169 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
Beam-helicity asymmetries for single-hadron production in semi-inclusive deep-inelastic scattering from unpolarized hydrogen and deuterium targets
A measurement of beam-helicity asymmetries for single-hadron production in
deep-inelastic scattering is presented. Data from the scattering of 27.6 GeV
electrons and positrons off gaseous hydrogen and deuterium targets were
collected by the HERMES experiment. The asymmetries are presented separately as
a function of the Bjorken scaling variable, the hadron transverse momentum, and
the fractional energy for charged pions and kaons as well as for protons and
anti-protons. These asymmetries are also presented as a function of the three
aforementioned kinematic variables simultaneously
Transverse-target-spin asymmetry in exclusive -meson electroproduction
Hard exclusive electroproduction of mesons is studied with the
HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and
electron beams off a transversely polarized hydrogen target. The amplitudes of
five azimuthal modulations of the single-spin asymmetry of the cross section
with respect to the transverse proton polarization are measured. They are
determined in the entire kinematic region as well as for two bins in photon
virtuality and momentum transfer to the nucleon. Also, a separation of
asymmetry amplitudes into longitudinal and transverse components is done. These
results are compared to a phenomenological model that includes the pion pole
contribution. Within this model, the data favor a positive
transition form factor.Comment: DESY Report 15-14
Semiexclusive pionic double charge exchange on 4 He
The semiexclusive reaction {}^{4}\mathrm{He}({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}}pp)pp has been studied at pion kinetic energies of 105 MeV and 115 MeV. Signatures from the production of the hypothetical \ensuremath{\pi}\mathrm{NN} resonance {d}^{\ensuremath{'}} have been searched for in the invariant mass {M}_{\ensuremath{\pi}\mathrm{pp}} spectra. No hint for a dominant {d}^{\ensuremath{'}} production, as anticipated from double charge exchange excitation functions on nuclei, has been found. The data are satisfactorily described by sequential single charge exchange
Longitudinal double-spin asymmetries in semi-inclusive deep-inelastic scattering of electrons and positrons by protons and deuterons
A comprehensive collection of results on longitudinal double-spin asymmetries is presented for charged pions and kaons produced in semi-inclusive deep-inelastic scattering of electrons and positrons on the proton and deuteron, based on the full HERMES data set. The dependence of the asymmetries on hadron transverse momentum and azimuthal angle extends the sensitivity to the flavor structure of the nucleon beyond the distribution functions accessible in the collinear framework. No strong dependence on those variables is observed. In addition, the hadron charge-difference asymmetry is presented, which under certain model assumptions provides access to the helicity distributions of valence quarks
pi+- p differential cross sections at low energies
Differential cross sections for pi- p and pi+ p elastic scattering were
measured at five energies between 19.9 and 43.3 MeV. The use of the CHAOS
magnetic spectrometer at TRIUMF, supplemented by a range telescope for muon
background suppression, provided simultaneous coverage of a large part of the
full angular range, thus allowing very precise relative cross section
measurements. The absolute normalisation was determined with a typical accuracy
of 5 %. This was verified in a simultaneous measurement of muon proton elastic
scattering. The measured cross sections show some deviations from phase shift
analysis predictions, in particular at large angles and low energies. From the
new data we determine the real part of the isospin forward scattering
amplitude.Comment: 13 pages, 5 figures. To appear in PL
Bose-Einstein correlations in hadron-pairs from lepto-production on nuclei ranging from hydrogen to xenon
Bose-Einstein correlations of like-sign charged hadrons produced in
deep-inelastic electron and positron scattering are studied in the HERMES
experiment using nuclear targets of H, H, He, He, N, Ne, Kr,
and Xe. A Gaussian approach is used to parametrize a two-particle correlation
function determined from events with at least two charged hadrons of the same
sign charge. This correlation function is compared to two different empirical
distributions that do not include the Bose-Einstein correlations. One
distribution is derived from unlike-sign hadron pairs, and the second is
derived from mixing like-sign pairs from different events. The extraction
procedure used simulations incorporating the experimental setup in order to
correct the results for spectrometer acceptance effects, and was tested using
the distribution of unlike-sign hadron pairs. Clear signals of Bose-Einstein
correlations for all target nuclei without a significant variation with the
nuclear target mass are found. Also, no evidence for a dependence on the
invariant mass W of the photon-nucleon system is found when the results are
compared to those of previous experiments
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