109 research outputs found
Radiative Proton Capture Studies at Intermediate Energies
This work was supported by National Science Foundation Grants PHY 76-84033A01, PHY 78-22774, and Indiana Universit
Radiative Proton Capture: Recent Results
Supported by the National Science Foundation and Indiana Universit
Radiative Capture Reactions at Intermediate Energies
This research was sponsored by the National Science Foundation Grant NSF PHY 87-1440
Low--Energy Behavior of Two--Point Functions of Quark Currents
We discuss vector, axial-vector, scalar and pseudoscalar two-point functions
at low and intermediate energies. We first review what is known from chiral
perturbation theory, as well as from a heat kernel expansion within the context
of the extended Nambu-Jona-Lasinio (ENJL) model of ref. \cite{12}. In this work
we derive then these two-point functions to all orders in the momenta and to
leading order in . We find an improved high-energy behaviour and a
general way of parametrizing them that shows relations between some of the
two-point functions, which are also valid in the presence of gluonic
interactions. The similarity between the shape of the experimentally known
spectral functions and the ones we derive, is greatly improved with respect to
those predicted by the usual constituent quark like models. We also obtain the
scalar mass independent of the regularization scheme. In the end,
we calculate fully an example of a nonleptonic matrix element in the
ENJL--model, the electromagnetic mass difference and find good
agreement with the measured value.Comment: 37 pages + 6 uuencoded figures, CERN TH 6924/93, CPT-93/P.2917,
NORDITA 93/43-N,
QCD S Parameter from Inhomogeneous Bethe-Salpeter Equation
We calculate the low-energy parameter S in QCD, which is also known as
L_{10}, and the pion decay constant f_\pi using inhomogeneous Bethe-Salpeter
equation in improved ladder approximation. To extract these quantities we
calculate the ``V-A'' two-point function, \Pi_{VV}(q^2) - \Pi_{AA}(q^2), in
space-like region. We obtain S = 0.43 \sim 0.48, which is about 30% larger than
the experimental value. The calculated f_\pi is well consistent with the result
by solving the homogeneous Bethe-Salpeter equation for pion. We also evaluate
parameter in SU(3) gauge theory with N_D doublets of fermions in connection
with walking technicolor model, and find that the value of S/N_D hardly depends
on N_D.Comment: 22 pages (LaTeX), 6 PostScript figures are included as
uuencoded-compressed-tar file at the end (need 'epsf.tex' macro package),
KUNS-1270 HE(TH)94/0
Low-energy photon-photon collisions to two loops revisited
In view of ongoing experimental activities to determine the pion
polarizabilities, we have started to recalculate the available two-loop
expressions in the framework of chiral perturbation theory, because they have
never been checked before. We make use of the chiral Lagrangian at order p^6
now available, and of improved techniques to evaluate the two-loop diagrams.
Here, we present the result for the neutral pions. The cross section for the
reaction gamma+gamma->pi0+pi0 agrees with the earlier calculation within a
fraction of a percent. We present analytic results for the dipole and
quadrupole polarizabilities, and compare the latter with a recent evaluation
from data on gamma+gamma->pi0+pi0.Comment: 28 pages, 7-eps figures, 1 tabl
Duality violations and spectral sum rules
We study the issue of duality violations in the VV-AA vacuum polarization
function in the chiral limit. This is done with the help of a model with an
expansion in inverse powers of the number of colors, Nc, allowing us to
consider resonances with a finite width. Due to these duality violations, the
Operator Product Expansion (OPE) and the moments of the spectral function (e.g.
the Weinberg sum rules) do not match at finite momentum, and we analyze this
difference in detail. We also perform a comparative study of many of the
different methods proposed in the literature for the extraction of the OPE
parameters and find that, when applied to our model, they all fare quite
similarly. In fact, the model strongly suggests that a significant improvement
in precision can only be expected after duality violations are included. To
this end, we propose a method to parameterize these duality violations. The
method works quite well for the model, and we hope that it may also be useful
in future determinations of OPE parameters in QCD.Comment: 29 pages, 9 figures, LateX file. Small changes to match journal
versio
The Effects of Quantum Entropy on the Bag Constant
The effects of quantum entropy on the bag constant are studied at low
temperatures and small chemical potentials. The inclusion of the quantum
entropy of the quarks in the equation of state provides the hadronic bag with
an additional heat which causes a decrease in the effective latent heat inside
the bag. We have considered two types of baryonic bags, and
. In both cases we have found that the bag constant without the
quantum entropy almost does not change with the temperature and the quark
chemical potential. The contribution from the quantum entropy to the equation
of state clearly decreases the value of the bag constant.Comment: 7 pages, 2 figures (two parts each
Isospin breaking in the vector current of the nucleon
Extraction of the nucleon's strange form factors from experimental data
requires a quantitative understanding of the unavoidable contamination from
isospin violation. A number of authors have addressed this issue during the
past decade, and their work is reviewed here. The predictions from early models
are largely consistent with recent results that rely as much as possible on
input from QCD symmetries and related experimental data. The resulting bounds
on isospin violation are sufficiently precise to be of value to on-going
experimental and theoretical studies of the nucleon's strange form factors.Comment: 5 pages, 3 figures. Presented at the International Workshop "From
Parity Violation to Hadronic Structure and more...", Milos, Greece, 16-20 May
2006. Version 2 is only to update Refs. [21] and [25
Measurements of S and P-A Using the 12-C(p,p'y) Reaction at 150 MeV
This work was supported by the National Science Foundation Grant NSF PHY 81-14339 and by Indiana Universit
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