382 research outputs found
General properties of the pion production reaction in nuclear matter
The pion production reaction on was
studied at incident pion energies of = 240, 260, 280, 300, and
320 MeV. The experiment was performed using the pion-channel at TRIUMF,
and multiparticle events, () and
(), were detected with the CHAOS spectrometer. Results
are reported in the form of both differential and total cross sections, and are
compared to theoretical predictions and the reaction phase space. The present
investigation of the T-dependence of the
reaction complements earlier examinations of the A-dependence of the reaction,
which was measured using , , , , , and
targets at 280 MeV. Some general properties of the
pion-induced pion production reaction in nuclear matter will be presented,
based on the combined results of the two studies.Comment: 23 pages, Latex, accepted for publication in Nucl. Phys.
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
Hadrons in Nuclei: Experiments and Perspectives
The question of the origin of hadron masses is one major issue in the
understanding of the strong interaction. The challenge is addressed by
searching for indications of in-medium modifications of hadron properties and
studying hadrons in nuclei. The quest driving in-medium studies is to
understand the origin of hadron masses in the context of spontaneous chiral
symmetry breaking.
The experimental status of the modification of hadron properties in the
nuclear medium is discussed including experiments using hadron, heavy-ion, and
photon beams. Particular emphasis is put on the production of light mesons from
nuclei.
A number of experimental programs is underway to provide a detailed
comparison of properties of free hadrons and hadrons embedded in nuclei. The
existing experimental efforts are discussed and possibilities are introduced
for the new WASA-at-COSY facility, initially focussed on investigations of
symmetries and symmetry breaking, to contribute to the field.Comment: To appear in the proceedings of 23rd International Nuclear Physics
Conference (INPC 2007), Tokyo, Japan, 3-8 Jun 200
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
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
The pi -> pi pi process in nuclei and the restoration of chiral symmetry
The results of an extensive campaign of measurements of the pi -> pi pi
process in the nucleon and nuclei at intermediate energies are presented. The
measurements were motivated by the study of strong pi pi correlations in
nuclei. The analysis relies on the composite ratio C_{pi pi}^A, which accounts
for the clear effect of the nuclear medium on the (pi pi) system. The
comparison of the C_{pi pi}^A distributions for the (pi pi)_{I=J=0} and (pi
pi)_{I=0,J=2} systems to the model predictions indicates that the C_{pi pi}^A
behavior in proximity of the 2m_pi threshold is explainable through the partial
restoration of chiral symmetry in nuclei.Comment: accepted for publication in Nucl. Phys.
In-medium chiral perturbation theory beyond the mean-field approximation
An explicit expression of the generating functional of two-flavor low-energy
QCD with external sources in the presence of non-vanishing nucleon densities
has been derived recently [1]. Within this approach we derive power counting
rules for the calculation of in-medium pion properties. We develop the
so-called standard rules for residual nucleon energies of the order of the pion
mass and a modified scheme (non-standard counting) for vanishing residual
nucleon energies. We also establish the different scales for the range of
applicability of this perturbative expansion, which are \sqrt{6}\pi f_\pi\simeq
0.7 GeV for the standard and 6\pi^2 f_\pi^2/2m_N\simeq 0.27 GeV for
non-standard counting, respectively. We have performed a systematic analysis of
n-point in-medium Green functions up to and including next-to-leading order
when the standard rules apply. These include the in-medium contributions to
quark condensates, pion propagators, pion masses and couplings of the
axial-vector, vector and pseudoscalar currents to pions. In particular, we find
a mass shift for negatively charged pions in heavy nuclei that agrees with
recent determinations from deeply bound pionic Pb-207. We have also established
the absence of in-medium renormalization in the \pi^0 \to \gamma\gamma decay
amplitude up to the same order. The study of \pi\pi scattering requires the use
of the non-standard counting and the calculation is done at leading order. Even
at that order we establish new contributions not considered so far. We also
point towards further possible improvements of this scheme and touch upon its
relation to more conventional many-body approaches.Comment: 40 pages, 12 figures, version to appear in Ann. Phy
Low-energy pions in nuclear matter and 2pi photoproduction within a BUU transport model
A description of low-energy scattering of pions and nuclei within a BUU
transport model is presented. Implementing different scenarios of medium
modifications, the mean free path of pions in nuclear matter at low momenta and
pion absorption reactions on nuclei have been studied and compared to data and
to results obtained via quantum mechanical scattering theory. We show that even
in a regime of a long pionic wave length the semi-classical transport model is
still a reliable framework for pion kinetic energies greater than ~20-30 MeV.
Results are presented on pion-absorption cross sections in the regime of 10 MeV
< E(kin) < 130 MeV and on photon-induced double-pion production at incident
beam energies of 400-500 MeV.Comment: 22 pages, 20 figures Replaced with a revised version. Accepted for
publication in EPJ A. Added a short section on pion reaction and charge
exchange cross-section
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
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