matching the Spectral Energy Distribution and p Mode Oscillation Frequencies of the Rapidly Rotating Delta Scuti Star ? Ophiuchi with a 2D Rotating Stellar Model
Spectral energy distributions are computed using 2D rotating stellar models
and NLTE plane parallel model atmospheres. A rotating, 2D stellar model has
been found which matches the observed ultraviolet and visible spectrum of ?
Oph. The SED match occurs for the interferometrically deduced surface shape and
inclination, and is different from the SED produced by spherical models. The p
mode oscillation frequencies in which the latitudinal variation is modelled by
a linear combination of eight Legendre polynomials were computed for this
model. The five highest and seven of the nine highest amplitude modes show
agreement between computed axisymmetric, equatorially symmetric mode
frequencies and the mode frequencies observed by MOST to within the
observational error. Including nonaxisymmetric modes up through |m| = 2 and
allowing the possibility that the eight lowest amplitude modes could be
produced by modes which are not equatorially symmetric produces matches for 24
out of the 35 MOST modes to within the observational error and another eight
modes to within twice the observational error. The remaining three observed
modes can be fit within 4.2 times the observational error, but even these may
be fit to within the observational error if the criteria for computed modes are
expanded.Comment: Accepted by Ap
On the puzzle of Bremsstrahlung as described by coaccelerated observers
We consider anew some puzzling aspects of the equivalence of the quantum
field theoretical description of Bremsstrahlung from the inertial and
accelerated observer's perspectives. More concretely, we focus on the seemingly
paradoxical situation that arises when noting that the radiating source is in
thermal equilibrium with the thermal state of the quantum field in the wedge in
which it is located, and thus its presence does not change there the state of
the field, while it clearly does not affect the state of the field on the
opposite wedge. How then is the state of the quantum field on the future wedge
changed, as it must in order to account for the changed energy momentum tensor
there? This and related issues are carefully discussed.Comment: 29 pages, 1 figure; Revtex, minor changes, PACS correcte
A Far-UV Variability Survey of the Globular Cluster M80
We have searched for variable sources in the core region of M80, using far
ultra-violet data taken with the Advanced Camera for Surveys on board the
Hubble Space Telescope. We found three sources that exhibit strong signs of
variability in our data. Among these is source TDK1, which we believe to be an
RR Lyrae star that reached maximum brightness during our observations. The
light curve shows a >3 mag FUV brightening over the course of ~5 hours, with an
estimated peak brightness of ~16.7 mag, followed by a decrease to ~20 mag.
Archival optical data obtained with WFPC2 confirm that TDK1 is variable in all
wavebands. TDK1's SED is reasonably fit by a star with temperature T(eff)=6700K
and radius R=4.2R(sun), consistent with the suggestion that it is an RR Lyrae.
Based on the photometric and variability characteristics of the other two
variables, we suggest that TDK2 is likely to be an SX Phoenicis star with ~55
minutes period, and TDK3 is likely another RR Lyrae. Finally, we briefly
discuss the FUV counterparts to two previously known variables in M80, the
classical nova T Sco and the dwarf nova DN1.Comment: 12 pages, 9 figures and 3 tables. Accepted for publication in MNRAS
- …