7,669 research outputs found
New insights into the spin structure of the nucleon
We analyze the low-energy spin structure of the nucleon in a covariant
effective field theory with explicit spin-3/2 degrees of freedom to third order
in the small scale expansion. Using the available data on the strong and
electromagnetic width of the Delta-resonance, we give parameter-free
predictions for various spin-polarizabilities and moments of spin structure
functions. We find an improved description of the nucleon spin structure at
finite photon virtualities for some observables and point out the necessity of
a fourth order calculation.Comment: 13 pages, 6 figure
Improved analysis of neutral pion electroproduction off deuterium in chiral perturbation theory
Near threshold neutral pion electroproduction on the deuteron is studied in
the framework of heavy baryon chiral perturbation theory. We include the
next-to-leading order corrections to the three-body contributions. We find an
improved description of the total and differential cross section data measured
at MAMI. We also obtain more precise values for the threshold S-wave
multipoles. We discuss in detail the theoretical uncertainties of the
calculation.Comment: 14 pp, 11 figs, uses svjour.cl
Subleading contributions to the chiral three-nucleon force II: Short-range terms and relativistic corrections
We derive the short-range contributions and the leading relativistic
corrections to the three-nucleon force at next-to-next-to-next-to-leading order
in the chiral expansion.Comment: 14 pages, 4 figure
Scaling studies of QCD with the dynamical HISQ action
We study the lattice spacing dependence, or scaling, of physical quantities
using the highly improved staggered quark (HISQ) action introduced by the
HPQCD/UKQCD collaboration, comparing our results to similar simulations with
the asqtad fermion action. Results are based on calculations with lattice
spacings approximately 0.15, 0.12 and 0.09 fm, using four flavors of dynamical
HISQ quarks. The strange and charm quark masses are near their physical values,
and the light-quark mass is set to 0.2 times the strange-quark mass. We look at
the lattice spacing dependence of hadron masses, pseudoscalar meson decay
constants, and the topological susceptibility. In addition to the commonly used
determination of the lattice spacing through the static quark potential, we
examine a determination proposed by the HPQCD collaboration that uses the decay
constant of a fictitious "unmixed s bar s" pseudoscalar meson. We find that the
lattice artifacts in the HISQ simulations are much smaller than those in the
asqtad simulations at the same lattice spacings and quark masses.Comment: 36 pages, 11 figures, revised version to be published. Revisions
include discussion of autocorrelations and several clarification
Universal decay of scalar turbulence
The asymptotic decay of passive scalar fields is solved analytically for the
Kraichnan model, where the velocity has a short correlation time. At long
times, two universality classes are found, both characterized by a distribution
of the scalar -- generally non-Gaussian -- with global self-similar evolution
in time. Analogous behavior is found numerically with a more realistic flow
resulting from an inverse energy cascade.Comment: 4 pages, 3 Postscript figures, submitted to PR
Renormalization of the Lattice HQET Isgur-Wise Function
We compute the perturbative renormalization factors required to match to the
continuum Isgur-Wise function, calculated using lattice Heavy Quark Effective
Theory. The velocity, mass, wavefunction and current renormalizations are
calculated for both the forward difference and backward difference actions for
a variety of velocities. Subtleties are clarified regarding tadpole
improvement, regulating divergences, and variations of techniques used in these
renormalizations.Comment: 28 pages, 0 figures, LaTeX. Final version accepted for publication in
Phys. Rev. D. (Minor changes.
Nuclear forces with Delta-excitations up to next-to-next-to-leading order I: peripheral nucleon-nucleon waves
We study the two-nucleon force at next-to-next-to-leading order in a chiral
effective field theory with explicit Delta degrees of freedom. Fixing the
appearing low-energy constants from a next-to-leading order calculation of
pion-nucleon threshold parameters, we find an improved convergence of most
peripheral nucleon-nucleon phases compared to the theory with pions and
nucleons only. In the delta-full theory, the next-to-leading order corrections
are dominant in most partial waves considered.Comment: 16 pages, 10 figure
Isolation of a genomic clone partially encoding human hypoxanthine phosphoribosyltransferase.
Acute traumatic aortic rupture: early stent-graft repair
Objective: Prospective evaluation of early stent-graft repair of acute traumatic aortic rupture. Methods: Twelve patients with acute traumatic aortic rupture of the descending aorta, out of a series of 337 endovascular aortic procedures, were treated by implantation of self-expanding stent-grafts. The procedures were performed within a mean post-injury time-period of 5±7 days (median: 1 day). The feasibility of stent-grafting was assessed by CT scanning and echography. Implantation was performed under local (n=6), or general anesthesia (n=6) if patients were already intubated (n=5) or required a common iliac artery access (n=1). Results: The immediate technical success rate was 100%. There were no post-procedure complications in all but one patient, who died 12 h postoperatively (8% mortality). Complete sealing of the aortic rupture in the remaining 11 patients was confirmed by postoperative CT scans. There were no intervention-related morbidity or mortality during the mean follow-up of 17 months. One patient with peri-graft leakage was successfully repaired with an additional stent-graft 12 months postoperatively. Conclusion: Non-delayed or early stent-grafting in acute traumatic rupture of the descending aorta is feasible. This technique seems to be a valuable option, in particular when associated lesions may interfere with the surgical outcome. Immediate post-procedural CT scanning and/or echography should be performed, in order to rule out residual leakag
Emission times and opacities from interferometry in non-central Relativistic Nuclear Collisions
The nuclear overlap zone in non-central relativistic heavy ion collisions is
azimuthally very asymmetric. By varying the angle between the axes of
deformation and the transverse direction of the pair momenta, the transverse
HBT radii oscillate in a characteristic way. It is shown that these
oscillations allow determination of source sizes, deformations as well as the
opacity and duration of emission of the source created in any non-central high
energy nuclear collisions. The behavior of the physical quantities with
centrality of the collisions is discussed --- in particular changes caused by a
possible phase transition to a quark-gluon plasma.Comment: Revised version, to appear in Phys. Rev. Letter
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