299,945 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 $B - \bar B$ Oscillations

We present an analysis, using quenched configurations at $6/g^2=$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
$B$ 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 $V_{td}$ once the mixing
parameter $x_s$ for \Bs-\bar\Bs is measured experimentally. We also determine
values for the corresponding B parameters, $B_{bs}(2 \rm{GeV})=B_{bd}(2
\rm{GeV})=1.02(13)$, 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 $K\to\pi\pi$ Decays in Quenched and Unquenched Chiral Perturbation Theory

We calculate the logarithmic corrections to the matrix elements for
$K^+\to\pi^+$ and $K\to$vacuum (which are used on the lattice to determine
$K\to\pi\pi$ 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 $K\to\pi\pi$
amplitudes. In particular, we address effects from choosing $m_s=m_d$ 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 $k^2$-dependence of the
multipole $L_{0+}^{(+)}$ at threshold is given by the nucleon scalar form
factor, namely $\sigma(k^2-M_\pi^2 ) /(3 \pi M_\pi F_\pi )$. 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
$\gamma p \to \pi^0 \pi^0 p$ to order ${\cal O}(M_\pi^2)$. 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 $\sigma$--term: The size of the remainder at the Cheng--Dashen point

We calculate the one--loop contributions of order $M_\pi^4$ to the difference
$\Delta_R$ between the on--shell pion--nucleon scattering amplitude
$\bar{D}^+(0,2M_\pi^2)$ at the Cheng--Dashen point $\nu =0$, $t=2M_\pi^2$ and
the scalar form factor $\sigma (2M_\pi^2)$ in the framework of heavy baryon
chiral perturbation theory. We proof that to this order $\Delta_R$ contains
$no$ chiral logarithms and therefore it vanishes simply as $M_\pi^4$ in the
chiral limit. Numerically, we find as an upper limit $\Delta_R \simeq 2\,$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 $6/g^2=5.7$ 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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