22,136 research outputs found
A new possibility to monitor collisions of relativistic heavy ions at LHC and RHIC
We consider the radiation of particles of one bunch in the collective field
of the oncoming bunch, called coherent bremsstrahlung (CBS). The main
characteristics of CBS for LHC (in the Pb-Pb mode) and for RHIC are calculated.
At LHC about photons per second are expected for
photon energies eV. It seems that CBS
can be a potential tool for optimizing collisions and for measuring beam
parameters. The bunch length can be found from the critical energy of the CBS
spectrum; the transverse bunch size is related to the photon rate. A specific
dependence of photon rate on the impact parameter between the beams allows for
a fast control over the beam displacement.Comment: 9 pages + 4 figures, latex with poscript figures uuencode
Ab-initio coupled-cluster effective interactions for the shell model: Application to neutron-rich oxygen and carbon isotopes
We derive and compute effective valence-space shell-model interactions from
ab-initio coupled-cluster theory and apply them to open-shell and neutron-rich
oxygen and carbon isotopes. Our shell-model interactions are based on
nucleon-nucleon and three-nucleon forces from chiral effective-field theory. We
compute the energies of ground and low-lying states, and find good agreement
with experiment. In particular our calculations are consistent with the N=14,
16 shell closures in oxygen-22 and oxygen-24, while for carbon-20 the
corresponding N=14 closure is weaker. We find good agreement between our
coupled-cluster effective-interaction results with those obtained from standard
single-reference coupled-cluster calculations for up to eight valence neutrons
Effective Operators for Double-Beta Decay
We use a solvable model to examine double-beta decay, focusing on the
neutrinoless mode. After examining the ways in which the neutrino propagator
affects the corresponding matrix element, we address the problem of finite
model-space size in shell-model calculations by projecting our exact wave
functions onto a smaller subspace. We then test both traditional and more
recent prescriptions for constructing effective operators in small model
spaces, concluding that the usual treatment of double-beta-decay operators in
realistic calculations is unable to fully account for the neglected parts of
the model space. We also test the quality of the Quasiparticle Random Phase
Approximation and examine a recent proposal within that framework to use
two-neutrino decay to fix parameters in the Hamiltonian. The procedure
eliminates the dependence of neutrinoless decay on some unfixed parameters and
reduces the dependence on model-space size, though it doesn't eliminate the
latter completely.Comment: 10 pages, 8 figure
Determination of biaxial creep strength of T-111 tantalum alloy
Biaxial creep strength of T-111 tantalum alloy tubing in high temperature, high vacuum environmen
Random-phase-approximation-based correlation energy functionals: Benchmark results for atoms
The random phase approximation (RPA) for the correlation energy functional of
density functional theory has recently attracted renewed interest. Formulated
in terms of the Kohn-Sham (KS) orbitals and eigenvalues, it promises to resolve
some of the fundamental limitations of the local density and generalized
gradient approximations, as for instance their inability to account for
dispersion forces. First results for atoms, however, indicate that the RPA
overestimates correlation effects as much as the orbital-dependent functional
obtained by a second order perturbation expansion on the basis of the KS
Hamiltonian. In this contribution, three simple extensions of the RPA are
examined, (a) its augmentation by an LDA for short-range correlation, (b) its
combination with the second order exchange term, and (c) its combination with a
partial resummation of the perturbation series including the second order
exchange. It is found that the ground state and correlation energies as well as
the ionization potentials resulting from the extensions (a) and (c) for closed
sub-shell atoms are clearly superior to those obtained with the unmodified RPA.
Quite some effort is made to ensure highly converged RPA data, so that the
results may serve as benchmark data. The numerical techniques developed in this
context, in particular for the inherent frequency integration, should also be
useful for applications of RPA-type functionals to more complex systems.Comment: 11 pages, 7 figure
Comment: Superconducting transition in Nb nanowires fabricated using focused ion beam
In a recent paper Tettamanzi et al (2009 Nanotechnology \bf{20} 465302)
describe the fabrication of superconducting Nb nanowires using a focused ion
beam. They interpret their conductivity data in the framework of thermal and
quantum phase slips below . In the following we will argue that their
analysis is inappropriate and incomplete, leading to contradictory results.
Instead, we propose an interpretation of the data within a SN proximity model.Comment: 3 pages, 1 figure accepted in Nanotechnolog
Axial charges of octet and decuplet baryons
We present a study of axial charges of baryon ground and resonant states with
relativistic constituent quark models. In particular, the axial charges of
octet and decuplet , , , , , and
baryons are considered. The theoretical predictions are compared to existing
experimental data and results from other approaches, notably from lattice
quantum chromodynamics and chiral perturbation theory. The relevance of axial
charges with regard to -dressing and spontaneous chiral-symmetry breaking
is discussed
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