331 research outputs found
Electromagnetic Polarizabilities: Lattice QCD in Background Fields
Chiral perturbation theory makes definitive predictions for the extrinsic
behavior of hadrons in external electric and magnetic fields. Near the chiral
limit, the electric and magnetic polarizabilities of pions, kaons, and nucleons
are determined in terms of a few well-known parameters. In this limit, hadrons
become quantum mechanically diffuse as polarizabilities scale with the inverse
square-root of the quark mass. In some cases, however, such predictions from
chiral perturbation theory have not compared well with experimental data.
Ultimately we must turn to first principles numerical simulations of QCD to
determine properties of hadrons, and confront the predictions of chiral
perturbation theory. To address the electromagnetic polarizabilities, we
utilize the background field technique. Restricting our attention to
calculations in background electric fields, we demonstrate new techniques to
determine electric polarizabilities and baryon magnetic moments for both
charged and neutral states. As we can study the quark mass dependence of
observables with lattice QCD, the lattice will provide a crucial test of our
understanding of low-energy QCD, which will be timely in light of ongoing
experiments, such as at COMPASS and HI\gamma S.Comment: 3 pages, talk given by B. C. Tiburzi at PANIC 201
A Lattice Test of 1/N_c Baryon Mass Relations
1/N_c baryon mass relations are compared with lattice simulations of baryon
masses using different values of the light-quark masses, and hence different
values of SU(3) flavor-symmetry breaking. The lattice data clearly display both
the 1/N_c and SU(3) flavor-symmetry breaking hierarchies. The validity of 1/N_c
baryon mass relations derived without assuming approximate SU(3)
flavor-symmetry also can be tested by lattice data at very large values of the
strange quark mass. The 1/N_c expansion constrains the form of discretization
effects; these are suppressed by powers of 1/N_c by taking suitable
combinations of masses. This 1/N_c scaling is explicitly demonstrated in the
present work.Comment: 13 pages, 20 figures; v2 version to be published in PR
Extracting Electric Polarizabilities from Lattice QCD
Charged and neutral, pion and kaon electric polarizabilities are extracted
from lattice QCD using an ensemble of anisotropic gauge configurations with
dynamical clover fermions. We utilize classical background fields to access the
polarizabilities from two-point correlation functions. Uniform background
fields are achieved by quantizing the electric field strength with the proper
treatment of boundary flux. These external fields, however, are implemented
only in the valence quark sector. A novel method to extract charge particle
polarizabilities is successfully demonstrated for the first time.Comment: 17 pages, 6 figures, a few clarifications added, published versio
Electromagnetic and spin polarisabilities in lattice QCD
We discuss the extraction of the electromagnetic and spin polarisabilities of
nucleons from lattice QCD. We show that the external field method can be used
to measure all the electromagnetic and spin polarisabilities including those of
charged particles. We then turn to the extrapolations required to connect such
calculations to experiment in the context of finite volume chiral perturbation
theory. We derive results relevant for lattice simulations of QCD,
partially-quenched QCD and quenched QCD. Our results for the polarisabilities
show a strong dependence on the lattice volume and quark masses, typically
differing from the infinite volume limit by ~10% for current lattice volumes
and quark masses.Comment: Minor change
Present Constraints on the H-dibaryon at the Physical Point from Lattice QCD
The current constraints from lattice QCD on the existence of the H-dibaryon
are discussed. With only two significant lattice QCD calculations of the
H-dibaryon binding energy at approximately the same lattice spacing, the forms
of the chiral and continuum extrapolations to the physical point are not
determined. In this brief report, we consider the constraints on the H-dibaryon
imposed by two simple chiral extrapolations. In both instances, the
extrapolation to the physical pion mass allows for a bound H-dibaryon or a
near-threshold scattering state. Further lattice QCD calculations are required
to clarify this situation.Comment: 8 pages, 2 figures, 1 table; revised for the journa
Taming the Pion Cloud of the Nucleon
We present a light-front determination of the pionic contribution to the
nucleon self-energy, , to second-order in pion-baryon coupling
constants that allows the pion-nucleon vertex function to be treated in a
model-independent manner constrained by experiment. The pion mass
dependence of is consistent with chiral perturbation theory
results for small values of and is also linearly dependent on for
larger values, in accord with the results of lattice QCD calculations. The
derivative of with respect to yields the dominant
contribution to the pion content, which is consistent with the
difference observed experimentally in the violation of the
Gottfried sum rule.Comment: 11 pages, 3 figure
High Statistics Analysis using Anisotropic Clover Lattices: (IV) Volume Dependence of Light Hadron Masses
The volume dependence of the octet baryon masses and relations among them are
explored with Lattice QCD. Calculations are performed with n_f=2+1 clover
fermion discretization in four lattice volumes, with spatial extent L ~ 2.0,
2.5, 3.0 and 3.9 fm, with an anisotropic lattice spacing of b_s ~ 0.123 fm in
the spatial direction, and b_t = b_s/3.5 in the time direction, and at a pion
mass of m_pi ~ 390 MeV. The typical precision of the ground-state baryon mass
determination is ~0.2%, enabling a precise exploration of the volume dependence
of the masses, the Gell-Mann--Okubo mass relation, and of other mass
combinations. A comparison of the volume dependence with the predictions of
heavy baryon chiral perturbation theory is performed in both the SU(2)_L X
SU(2)_R and SU(3)_L X SU(3)_R expansions. Predictions of the three-flavor
expansion for the hadron masses are found to describe the observed volume
dependences reasonably well. Further, the Delta-N-pi axial coupling constant is
extracted from the volume dependence of the nucleon mass in the two-flavor
expansion, with only small modifications in the three-flavor expansion from the
inclusion of kaons and etas. At a given value of m_pi L, the finite-volume
contributions to the nucleon mass are predicted to be significantly smaller at
m_pi ~ 140 MeV than at m_pi ~ 390 MeV due to a coefficient that scales as ~
m_pi^3. This is relevant for the design of future ensembles of lattice
gauge-field configurations. Finally, the volume dependence of the pion and kaon
masses are analyzed with two-flavor and three-flavor chiral perturbation
theory.Comment: 34 pages, 45 figure
Light Nuclei and Hypernuclei from Quantum Chromodynamics in the Limit of SU(3) Flavor Symmetry
The binding energies of a range of nuclei and hypernuclei with atomic number
A <= 4 and strangeness |s| <= 2, including the deuteron, di-neutron,
H-dibaryon, 3He, Lambda 3He, Lambda 4He, and Lambda Lambda 4He, are calculated
in the limit of flavor-SU(3) symmetry at the physical strange quark mass with
quantum chromodynamics (without electromagnetic interactions). The nuclear
states are extracted from Lattice QCD calculations performed with n_f=3
dynamical light quarks using an isotropic clover discretization of the
quark-action in three lattice volumes of spatial extent L ~ 3.4 fm, 4.5 fm and
6.7 fm, and with a single lattice spacing b ~ 0.145 fm.Comment: 35 pages, 45 figures Increased statistics, enhanced discussion, fixed
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