4,599 research outputs found
One-loop conformal anomaly in an implicit momentum space regularization framework
In this paper we consider matter fields in a gravitational background in
order to compute the breaking of the conformal current at one-loop order.
Standard perturbative calculations of conformal symmetry breaking expressed by
the non-zero trace of the energy-momentum tensor have shown that some violating
terms are regularization dependent, which may suggest the existence of spurious
breaking terms in the anomaly. Therefore, we perform the calculation in a
momentum space regularization framework in which regularization dependent terms
are judiciously parametrized. We compare our results with those obtained in the
literature and conclude that there is an unavoidable arbitrariness in the
anomalous term .Comment: in European Physical Journal C, 201
The Casimir spectrum revisited
We examine the mathematical and physical significance of the spectral density
sigma(w) introduced by Ford in Phys. Rev. D38, 528 (1988), defining the
contribution of each frequency to the renormalised energy density of a quantum
field. Firstly, by considering a simple example, we argue that sigma(w) is well
defined, in the sense of being regulator independent, despite an apparently
regulator dependent definition. We then suggest that sigma(w) is a spectral
distribution, rather than a function, which only produces physically meaningful
results when integrated over a sufficiently large range of frequencies and with
a high energy smooth enough regulator. Moreover, sigma(w) is seen to be simply
the difference between the bare spectral density and the spectral density of
the reference background. This interpretation yields a simple `rule of thumb'
to writing down a (formal) expression for sigma(w) as shown in an explicit
example. Finally, by considering an example in which the sign of the Casimir
force varies, we show that the spectrum carries no manifest information about
this sign; it can only be inferred by integrating sigma(w).Comment: 10 pages, 4 figure
Improved estimate of electron capture rates on nuclei during stellar core collapse
Electron captures on nuclei play an important role in the dynamics of the
collapsing core of a massive star that leads to a supernova explosion. Recent
calculations of these capture rates were based on microscopic models which
account for relevant degrees of freedom. Due to computational restrictions such
calculations were limited to a modest number of nuclei, mainly in the mass
range A=45-110. Recent supernova simulations show that this pool of nuclei,
however, omits the very neutron-rich and heavy nuclei which dominate the
nuclear composition during the last phase of the collapse before neutrino
trapping. Assuming that the composition is given by Nuclear Statistical
Equilibrium we present here electron capture rates for collapse conditions
derived from individual rates for roughly 2700 individual nuclei. For those
nuclei which dominate in the early stage of the collapse, the individual rates
are derived within the framework of microscopic models, while for the nuclei
which dominate at high densities we have derived the rates based on the Random
Phase Approximation with a global parametrization of the single particle
occupation numbers. In addition, we have improved previous rate evaluations by
properly including screening corrections to the reaction rates into account.Comment: 32 pages, 13 figures, 1 table; elsart; to appear in Nuclear Physics
Hubbard-model description of the high-energy spin-spectral-weight distribution in La(2)CuO(4)
The spectral-weight distribution in recent neutron scattering experiments on
the parent compound LaCuO (LCO), which are limited in energy range to
about 450\,meV, is studied in the framework of the Hubbard model on the square
lattice with effective nearest-neighbor transfer integral and on-site
repulsion . Our study combines a number of numerical and theoretical
approaches, including, in addition to standard treatments, density matrix
renormalization group calculations for Hubbard cylinders and a suitable spinon
approach for the spin excitations. Our results confirm that the
magnitude suitable to LCO corresponds to intermediate values smaller than
the bandwidth , which we estimate to be eV for
. This confirms the unsuitability of the conventional linear
spin-wave theory. Our theoretical studies provide evidence for the occurrence
of ground-state d-wave spinon pairing in the half-filled Hubbard model on the
square lattice. This pairing applies only to the rotated-electron spin degrees
of freedom, but it could play a role in a possible electron d-wave pairing
formation upon hole doping. We find that the higher-energy spin spectral weight
extends to about 566 meV and is located at and near the momentum .
The continuum weight energy-integrated intensity vanishes or is extremely small
at momentum . This behavior of this intensity is consistent with that
of the spin waves observed in recent high-energy neutron scattering
experiments, which are damped at the momentum . We suggest that future
LCO neutron scattering experiments scan the energies between 450 meV and 566
meV and momenta around .Comment: 23 pages, 5 figure
Scalar Casimir Effect on a D-dimensional Einstein Static Universe
We compute the renormalised energy momentum tensor of a free scalar field
coupled to gravity on an (n+1)-dimensional Einstein Static Universe (ESU),
RxS^n, with arbitrary low energy effective operators (up to mass dimension
n+1). A generic class of regulators is used, together with the Abel-Plana
formula, leading to a manifestly regulator independent result. The general
structure of the divergences is analysed to show that all the gravitational
couplings (not just the cosmological constant) are renormalised for an
arbitrary regulator. Various commonly used methods (damping function,
point-splitting, momentum cut-off and zeta function) are shown to, effectively,
belong to the given class. The final results depend strongly on the parity of
n. A detailed analytical and numerical analysis is performed for the behaviours
of the renormalised energy density and a quantity `sigma' which determines if
the strong energy condition holds for the `quantum fluid'. We briefly discuss
the quantum fluid back-reaction problem, via the higher dimensional Friedmann
and Raychaudhuri equations, observe that equilibrium radii exist and unveil the
possibility of a `Casimir stabilisation of Einstein Static Universes'.Comment: 37 pages, 15 figures, v2: minor changes in sections 1, 2.5, 3 and 4;
version published in CQ
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