48 research outputs found
Continuum Moment Equations on the Lattice
An analysis is given as to why one can not directly evaluate continuum moment
equations, i.e., equations involving powers of the position variable times
charge, current, or energy/momentum operators, on the lattice. I examine two
cases: a three point function evaluation of the nucleon magnetic moment and a
four point function (charge overlap) evaluation of the pseudoscalar charge
radius.Comment: 9 pages; 1 ps figur
Lattice calculation of SU(3) flavor breaking ratios in B - anti-B mixing
We present an unquenched lattice calculation for the SU(3) flavor breaking
ratios of the heavy-light decay constants and the matrix
elements. The calculation was performed on lattices with two
dynamical flavors of domain-wall quarks and inverse lattice spacing GeV. Heavy quarks were implemented using an improved lattice
formulation of the static approximation. In the infinite heavy-quark mass limit
we obtain , , where the first error is statistical and the second systematic.Comment: 23 pages, 8 figures, RevTeX4; mentioned existence of 1/m_b
corrections, minor changes improving readabilit
Residual Chiral Symmetry Breaking in Domain-Wall Fermions
We study the effective quark mass induced by the finite separation of the
domain walls in the domain-wall formulation of chiral fermion as the function
of the size of the fifth dimension (), the gauge coupling and the
physical volume . We measure the mass by calculating the small eigenvalues
of the hermitian domain-wall Dirac operator ( in the
topologically-nontrivial quenched SU(3) gauge configurations. We find that the
induced quark mass is nearly independent of the physical volume, decays
exponentially as a function of , and has a strong dependence on the size
of quantum fluctuations controlled by . The effect of the choice of the
lattice gluon action is also studied.Comment: 12 pages, 7 figure
Baryon magnetic moments in the background field method
We present a calculation of the magnetic moments for the baryon octet and
decuplet using the background-field method and standard Wilson gauge and
fermion actions in the quenched approximation of lattice QCD. Progressively
smaller static magnetic fields are introduced on a lattice at beta=6.0
and the pion mass is probed down to about 500 MeV. Magnetic moments are
extracted from the linear response of the masses to the background field.Comment: 15 pages, 7 figures, 1 table, to appear in Phys. Lett.
Relativistic effects in electromagnetic nuclear responses in the quasi-elastic delta region
A new non-relativistic expansion in terms of the nucleon's momentum inside
nuclear matter of the current for isobar electro-excitation from the nucleon is
performed. Being exact with respect to the transferred energy and momentum,
this yields new current operators which retain important aspects of relativity
not taken into account in the traditional non-relativistic reductions. The
transition current thus obtained differs from the leading order of the
traditional expansion by simple multiplicative factors. These depend on the
momentum and energy transfer and can be easily included together with
relativistic kinematics in non-relativistic, many-body models of isobar
electro-excitation in nuclei. The merits of the new current are tested by
comparing with the unexpanded electromagnetic nuclear responses in the isobar
peak computed in a relativistic Fermi gas framework. The sensitivity of the
relativistic responses to the isobar's magnetic, electric and Coulomb form
factors and the finite width of the isobar is analyzed.Comment: 26 pages plus 6 figure
Generalized Parton Distributions and the Spin Structure of the Nucleon
Generalized parton distributions are a new type of hadronic observables which
has recently stimulated great interest among theorists and experimentalists
alike. Introduced to delineate the spin structure of the nucleon, the orbital
angular momentum of quarks in particular, the new distributions contain vast
information about the internal structure of the nucleon, with the usual
electromagnetic form factors and Feynman parton distributions as their special
limits. While new perturbative QCD processes, such as deeply virtual Compton
scattering and exclusive meson production, have been found to measure the
distributions directly in experiments, lattice QCD offers a great promise to
provide the first-principle calculations of these interesting observables.Comment: 9 pages, plenary talk given at Lattice 2002, Cambridge, MA, US
Role of 2p-2h MEC excitations in superscaling
Following recent studies of inclusive electron scattering from nuclei at high
energies which focused on two-nucleon emission mediated by meson-exchange
currents, in this work the superscaling behavior of such contributions is
investigated. Comparisons are made with existing data below the quasielastic
peak where at high momentum transfers scaling of the second kind is known to be
excellent and scaling of the first kind is good, in the proximity of the peak
where both 1p-1h and 2p-2h contributions come into play, and above the peak
where inelasticity becomes important and one finds scaling violations of the
two kinds.Comment: 27 pages, 12 figures; references adde