802 research outputs found
Existence of a Semiclassical Approximation in Loop Quantum Gravity
We consider a spherical symmetric black hole in the Schwarzschild metric and
apply Bohr-Sommerfeld quantization to determine the energy levels. The
canonical partition function is then computed and we show that the entropy
coincides with the Bekenstein-Hawking formula when the maximal number of states
for the black hole is the same as computed in loop quantum gravity, proving in
this case the existence of a semiclassical limit and obtaining an independent
derivation of the Barbero-Immirzi parameter.Comment: 6 pages, no figures. Final version accepted for publication in
General Relativity and Gravitatio
A Perturbative Calculation of the Electromagnetic Form Factors of the Deuteron
Making use of the effective field theory expansion recently developed by the
authors, we compute the electromagnetic form factors of the deuteron
analytically to next-to-leading order (NLO). The computation is rather simple,
and involves calculating several Feynman diagrams, using dimensional
regularization. The results agree well with data and indicate that the
expansion is converging. They do not suffer from any ambiguities arising from
off-shell versus on-shell amplitudes.Comment: 22 pages, 8 figures. Discussion of effective range theory added,
typos correcte
A renormalisation group approach to two-body scattering in the presence of long-range forces
We apply renormalisation-group methods to two-body scattering by a
combination of known long-range and unknown short-range potentials. We impose a
cut-off in the basis of distorted waves of the long-range potential and
identify possible fixed points of the short-range potential as this cut-off is
lowered to zero. The expansions around these fixed points define the power
countings for the corresponding effective field theories. Expansions around
nontrivial fixed points are shown to correspond to distorted-wave versions of
the effective-range expansion. These methods are applied to scattering in the
presence of Coulomb, Yukawa and repulsive inverse-square potentials.Comment: 22 pages (RevTeX), 4 figure
Spectral Properties of the k-Body Embedded Gaussian Ensembles of Random Matrices
We consider spinless Fermions in degenerate single-particle
levels interacting via a -body random interaction with Gaussian probability
distribution and in the limit to infinity (the embedded -body
random ensembles). We address the cases of orthogonal and unitary symmetry. We
derive a novel eigenvalue expansion for the second moment of the Hilbert-space
matrix elements of these ensembles. Using properties of the expansion and the
supersymmetry technique, we show that for , the average spectrum has
the shape of a semicircle, and the spectral fluctuations are of Wigner-Dyson
type. Using a generalization of the binary correlation approximation, we show
that for , the spectral fluctuations are Poissonian. This is
consistent with the case which can be solved explicitly. We construct
limiting ensembles which are either fully integrable or fully chaotic and show
that the -body random ensembles lie between these two extremes. Combining
all these results we find that the spectral correlations for the embedded
ensembles gradually change from Wigner-Dyson for to Poissonian for .Comment: 44 pages, 3 postscript figures, revised version including a new proof
of one of our main claim
Nucleon propagation through nuclear matter in chiral effective field theory
We treat the propagation of nucleon in nuclear matter by evaluating the
ensemble average of the two-point function of nucleon currents in the framework
of the chiral effective field theory. We first derive the effective parameters
of nucleon to one loop. The resulting formula for the effective mass was known
previously and gives an absurd value at normal nuclear density. We then modify
it following Weinberg's method for the two-nucleon system in the effective
theory. Our results for the effective mass and the width of nucleon are
compared with those in the literature.Comment: 11 pages including 4 figures. To appear in Eur. J. Phys.
ON COMPUTER SIMULATION AS A COMPONENT IN INFORMATION SYSTEMS RESEARCH
Computer simulation is widely regarded as a useful activity during various phases of research. However, depending on its context, the meaning, definition, and focus of the term can vary: In traffic planning, for example, simulation is used to determine useful configurations of a road network, thus focusing on the environment. An entirely different perspective is used within multi-agent systems. In such settings, the environment of the agents remains static, while the interesting research questions concern the behavior of the agents themselves. The research focuses on the microscopic level and the resulting emergent behavior. This article puts such diverse meanings in the context of a research process that treats descriptive and prescriptive research as two sides of the same coin. We develop a framework to classify different types of simulation, based on the actual research activity they are intended to be used for. Two case studies supplement the framework
Coupled-channel effective field theory and proton-Li scattering
We apply the renormalisation group (RG) to analyse scattering by short-range
forces in systems with coupled channels. For two S-wave channels, we find three
fixed points, corresponding to systems with zero, one or two bound or virtual
states at threshold. We use the RG to determine the power countings for the
resulting effective field theories. In the case of a single low-energy state,
the resulting theory takes the form of an effective-range expansion in the
strongly interacting channel. We also extend the analysis to include the
effects of the Coulomb interaction between charged particles. The approach is
then applied to the coupled Li and Be channels which couple to
a state of Be very close to the Be threshold. At
next-to-leading order, we are able to get a good description of the Li
phase shift and the Be(n,p)Li cross section using four parameters.
Fits at one order higher are similarly good but the available data are not
sufficient to determine all five parameters uniquely.Comment: 22 pages, 2 figures, RevTeX4, typos corrected, accepted for
publication in European Physical Journal
Observation of Parity Violation in the Omega-minus -> Lambda + K-minus Decay
The alpha decay parameter in the process Omega-minus -> Lambda + K-minus has
been measured from a sample of 4.50 million unpolarized Omega-minus decays
recorded by the HyperCP (E871) experiment at Fermilab and found to be [1.78 +/-
0.19(stat) +/- 0.16(syst)]{\times}10^{-2}. This is the first unambiguous
evidence for a nonzero alpha decay parameter, and hence parity violation, in
the Omega-minus -> Lambda + K-minus decay.Comment: 10 pages, 7 figure
Deconstructing 1S0 nucleon-nucleon scattering
A distorted-wave method is used to analyse nucleon-nucleon scattering in the
1S0 channel. Effects of one-pion exchange are removed from the empirical phase
shift to all orders by using a modified effective-range expansion. Two-pion
exchange is then subtracted in the distorted-wave Born approximation, with
matrix elements taken between scattering waves for the one-pion exchange
potential. The residual short-range interaction shows a very rapid energy
dependence for kinetic energies above about 100 MeV, suggesting that the
breakdown scale of the corresponding effective theory is only 270MeV. This may
signal the need to include the Delta resonance as an explicit degree of freedom
in order to describe scattering at these energies. An alternative strategy of
keeping the cutoff finite to reduce large, but finite, contributions from the
long-range forces is also discussed.Comment: 10 pages, 2 figures (introduction revised, references added; version
to appear in EPJA
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