39,506 research outputs found
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
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
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
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
A detector of small harmonic displacements based on two coupled microwave cavities
The design and test of a detector of small harmonic displacements is
presented. The detector is based on the principle of the parametric conversion
of power between the resonant modes of two superconducting coupled microwave
cavities. The work is based on the original ideas of Bernard, Pegoraro, Picasso
and Radicati, who, in 1978, suggested that superconducting coupled cavities
could be used as sensitive detectors of gravitational waves, and on the work of
Reece, Reiner and Melissinos, who, {in 1984}, built a detector of this kind.
They showed that an harmonic modulation of the cavity length l produced an
energy transfer between two modes of the cavity, provided that the frequency of
the modulation was equal to the frequency difference of the two modes. They
achieved a sensitivity to fractional deformations of dl/l~10^{-17} Hz^{-1/2}.
We repeated the Reece, Reiner and Melissinos experiment, and with an improved
experimental configuration and better cavity quality, increased the sensitivity
to dl/l~10^{-20} Hz^{-1/2}. In this paper the basic principles of the device
are discussed and the experimental technique is explained in detail. Possible
future developments, aiming at gravitational waves detection, are also
outlined.Comment: 28 pages, 12 eps figures, ReVteX. \tightenlines command added to
reduce number of pages. The following article has been accepted by Review of
Scientific Instruments. After it is published, it will be found at
http://link.aip.org/link/?rs
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
The Reaction at Threshold
We consider the chiral expansion for the reaction in
heavy baryon chiral perturbation theory. To order we derive novel
low--energy theorems that compare favorably with recent determinations of the
total cross sections for and .Comment: 7 pp, LateX (uses epsf.sty), 3 figures appended as ps files (split
off as ppnf1.ps,ppnf2.ps,ppnf3.ps), CRN 94/1
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