C. Wilkin

Elastic proton-deuteron scattering at large angles

The large angle elastic proton-deuteron differential cross 1 GeV is calculated from a triangle graph which includes as input the pp → πd amplitude. The predicted cross sections are rather low, but the shapes are in good agreement with the experimental data. A maximum in the 180° cross section is predicted for a proton kinetic energy of 700 MeV.

NEAR-THRESHOLD PRODUCTION OF ETA-MESONS

It is shown that the striking energy variation in the [ital pd][r arrow][sup 3]He [eta] cross section near threshold is probably due to a final state interaction associated with a large (complex) [eta][minus][sup 3]He scattering length. The consequences of this hypothesis are studied for the production of the meson in the [eta][minus][sup 4]He and [eta][minus][sup 7]Be channels.

Lineshape of the Λ(1405) hyperon measured through its Σ0π0 decay

The pp --> pK(+)Y(0) reaction has been studied for hyperon masses m (Y-0) <= 1540 MeV/c(2) at COSY-Julich by using a 3.65 GeV/c circulating proton beam incident on an internal hydrogen target. Final states comprising two protons, one positively charged kaon and one negatively charged pion have been identified with the ANKE spectrometer. Such configurations are sensitive to the production of the ground state Lambda and Sigma(0) hyperons as well as the Sigma(0)(1385) and Lambda(1405) resonances. Applying invariant- and missing-mass techniques, the two overlapping excited states could be well separated, though with limited statistics. The shape and position of the Lambda(1405) distribution, reconstructed cleanly in the Sigma(0)pi(0) channel, are similar to those found from other decay modes and there is no obvious mass shift. This finding constitutes a challenging test for models that predict Lambda( 1405) to be a two-state resonance

Precision study of the eta He-3 system using the dp ---> He-3 eta reaction

The differential and total cross sections for the d+p->3He+eta reaction have been measured in a high precision high statistics COSY-ANKE experiment near threshold using a continuous beam energy ramp up to an excess energy Q of 11.3 MeV with essentially 100% acceptance. The kinematics allowed the mean value of Q to be determined to about 9 keV. Evidence is found for the effects of higher partial waves for Q>4 MeV. The very rapid rise of the total cross section to its maximum value within 0.5 MeV of threshold implies a very large eta-3He scattering length and hence the presence of a quasi-bound state extremely close to threshold.

η-helium quasi-bound states

Abstract The cross section and tensor analysing power t20 of the d d → 4 He η reaction have been measured at six c.m. momenta, 10 ⩽ pη ⩽ 90 MeV/c. The threshold value of t20 is consistent with 1 2 , which follows from parity conservation and Bose symmetry. The much slower momentum variation observed for the reaction amplitude, as compared to that for the analogous pd → 3 He η case, strongly suggests the existence of a quasi-bound state in the η4He system and simple optical model fits tentatively indicate that this might also be the case for η3He.

ELASTIC PION--DEUTERON SCATTERING AT HIGH ENERGIES.

Abstract Elastic pion-deuteron scattering at 3.65 and 3.75 (GeV/ c ) is analysed on the basis of Glauber theory, particular attention being paid to the deuteron D-state and to the spin dependence and phase variation of the pion-nucleon amplitudes. The good overall agreement between theory and experiment would be further improved with a larger value of the deuteron quadrupole form factor than that given by a Hamada-Johnston type wave function.

A study of charge independence and symmetry from π+ and π− total cross sections on hydrogen and deuterium near the 3, 3 resonance

The total cross sections for π++-p and π±d scattering have been measured in the energy range 70–370 MeV in a classical transmission experiment using multiwire proportional chambers. The hydrogen data agree quite well with earlier measurements. After correcting our results for the direct effects of the Coulomb potential, we find energy-dependent differences of a few percent between the π+d and π−d cross sections. This charge symmetry violation can be parameterised in terms of mass and width differences between the Δ-isobars in agreement with the predictions of the quark model.

Comparison of Lambda and Sigma**0 threshold production in proton proton collisions

Threshold measurements of the associated strangeness production reactions pp --> p K(+) Lambda and pp --> p K(+) Sigma(0) are presented. Although slight differences in the shapes of the excitation functions are observed, the most remarkable feature of the data is that at the same excess energy the total cross section for the Sigma(0) production appears to be about a factor of 28 smaller than the one for the Lambda particle. It is concluded that strong Sigma(0)-p final state interactions, and in particular the Sigma-N --> Lambda-p conversion reaction, are the likely cause of the depletion for the yield in the Sigma signal. This hypothesis is in line with other experimental evidence in the literature.

The reaction pd→3Heη near threshold

Abstract The cross section and deuteron tensor analysing power of the reaction pd→ 3 Heη are studied near threshold in a two-step model, where a pion beam, created in a NN→dπ reaction on one of the nucleons in the target deuteron, is converted into an η-meson on the second nucleon via a πN→ ηN process. By taking into account the strong final-state interaction of the η with the 3 He nucleus, the energy dependence of the cross section is well reproduced, though its predicted absolute value is a factor of 2.5 too low. The analysing power is much more sensitive to the assumptions in the model.

The pp → ppη reaction near threshold

Abstract The differential and total cross sections for pp → pp η are evaluated close to threshold in a model where the S 11 nucleon isobar is excited through the exchange of ρ, π and η mesons. The preliminary data are not incompatible with these estimates.