19,595 research outputs found
Chiral extrapolation of nucleon magnetic form factors
The extrapolation of nucleon magnetic form factors calculated within lattice
QCD is investigated within a framework based upon heavy baryon chiral
effective-field theory. All one-loop graphs are considered at arbitrary
momentum transfer and all octet and decuplet baryons are included in the
intermediate states. Finite range regularisation is applied to improve the
convergence in the quark-mass expansion. At each value of the momentum transfer
(), a separate extrapolation to the physical pion mass is carried out as a
function of alone. Because of the large values of involved, the
role of the pion form factor in the standard pion-loop integrals is also
investigated. The resulting values of the form factors at the physical pion
mass are compared with experimental data as a function of and demonstrate
the utility and accuracy of the chiral extrapolation methods presented herein.Comment: 19 pages, 10 figure
On the pion cloud of the nucleon
We evaluate the two--pion contribution to the nucleon electromagnetic form
factors by use of dispersion analysis and chiral perturbation theory. After
subtraction of the rho--meson component, we calculate the distributions of
charge and magnetization in coordinate space, which can be interpreted as the
effects of the pion cloud. We find that the charge distribution of this pion
cloud effect peaks at distances of about 0.3 fm. Furthermore, we calculate the
contribution of the pion cloud to the isovector charges and radii of the
nucleon.Comment: 7 pages, latex, 3 ps figures, minor change
Enhanced chiral logarithms in partially quenched QCD
I discuss the properties of pions in ``partially quenched'' theories, i.e.
those in which the valence and sea quark masses, and , are
different. I point out that for lattice fermions which retain some chiral
symmetry on the lattice, e.g. staggered fermions, the leading order prediction
of the chiral expansion is that the mass of the pion depends only on , and
is independent of . This surprising result is shown to receive corrections
from loop effects which are of relative size , and which thus
diverge when the valence quark mass vanishes. Using partially quenched chiral
perturbation theory, I calculate the full one-loop correction to the mass and
decay constant of pions composed of two non-degenerate quarks, and suggest
various combinations for which the prediction is independent of the unknown
coefficients of the analytic terms in the chiral Lagrangian. These results can
also be tested with Wilson fermions if one uses a non-perturbative definition
of the quark mass.Comment: 14 pages, 3 figures, uses psfig. Typos in eqs (18)-(20) corrected
(alpha_4 is replaced by alpha_4/2
Physical Nucleon Properties from Lattice QCD
We demonstrate that the extremely accurate lattice QCD data for the mass of
the nucleon recently obtained by CP-PACS, combined with modern chiral
extrapolation techniques, leads to a value for the mass of the physical nucleon
which has a systematic error of less than one percent.Comment: 4 pages, 2 figure
Extinction and dust/gas ratio in LMC molecular clouds
Aims. The goal of this paper is to measure the dust content and distribution in the Large Magellanic Cloud (LMC) by comparing extinction maps produced in the near-infrared wavelengths and the spatial distribution of the neutral and molecular gas, as traced by Hi and CO observations.
Methods. In order to derive an extinction map of the LMC, we have developed a new method to measure the color excess of dark clouds, using the 2MASS all-sky survey. Classical methods to measure the color excess (including the NICE method) tend to underestimate the true color excess if the clouds are significantly contaminated by unreddened foreground stars, as is the case in the LMC. We propose a new method that uses the color of the X percentile reddest stars and which is robust against such contamination. Using this method, it is possible to infer the positions of dark clouds with respect to the star distribution by comparing the observed color excess as a function of the percentile used and that predicted by a model.
Results. On the basis of the resulting extinction map, we perform a correlation analysis for a set of dark molecular clouds. Assuming similar infrared absorption properties for the dust in the neutral and molecular phases, we derive the absorption-to-column density ratio AV/NH and the CO-to-H2 conversion factor X_(CO). We show that AV/NH increases from the outskirts of the LMC towards the 30 Dor star-forming region. This can be explained either by a systematic increase of the dust abundance, or by the presence of an additional gas component not traced by Hi or CO, but strongly correlated to the Hi distribution. If dust abundance is allowed to vary, the derived X_(CO) factors for the selected regions are several times lower than those derived from a virial analysis of the CO data. This could indicate that molecular clouds in the LMC are not gravitationally bound, or that they are bounded by substantial external pressure. However, the X_(CO) values derived from absorption can be reconciled with the virial results assuming a constant value for the dust abundance and the existence of an additional, unseen gas component. These results are in agreement with those derived for the LMC from diffuse far-infrared emission
Chiral Prediction for the Scattering Length to Order
We evaluate the S-wave pion--nucleon scattering length in the framework
of heavy baryon chiral perturbation theory up--to--and--including terms of
order . We show that the order piece of the isovector
amplitude at threshold, , vanishes exactly. We predict for the
isovector scattering length, .Comment: 5 pp, LaTeX file, 2 figures (appended as separate compressed tar
file, amin.uu
A Fast Algorithm for Simulating the Chordal Schramm-Loewner Evolution
The Schramm-Loewner evolution (SLE) can be simulated by dividing the time
interval into N subintervals and approximating the random conformal map of the
SLE by the composition of N random, but relatively simple, conformal maps. In
the usual implementation the time required to compute a single point on the SLE
curve is O(N). We give an algorithm for which the time to compute a single
point is O(N^p) with p<1. Simulations with kappa=8/3 and kappa=6 both give a
value of p of approximately 0.4.Comment: 17 pages, 10 figures. Version 2 revisions: added a paragraph to
introduction, added 5 references and corrected a few typo
Capture rate and neutron helicity asymmetry for ordinary muon capture on hydrogen
Applying heavy-baryon chiral perturbation theory to ordinary muon capture
(OMC) on a proton, we calculate the capture rate and neutron helicity asymmetry
up to next-to-next-to-leading order. For the singlet hyperfine state, we obtain
the capture rate Gamma_0 = 695 sec^{-1} while, for the triplet hyperfine state,
we obtain the capture rate Gamma_1 = 11.9 sec^{-1} and the neutron asymmetry
alpha_1 = 0.93. If the existing formalism is used to relate these atomic
capture rates to Gamma_{liq}, the OMC rate in liquid hydrogen, then Gamma_{liq}
corresponding to our improved values of Gamma_0 and Gamma_1 is found to be
significantly larger than the experimental value, primarily due to the updated
larger value of g_A. We argue that this apparent difficulity may be correlated
to the specious anomaly recently reported for mu^- + p to n + nu_mu + gamma,
and we suggest a possibility to remove these two "problems" simply and
simultaneously by reexamining the molecular physics input that underlies the
conventional analysis of Gamma_{liq}.Comment: 14 pages, 1 figur
The Strangeness Radius and Magnetic Moment of the Nucleon Revisited
We update Jaffe's estimate of the strange isoscalar radius and magnetic
moment of the nucleon. We make use of a recent dispersion--theoretical fit to
the nucleon electromagnetic form factors and an improved description of
symmetry breaking in the vector nonet. We find ~n.m.
and ~fm. The strange formfactor follows
a dipole with a cut--off mass of 1.46~GeV, . These numbers should be considered as upper limits on the
strange vector current matrix--elements in the nucleon.Comment: 8 pp, LaTeX, uses epsf, 1 figure in separate fil
Damage spreading and coupling in Markov chains
In this paper, we relate the coupling of Markov chains, at the basis of
perfect sampling methods, with damage spreading, which captures the chaotic
nature of stochastic dynamics. For two-dimensional spin glasses and hard
spheres we point out that the obstacle to the application of perfect-sampling
schemes is posed by damage spreading rather than by the survey problem of the
entire configuration space. We find dynamical damage-spreading transitions
deeply inside the paramagnetic and liquid phases, and show that critical values
of the transition temperatures and densities depend on the coupling scheme. We
discuss our findings in the light of a classic proof that for arbitrary Monte
Carlo algorithms damage spreading can be avoided through non-Markovian coupling
schemes.Comment: 6 pages, 8 figure
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