145 research outputs found
Quark-quark correlations and baryon electroweak observables
The simple independent quark models have difficulties explaining
simultaneously the totality of the known hyperon magnetic moments and hyperon
semi-leptonic decay rates. We show that both the Goldstone boson loop
contributions and the two-quark effective exchange currents are essential in
explaining these observables.Comment: 7 pages, 1 figur
The proton spin sum rule chiral bag prediction, an update
We reevaluate a quark model prediction using the new QCD evolution function
calculated to the 3 loop order and conclude that this model compares favorably
with the new experimental results.Comment: 10 pages, 2 figures available by request, give fax numbe
Neutron beta decay in effective field theory
Radiative corrections to the lifetime and angular correlation coefficients of
neutron beta-decay are evaluated in effecitive field theory. We also evaluate
the lowest order nucleon recoil corrections, including weak-magnetism. Our
results agree with those of the long-range and model-independent part of
previous calculations. In an effective theory the model-dependent radiative
corrections are replaced by well-defined low-energy constants. The effective
field theory allows a systematic evaluation of higher order corrections to our
results to the extent that the relevant low-energy constants are known.Comment: 13 pages, 1 figure; two references added, minor correctio
Toy Model for Pion Production in Nucleon-Nucleon Collisions
We develop a toy model of pion production in nucleon-nucleon collisions that
reproduces some of the features of the chiral Lagrangian calculations. We
calculate the production amplitude and examine some common approximations.Comment: 9 pages, 3 figure
Chiral Perturbation Theory and the pp -> pp pi0 Reaction Near Threshold
A chiral-perturbative consideration of the near-threshold pp -> pp pi0
reaction indicates that the pion-rescattering term has a substantial energy and
momentum dependence. The existing calculations that incorporate this dependence
give pion rescattering contributions significantly larger than those of the
conventional treatment, and this enhanced rescattering term interferes
destructively with the one-body impulse term, leading to theoretical cross
sections that are much smaller than the observed values. However, since the
existing calculations are based on coordinate-space representation, they
involve a number of simplifying assumptions about the energy-momentum flow in
the rescattering diagram, even though the delicate interplay between the
one-body and two-body terms makes it desirable to avoid these kinematical
assumptions. We carry out here a momentum-space calculation that retains the
energy-momentum dependence of the vertices as predicted by chiral perturbation
theory. Our improved treatment increases the rescattering amplitude by a factor
of 3 over the value obtained in the r-space calculations. The pp -> pp pi0
transition amplitude, which is now dominated by the rescattering term, leads to
the cross section much larger than what was reported in the approximate r-space
calculations. Thus, the extremely small cross sections obtained in the previous
chiral perturbative treatments of this reaction should be considered as an
accidental consequence of the approximations employed rather than a general
feature.Comment: 25 pages,REVTEX, 5 ps figure
A next-to-next-to-leading-order transition operator in chiral perturbation theory
We present a systematic analysis of next-to-next-to-leading-order diagrams
that contribute to the production at threshold. Analytic
expressions are given for the effective transition operators, and the relative
importance of various types of diagrams is discussed. The
vertex-correction-type graphs are found to give only small corrections to lower
order graphs in conformity with expectations. By contrast, we find very large
contributions from the two-pion graphs which can be interpreted as a part of
effective -meson exchange diagrams. The recoil correction to the pion
rescattering diagram also turns out to be large.Comment: 15 pages, 6 (eps) figures, corrected the sign of type VII graph in
Table II, changed one sentence in the text accordingl
Neutrino-deuteron reactions at solar neutrino energies
In interpreting the SNO experiments, very accurate estimates of the \nu d
reaction cross sections are of great importance. We improve the previous
estimates of our group by updating some of its inputs and by taking into
account the results of a recent effective-field-theoretical calculation. The
new cross sections are slightly (\sim 1%) larger than the previously reported
values. We present arguments that lead to the conclusion that it is reasonable
to assign 1% uncertainty to the \nu d cross sections reported here.Comment: 13 pages, 1 figur
Two-pion-exchange contributions to the pp\to pp\pi^0 reaction
Our previous study of the near-threshold pp\to pp\pi^0 reaction based on a
hybrid nuclear effective field theory is further elaborated by examining the
momentum dependence of the relevant transition operators. We show that the
two-pion exchange diagrams give much larger contributions than the one-pion
exchange diagram, even though the former is of higher order in the Weinberg
counting scheme. The relation between our results and an alternative counting
scheme, the momentum counting scheme, is also discussed.Comment: 11 pages, minor change, a few references are adde
Chiral Perturbation Approach to the pp -> pp pi0 Reaction Near Threshold
The usual theoretical treatments of the near-threshold reaction are based on various phenomenological Lagrangians. In this
work we examine the relationship between these approaches and a systematic
chiral perturbation method. Our chiral perturbation calculation indicates that
the pion rescattering term should be significantly enhanced as compared with
the traditional phenomenological treatment, and that this term should have
substantial energy and momentum dependence. An important consequence of this
energy-momentum dependence is that, for a representative threshold kinematics
and within the framework of our semiquantitative calculation, the rescattering
term interferes destructively with the Born-term in sharp contrast to the
constructive interference obtained in the conventional treatment. This
destructive interference makes theoretical cross sections for much smaller than the experimental values, a feature that suggests the
importance of the heavy-meson exchange contributions to explain the
experimental data.Comment: 35 pages (REVTeX), 5 figures as 1 PostScript file acknowledgement
changed, reference added, Phys.Rev.C (in print
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