Isoscalar off-shell effects in threshold pion production from pd collisions

We test the presence of pion-nucleon isoscalar off-shell effects in the $pd\to \pi^+ t$ reaction around the threshold region. We find that these effects significantly modify the production cross section and that they may provide the missing strength needed to reproduce the data at threshold.Comment: 6 pages, REVTeX, twocolumn, including 3 figures (Postscript), uses psfig, updated and extended versio

The pd <--> pi+ t reaction around the Delta resonance

The pd pi+ t process has been calculated in the energy region around the Delta-resonance with elementary production/absorption mechanisms involving one and two nucleons. The isobar degrees of freedom have been explicitly included in the two-nucleon mechanism via pi-- and rho-exchange diagrams. No free parameters have been employed in the analysis since all the parameters have been fixed in previous studies on the simpler pp pi+ d process. The treatment of the few-nucleon dynamics entailed a Faddeev-based calculation of the reaction, with continuum calculations for the initial p-d state and accurate solutions of the three-nucleon bound-state equation. The integral cross-section was found to be quite sensitive to the NN interaction employed while the angular dependence showed less sensitivity. Approximately a 4% effect was found for the one-body mechanism, for the three-nucleon dynamics in the p-d channel, and for the inclusion of a large, possibly converged, number of three-body partial states, indicating that these different aspects are of comparable importance in the calculation of the spin-averaged observables.Comment: 40 Pages, RevTex, plus 5 PostScript figure

Spin observables for the pd <-> pi+ t process around the Delta resonance

The proton analyzing power Ay0 and the deuteron tensor analyzing power T20 are evaluated for the pd pi+ t process, in the energy region around and above the Delta resonance. These calculations extend a previous analysis of the excitation function and differential cross-section, based on a model embodying one-- and two-body p-wave absorption mechanisms and isobar excitation. The three-nucleon bound state and the pd scattering state are evaluated through Faddeev techniques for both the Bonn and Paris potentials. The spin variables exhibit a greater sensitivity to the number of included three-nucleon partial waves than the cross-sections, while the role played by the initial-- or final-state interactions appears to be small. The results for the tensor analyzing power at backward angles show a non-negligible dependence on the potentials employed, consistently with what has been previously found for the cross-sections. The calculation of spin observables gives a clear indication that other reaction mechanisms (presumably s-wave two-body absorption) have to be included in the model, in order to reproduce the experimental data below the Delta-resonance, in analogy with the simpler pp pi+ d process.Comment: 14 pages, REVTeX, plus 6 figs., PostScript (PRC, to be published

Total and differential cross sections of η-production in proton–deuteron fusion for excess energies between Qη = 13 MeV and Qη = 81 MeV

New data on both total and differential cross sections of the production of η mesons in proton–deuteron fusion to He3η in the excess energy region 13.6MeV≤Qη≤80.9MeV are presented. These data have been obtained with the WASA-at-COSY detector setup located at the Forschungszentrum Jülich, using a proton beam at 15 different beam momenta between pp=1.60GeV/c and pp=1.74GeV/c. While significant structure of the total cross section is observed in the energy region 20MeV≲Qη≲60MeV, a previously reported sharp variation around Qη≈50MeV cannot be confirmed. Angular distributions show the typical forward-peaking that was noted earlier. For the first time, it is possible to study the development of these angular distributions with rising excess energy over a wide interval

STUDY OF THE np→ppπ- AND np→d π+ π- REACTIONS WITH THE VERTEX DETECTOR ARCOLE

Nous présentons des résultats préliminaires de l'étude des réactions np→ppπ- et np→d π+ π- obtenus avec un détecteur d'angle solide 4π permettant la reconstruction complète de la cinématique événements par événements.We present preliminary results on the study of the np→ppπ- AND np→d π+ π- reactions obtained with a quasi-4π detector. Full kinematics is obtained event by event

MEASUREMENTS OF ANGULAR DISTRIBUTIONS OF ABSOLUTE CROSS SECTIONS AND ANALYZING POWERS OF THE REACTION pp→d π+ BETWEEN 1.3 AND 2.4 GeV

Nous avons mesuré les distributions angulaires des sections efficaces absolues, des pouvoirs d'analyse et des coefficients de transfert de spin Knn, de la réaction pp→d π+ pour 6 énergies incidentes du proton entre 1.3 et 2.4 GeV. La première phase de l'analyse concernant les distributions angulaires des Ayo et des sections efficaces absolues est achevée. Les distributions de Ayo présentées en fonction de √s pour t ou égaux à zéro ou pour θπ(c.m.) égal à 90° confirment le phénomène résonnant vu dans une précédente expérience. Dans cette même représentation, nous montrons pour la première fois un comportement non monotone de la section efficace (pour t = 0) autour de √s=2700 MeV.We have measured angular distributions of absolute cross sections, analyzing powers and the spin transfer parameters Knn of the reaction pp→d π+ at 6 incident proton energies between 1.3 and 2.4 GeV. In a first phase the data were analyzed to obtain angular distributions of absolute cross sections and analyzing powers Ayo. Distributions of Ayo where the Mandelstam parameters t or u are equal to zero or the angle θπ(c.m.) equals 90° as a function of √s confirm the resonance like behaviour suggested by an earlier experiment. For the first time deviations from a smooth behaviour are also found in the √s distribution (at t = 0) of the absolute cross section