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ABSOLUTE TOTAL CROSS SECTIONS.
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.
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.
Hyperon production in the threshold region was studied in the reaction pp→K+Λp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering the full phase-space, were taken at three different beam momenta pbeam=2.59 , 2.68 and 2.85 GeV/ c (corresponding to excess energies of ɛ=85 , 115 and 171 MeV). Total cross-sections were deduced to be 7.4±0.5 μb , 8.6±0.6 μb and 16.5±0.4 μb , respectively. Differential observables including Dalitz plots were obtained. From the investigation of the Dalitz plot at pbeam=2.85 GeV/c a dominant contribution of the N∗(1650) -resonance to the reaction mechanism was found. In addition the pΛ -final-state interaction turned out to have a significant influence on the Dalitz plot distribution even 171 MeV above threshold.
Measured total cross sections.
Distribution of the invariant mass of the P-LAMBDA subsystem at beam momentum 2.85 GeV.
Distribution of the invariant mass of the K-LAMBDA subsystem at beam momentum 2.85 GeV.
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 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).
The experimental results for the (π+,π−) reaction on nuclei obtained in recent years reveal clear systematic features of this reaction. New data on 7Li, 12C, 16O, and 56Fe supplementing the existing data base are presented. The data on 12C are partly at variance with previous results. The dependence of the cross sections on incident energy, scattering angle, and on the target mass is discussed for transitions leading to the ground state of the final nucleus or to the double isobaric analog state.
GST means nonanalog ground state.
GST means nonanalog ground state.
GST means nonanalog ground state.
Kinematically complete measurements of the $pp\to pp\pi^{0}$ reaction were performed for beam energies in the range $292 - 298 $MeV. By detecting both protons in coincidence with the large acceptance COSY-TOF spectrometer set up at an external beam line of the proton synchrotron COSY-Julich, total and differential cross sections and energy distributions were obtained. A strong enhancement is observed in the Dalitz plots resulting from the final state interaction between the outgoing proton pair: the data are well reproduced by Monte Carlo simulations with standard parameters for scattering length $a_0$=-7.83 fm and effective range $r_0$=2.8 fm. The total cross sections exceed the ones measured recently in internal target experiments at IUCF and CELSIUS by roughly 50%. Arguments are presented which link this discrepancy with the effect of the final state interaction pushing yield into the very small-angle region and the near-impossibility of an internal target experiment to cover just this range.
Total P P --> P P PI0 cross section.
The reaction pp -> pp eta was measured at excess energies of 15 and 41 MeV at an external target of the Juelich Cooler Synchrotron COSY with the Time of Flight Spectrometer. About 25000 events were measured for the excess energy of 15 MeV and about 8000 for 41 MeV. Both protons of the process pp eta were detected with an acceptance of nearly 100% and the eta was reconstructed by the missing mass technique. For both excess energies the angular distributions are found to be nearly isotropic. In the invariant mass distributions strong deviations from the pure phase space distributions are seen.
Angular distribution of the ETA in the CM frame.
Angular distribution of the P P momentum in the CM frame.
Squared P P invariant mass distribution.
The analyzing power,$A_{oono}$, and the polarization transfer observables$K_{onno}$,$K_{os''so}$
Position 'A' (see text for explanation).
Position 'A' (see text for explanation).
Position 'A' (see text for explanation).
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