The results from the first kinematically complete measurement of the dd --> 4Hepipi reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar pipi-invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low pipi-invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent NN--> dpi sub-processes.
Total cross section for neutral and charged pion channels.
Invariant PI0 PI0 mass distribution at deuteron kinetic energy 1.029 GeV.
Invariant PI+ PI- mass distribution at deuteron kinetic energy 1.029 GeV.
The pd→pdη reaction has been studied in a kinematically complete experiment at five beam energies 927, 961, 996, 1032, and 1096MeV. In contrast to our measurement of the pd→He3η at the same energies, all the c.m. angular distributions are consistent with isotropy. The energy dependence of the total cross section seems to follow a three-body phase space as modified by a proton-deuteron final-state interaction, and such an extrapolation is consistent with other near-threshold data. The distributions of the pd and pη invariant mass at fixed beam energy are both close to those of phase space. However, this is not the case for the dη invariant mass, which shows significant structure in the first few MeV above threshold. This behavior is similar to that observed in the energy variation of the pn→dη total cross section and is the sign of a large η-deuteron scattering length that has been predicted in many theoretical models.
Total cross section as a function of the incident proton kinetic energy and the excess energy Q given by SQRT(S)-M(P)-M(D)-M(ETA).
Kinematically complete events have been studied for the reactions dp→dpπ0 and dp→dnπ+ at projectile energies between 437 and 559 MeV. The measurement covers a range of pion momenta η=pπ,c.m.max/mπc from near the production threshold (η=0.32) to η=0.86 close to the NN→NNπ threshold. The measurements were performed at the CELSIUS storage ring with the PROMICE/WASA setup. Angular and spectral distributions of the charged ejectiles as well as total cross sections are decomposed into the fractions that can be attributed to a quasifree NN→dπ process with a spectator nucleon, and to a process involving all three nucleons. The quasifree contribution increases with energy and dominates from the NN→NNπ threshold on. The results are compared to calculations with a spectator model with and without dp final state interactions.
Two first points on energy correspond to different luminosities.
The quasifree p+n→d+η reaction cross section has been measured at the threshold using 1295 MeV protons in the CELSIUS storage ring and an internal cluster-jet deuterium target. The kinematics is chosen such that the target proton can be assumed to be a spectator. The Fermi momentum of the target neutron is used to extract the energy dependence of the cross section by reconstructing the kinematics on an event-by-event basis. The data cover excess energies from threshold to 10 MeV in the center of mass of the final dη system. Approaching the threshold the cross section is enhanced compared to what is expected from phase space. This behavior is typical for a strong final-state interaction.
Cross section as a function of the C.M. excess energy.
Measurements of the vector analyzing power iT11 in πd elastic scattering at 49 MeV have been performed using a dynamically polarized target and a magnetic spectrometer. Data at seven π+ laboratory scattering angles between 50° and 130° were taken together with a complementary measurement at 60° for π−d elastic scattering. In general, we find agreement with models that include the πN P11 amplitude and disagreement with models that exclude or suppress it.
No description provided.
No description provided.
We report on measurements of the differential π±p cross section at pion energies Tπ=32.7, 45.1, and 68.6 MeV. The measurements, covering the angular range 25°≤θlab≤123°, have been carried out at the Paul-Scherrer-Institute (PSI) in Villigen, Switzerland, employing the magnet spectrometer LEPS. The absolute normalization of the π±p cross sections have been achieved by relating them to the electromagnetic cross sections of μ±12C scattering. The results are in agreement with those of our preceding measurements at Tπ=32.2 and 45.1 MeV insofar as they overlap with the region of the Coulomb nuclear interference investigated there. A comparison with the predictions of the Karlsruhe-Helsinki phase shift analysis KH80, which has formed the basis for the determination of the ‘‘experimental’’ σ term, reveals considerable deviations. These are most pronounced for the π+p cross sections at Tπ=32.7 and 45.1 MeV. Single energy partial wave fits result in S-wave contributions, which are about 1° lower in magnitude then those specified by the KH80 solution. The data at 68.6 MeV are in good agreement with the phase shift analysis.
Statistical and systematic errors are addet in quadrature.
Statistical and systematic errors are addet in quadrature.
Statistical and systematic errors are addet in quadrature.
The analyzing power Ay for π+p→ scattering at 68.3 MeV has been measured at the Paul Scherrer Institut with the magnetic spectrometer LEPS. The measurements cover the angular range 40°≤θlab≤70°. The protons have been polarized in a butanol target, operated in frozen spin mode. The S31 phase shift comes out by about 1° smaller than the Koch-Pietarinen [Nucl. Phys. A 336, 331 (1980)] phase shift analysis, supporting the necessity of an alternative dispersion analysis of πN scattering to determine the σ term and the πN coupling constant. © 1996 The American Physical Society.
The two data sets correspond to measurements with two different target compositions (see text).
The values of the pion nucleon (πN) σ term, as determined, on the one hand, from experimental pion nucleon scattering by means of dispersion relations and, on the other hand, from baryon masses by means of chiral perturbation theory, differ by 10 to 15 MeV. The origin of this discrepancy is not yet understood. If the difference between the two values is attributed to the scalar current of strange sea quark pairs within the proton, the contribution to the proton mass would be of the order of 120 MeV. The discrepancy may hint at either theoretical deficiencies or an inadequate πN database. In order to provide reliable experimental data we have measured angular distributions of elastic pion proton scattering at pion energies Tπ=32.2 and 44.6 MeV using the magnet spectrometer LEPS located at the Paul-Scherrer-Institute (PSI) in Villigen, Switzerland. From the data covering the region of the Coulomb nuclear interference, the real parts of the isospin-even forward scattering amplitude ReD+(t=0), have been determined as a function of energy. The results have been compared with the predictions of the Karlsruhe-Helsinki phase shift analysis KH80, revealing discrepancies most pronounced for the π+p data. The experimentally determined values for ReD+(t=0), however, support the KH80 prediction (which is based on πN data available in 1979).
Statistical and systematic errors are addet in quadrature.
Statistical and systematic errors are addet in quadrature.
None
ABSOLUTE TOTAL CROSS SECTIONS.
Integral cross sections for the elastic scattering of π + by protons into angles greater than 20° or 30° (lab) have been measured by the beam-attenuation technique over the energy range of 45–126MeV. The measurements are aimed at providing independent checks on the absolute normalization of differential cross sections, where discrepancies exist between different data sets. Comparisons with predictions made with existing phase shifts show very good agreement with the dispersion-relation constrained phase shifts of the Karlsruhe group.
Two targets (C=THIN) and (C=THICK) are used.
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