102 research outputs found
The interaction in nuclear matter from a study of the reactions
The pion-production reactions were studied on
, , , and nuclei at an incident pion energy
of =283 MeV. Pions were detected in coincidence using the CHAOS
spectrometer. The experimental results are reduced to differential cross
sections and compared to both theoretical predictions and the reaction phase
space. The composite ratio between the
invariant masses on nuclei and on the nucleon is also presented. Near the
threshold pion pairs couple to when produced in
the reaction channel. There is a marked near-threshold
enhancement of which is consistent with theoretical
predictions addressing the partial restoration of chiral symmetry in nuclear
matter. Furthermore, the behaviour of is well
described when the restoration of chiral symmetry is combined with standard
P-wave renormalization of pions in nuclear matter. On the other hand, nuclear
matter only weakly influences , which displays a flat
behaviour throughout the energy range regardless of .Comment: 30 pages, 16 figures, PS format, accepted for publication in Nucl.
Phys
results in nuclei
The Crystal Ball () collaboration at has recently presented results
regarding a study of the reaction on and , using a nearly 4 detector. Similar results, but for the
reaction on , , ,
and , have been published earlier by the collaboration at
. In this Brief Report a comparison of the results of the two
measurements is made, which shows that the and data share relevant
common features. In particular, the increase in strength as a function of A
seen in the near-threshold invariant mass spectra reported by the
group, is also seen in the CB data, when the results from
the two groups are compared in a way which accounts for the different
acceptances of the two experiments.Comment: 7 pages, 1 figure, accepted for publication in Phy. Rev. C - Brief
Repor
In-medium pi-pi Correlation Induced by Partial Restoration of Chiral Symmetry
We show that both the linear and the non-linear chiral models give an
enhancement of the pi-pi cross section near the 2pi threshold in the
scalar-iso-scalar (I=J=0) channel in nuclear matter. The reduction of the
chiral condensate, i.e., the partial chiral restoration in nuclear matter, is
responsible for the enhancement in both cases. We extract an effective
4pi-nucleon vertex which is responsible for the enhancement but has not been
considered in the non-liear models for in-medium pi-pi interaction. Relation of
this vertex and a next-to-leading order terms in the heavy-baryon chiral
lagrangian, L_piN^(2), is also discussed.Comment: 5 pages, 5 eps figure, REVTe
Enhancement of Threshold Cross Sections by In-Medium Final State Interactions
We address the problem of pion production in low energy -nucleus
collisions. For the production mechanism we assume a simple model consisting of
a coherent sum of single pion exchange and the excitation---followed by the
decay into two pions and a nucleon---of the resonance. The
production amplitude is modified by the final state interaction between the
pions calculated using the chirally improved J\"ulich meson exchange model
including the polarization of the nuclear medium by the pions. The model
reproduces well the experimentally observed cross
sections, especially the enhancement with increasing of the
mass distribution in the threshold region.Comment: 5 pages RevTeX, 3-eps figure
Spectral functions in the sigma-channel near the critical end point
Spectral functions in the -channel are investigated near the chiral
critical end point (CEP), that is, the point where the chiral phase transition
ceases to be first-ordered in the -plane of the QCD phase diagram. At
that point the meson becomes massless in spite of explicit breaking of
the chiral symmetry. It is expected that experimental signatures peculiar to
CEP can be observed through spectral changes in the presence of abnormally
light mesons. As a candidate, the invariant-mass spectrum for diphoton
emission is estimated with the chiral quark model incorporated. The results
show the characteristic shape with a peak in the low energy region, which may
serve as a signal for CEP. However, we find that the diphoton multiplicity is
highly suppressed by infrared behaviors of the meson. Experimentally,
in such a low energy region below the threshold of two pions, photons from
are major sources of the background for the signal.Comment: 12 pages, 8 figures, 1 figure replaced, minor modification
Surface and volume effects in the photoabsorption of nuclei
Recent experimental results for meson photoproduction from nuclei obtained
with TAPS at MAMI are analyzed in view of the suppression of the second nucleon
resonance region in total photoabsorption. The cross sections can be split into
a component from the low density surface region of nuclei and a component which
scales more like the nuclear volume. The energy dependence of the surface
component is similar to the deuteron cross section, it shows a clear signal for
the second resonance peak assigned to the excitation of the P11(1440),
D13(1520), and S11(1535). The volume component behaves differently, it is
lacking the second resonance peak and shows an enhancement at intermediate
photon energies.Comment: accepted for publication in Eur. J. Phys.
Recent progress on the chiral unitary approach to meson meson and meson baryon interactions
We report on recent progress on the chiral unitary approach, analogous to the
effective range expansion in Quantum Mechanics, which is shown to have a much
larger convergence radius than ordinary chiral perturbation theory, allowing
one to reproduce data for meson meson interaction up to 1.2 GeV. Applications
to physical processes so far unsuited for a standard chiral perturbative
approach are presented. Results for the extension of these ideas to the meson
baryon sector are discussed, together with applications to kaons in a nuclear
medium and atoms.Comment: Contribution to the KEK Tanashi Symposium on Physics of Hadrons and
Nuclei, Tokyo, December 1998, 10 pages, 3 postscript figures. To be published
as a special issue of Nuclear Physics
In-medium modifications of the interaction in photon-induced reactions
Differential cross sections of the reactions
and have been measured for several
nuclei (H,C, and Pb) at an incident-photon energy of
=400-460 MeV at the tagged-photon facility at MAMI-B using the TAPS
spectrometer. A significant nuclear-mass dependence of the
invariant-mass distribution is found in the channel. This
dependence is not observed in the channel and is
consistent with an in-medium modification of the interaction in the
==0 channel. The data are compared to -induced measurements and to
calculations within a chiral-unitary approach
Medium Modification of The Pion-Pion Interaction at Finite Density
We discuss medium modifications of the unitarized pion-pion interaction in
the nuclear medium. We incorporate both the effects of chiral symmetry
restoration and the influence of collective nuclear pionic modes originating
from the p-wave coupling of the pion to delta-hole configurations. We show in
particular that the dropping of the sigma meson mass significantly enhances the
low energy structure created by the in-medium collective pionic modes.Comment: 26 pages, 7 figures included, Latex fil
Polarization Properties of Low Energy Amplitude for Reaction
The theoretical study of cross sections for polarized--target measurements of
reactions gives evidence that the interplay between the
strong contribution from OPE mechanism and the one from isobar exchanges, which
is equally strong within isobar half--widths energy region, must result in
nontrivial polarization phenomena. The Monte--Carlo simulations for asymmetries
in reaction at ~MeV/c with the use of theoretical amplitudes found as solutions for
unpolarized data at ~MeV/c provide confirmations for
significant effect. The effect is capable to discriminate between the OPE and
isobar exchanges and it is sensitive to the OPE parameters in question. This
leads to the conclusion that the decisive analysis,
aiming at determination of --scattering lengths, must combine both
unpolarized data and polarization information. The appropriate measurements are
shown to be feasible at the already existing CHAOS spectrometer.Comment: 24 pages, Latex, 6 Postscript figures, uses epsf.st
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