138,605 research outputs found
Excited Baryons and Chiral Symmetry Breaking of QCD
N* masses in the spin-1/2 and spin-3/2 sectors are computed using two
non-perturbative methods: lattice QCD and QCD sum rules. States with both
positive and negative parity are isolated via parity projection methods. The
basic pattern of the mass splittings is consistent with experiments. The mass
splitting within the same parity pair is directly linked to the chiral symmetry
breaking QCD.Comment: contribution to Baryon 2002, 5 pages, 3 figure
Path Integrals in Lattice Quantum Chromodynamics
I discuss the use of path integrals to study strong-interaction physics from
first principles. The underlying theory is cast into path integrals which are
evaluated numerically using Monte Carlo methods on a space-time lattice.
Examples are given on progress related to nuclear physics.Comment: 6 pages, 5 figures. Talk given at the International Conference on
Path Integrals - New Trends and Perspectives, 23-28 September 2007, Dresden,
German
Magnetic Moments of and from QCD Sum Rules
QCD sum rules for the magnetic moments of and are
derived using the external field method. They are analyzed by a Monte-Carlo
based procedure, using realistic estimates of the QCD input parameters. The
results are consistent with the measured values, despite relatively large
errors that can be attributed mostly to the poorly-known vacuum susceptibility
. It is shown that a 30% level accuracy can be achieved in the derived
sum rules, provided the QCD input parameters are improved to the 10% level.Comment: 10 pages, RevTex, 1 PS figure embedded with psfig.st
Neutron electric polarizability
We use the background field method to extract the "connected" piece of the
neutron electric polarizability. We present results for quenched simulations
using both clover and Wilson fermions and discuss our experience in extracting
the mass shifts and the challenges we encountered when we lowered the quark
mass. For the neutron we find that as the pion mass is lowered below 500\MeV,
the polarizability starts rising in agreement with predictions from chiral
perturbation theory. For our lowest pion mass, m_\pi=320\MeV, we find that
\alpha_n = 3.8(1.3)\times 10^{-4}\fm^3, which is still only one third of the
experimental value. We also present results for the neutral pion; we find that
its polarizability turns negative for pion masses smaller than 500\MeV which
is puzzling.Comment: 11 pages, 8 figures, presented at the XXVII International Symposium
on Lattice Field Theory, July 26-31, 2009, Peking University, Beijing, Chin
Neutron electric polarizability from unquenched lattice QCD using the background field approach
A calculational scheme for obtaining the electric polarizability of the
neutron in lattice QCD with dynamical quarks is developed, using the background
field approach. The scheme differs substantially from methods previously used
in the quenched approximation, the physical reason being that the QCD ensemble
is no longer independent of the external electromagnetic field in the dynamical
quark case. One is led to compute (certain integrals over) four-point
functions. Particular emphasis is also placed on the physical role of constant
external gauge fields on a finite lattice; the presence of these fields
complicates the extraction of polarizabilities, since it gives rise to an
additional shift of the neutron mass unrelated to polarizability effects. The
method is tested on a SU(3) flavor-symmetric ensemble furnished by the MILC
Collaboration, corresponding to a pion mass of m_pi = 759 MeV. Disconnected
diagrams are evaluated using stochastic estimation. A small negative electric
polarizability of alpha =(-2.0 +/- 0.9) 10^(-4) fm^3 is found for the neutron
at this rather large pion mass; this result does not seem implausible in view
of the qualitative behavior of alpha as a function of m_pi suggested by Chiral
Effective Theory.Comment: 36 pages, 11 figures. Note added concerning analytic continuation in
the external electric field; some notation made more precis
Scattering phaseshift formulas for mesons and baryons in elongated boxes
We derive L\"{u}scher phaseshift formulas for two-particle states in boxes
elongated in one of the dimensions. Such boxes offer a cost-effective way of
varying the relative momentum of the particles. Boosted states in the elongated
direction, which allow wider access to energies, are also considered. The
formulas for the various scenarios (moving and zero-momentum states in cubic
and elongated boxes) are compared and relations between them are clarified. The
results are applicable to a wide set of meson-meson and meson-baryon elastic
scattering processes, with the two-particle system having equal or unequal
masses.Comment: 28 pages, 2 figures, 16 tables. Updated to match published versio
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