312,456 research outputs found
Threshold Two-Pion Photo- and Electroproduction: More neutrals than expected
We present an exploratory study of two pion photo-- and electroproduction off
the nucleon in the threshold region. To calculate the pertinent amplitudes, we
make use of heavy baryon chiral perturbation theory. We show that due to finite
chiral loops the production cross section for final states with two neutral
pions is considerably enhanced. The experimental implications are briefly
discussed.Comment: 23pp, plain TeX, 11 figures available upon request, CRN 94/1
SU(3) Flavor Breaking in Hadronic Matrix Elements for Oscillations
We present an analysis, using quenched configurations at 5.7, 5.85,
6.0, and 6.3 of the matrix element \MP\equiv\langle \bar P_{hl}|\bar h
\gamma_\mu (1-\gamma_5)l \bar h \gamma_\mu(1-\gamma_5)l|P_{hl}\rangle for
heavy-light pseudoscalar mesons. The results are extrapolated to the physical
meson states, \Bd and \Bs. We directly compute the ratio \MS/\MB, and
obtain the preliminary result \MS/\MB=1.54(13)(32). A precise value of this
SU(3) breaking ratio is important for determining once the mixing
parameter for \Bs-\bar\Bs is measured experimentally. We also determine
values for the corresponding B parameters, , which we cannot distinguish in the present analysis.Comment: Poster presented at LATTICE96(heavy quarks). LaTeX, uses espcrc2.sty
and epsf, 4 pages, 4 postscript figures include
On Decays in Quenched and Unquenched Chiral Perturbation Theory
We calculate the logarithmic corrections to the matrix elements for
and vacuum (which are used on the lattice to determine
amplitudes), in one-loop quenched and unquenched Chiral
Perturbation Theory. We find that these corrections can be large. We also
discuss, and present some results for, the direct determination of
amplitudes. In particular, we address effects from choosing and
vanishing external spatial momenta, finite volume and quenching. In the
quenched octet case, we find enhanced finite-volume contributions which may
make numerical estimates of this matrix element unreliable for large volumes.Comment: LATTICE99(Matrix Elements), LaTeX(espcrc2.sty), 3p, 1 ps figur
Low--Energy Theorems for Weak Pion Production
We derive novel low--energy theorems for single pion production off nucleons
through the isovector axial current. We find that the -dependence of the
multipole at threshold is given by the nucleon scalar form
factor, namely . The relation to
PCAC results for soft pions including electroweak form factors is also
clarified.Comment: 9 pp, TeX, 2 figures available as ps files, CRN 93-5
Double neutral pion photoproduction at threshold
We consider the chiral expansion of the threshold amplitude for the reaction
to order . We substantiate a
claim that this photoproduction channel is significantly enhanced close to
threshold due to pion loops. A precise measurement of the corresponding cross
sections is called for which allows to test chiral perturbation theory.Comment: 8 pp, LaTe
On the analysis of the pion--nucleon --term: The size of the remainder at the Cheng--Dashen point
We calculate the one--loop contributions of order to the difference
between the on--shell pion--nucleon scattering amplitude
at the Cheng--Dashen point , and
the scalar form factor in the framework of heavy baryon
chiral perturbation theory. We proof that to this order contains
chiral logarithms and therefore it vanishes simply as in the
chiral limit. Numerically, we find as an upper limit MeV.Comment: 7 pp, LaTeX, uses epsf, 1 figure in separate PS fil
Pion-nucleon scattering in an effective chiral field theory with explicit spin-3/2 fields
We analyze elastic-pion nucleon scattering to third order in the so-called
small scale expansion. It is based on an effective Lagrangian including pions,
nucleons and deltas as active degrees of freedom and counting external momenta,
the pion mass and the nucleon-delta mass splitting as small parameters. The
fermion fields are considered as very heavy. We present results for phase
shifts, threshold parameters and the sigma term. We discuss the convergence of
the approach. A detailed comparison with results obtained in heavy baryon
chiral perturbation theory to third and fourth order is also given.Comment: 44 pp, LaTeX2e, uses epsf, 6 fig
Towards the continuum limit with quenched staggered quarks
We extend previous work on finite-size effects with dynamical staggered
quarks to the quenched approximation. We again emphasize the large volume limit
that is of interest for spectrum calculations which may hope to approach the
experimental values. Relying on new calculations at and recent work
with weaker couplings, we extrapolate to the continuum limit and find a nucleon
to rho mass ratio in close agreement with the experimental value and the value
obtained by extrapolations from calculations with Wilson quarks. Additional
calculations that should be done to improve the reliability of the
extrapolation are discussed.Comment: 3 pages, PostScript, Contribution to Lattice '9
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