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Register a new userPhenomenological study of the pp -> pi+ pn reaction
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Fully constrained bubble chamber data on the pp -> pi+ pn and pp -> pi+ d reactions are used to investigate the ratio of the counting rates for the two processes as function of the pn excitation energy Q. Though it is important to include effects associated with the p-wave nature of pion production, the data are insufficient to establish unambiguously the dependence on Q. The angular distributions show the presence of higher partial waves which seem to be anomalously large at small Q. The dispersion relation method to determine scattering lengths is extended to encompass cases where, as for the pp -> pi+ pn reaction, there is a bound state and, in a test example, it is shown that the values deduced for the low energy neutron-proton scattering parameters are significantly influenced by the pion p-wave behavior.
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Fully constrained bubble chamber data on the pp -> pi+ pn and pp -> pi+ d reactions are used to investigate the ratio of the counting rates for the two processes at low pn excitation energies. Whereas the ratio is in tolerable agreement with that found in a high resolution spectrometer experiment, the angular distribution in the final pn rest frame shows that the deviation from the predictions of final state interaction theory must originate primarily from higher partial waves in the pn system. These considerations might also be significant for the determination of the S-wave Lambda:p scattering length from data on the pp -> K+ Lambda p reaction.
In order to establish links between p-wave pion production in nucleon-nucleon collisions and low energy three-nucleon scattering, an extensive programme of experiments on pion production is currently underway at COSY-ANKE. The final proton pair is measured at very low excitation energy, leading to an S-wave diproton, denoted here as {pp}_s. By using a deuterium target we have obtained data on the differential cross section and analysing power of the quasi-free pol{p}n -> {pp}_s pi^- reaction at 353 MeV. The spectator proton p_sp was either measured directly in silicon tracking telescopes or reconstructed using the momentum of a detected pi^-. Both observables can be described in terms of s-, p-, and d-wave pion production amplitudes. Taken together with the analogous data on the pol{p}p -> {pp}_s pi^0 reaction, full partial wave decompositions of both processes were carried out.
The cross section for prompt neutral and charged three pion production in pp interactions was measured at excess energies in the range 160 - 217 MeV. That comprises the first measurement of the pp->pp pi0pi0pi0 reaction and the comparison with the pp->pp pi+pi-pi0 reaction, in a very direct way. The experiment was performed above the eta meson production threshold and the cross section normalization was obtained from a concurrent measurement of the reaction pp->pp eta with the eta decaying into 3 pions. Since the same final states are selected, the measurement has a low systematical error. The measured cross section ratio sigma(pp->pp pi+pi-pi0)/sigma(pp->pp pi0pi0pi0) is compared to predictions of dominance of different isobars in the intermediate state.
The cross sections for the pp -> ppK+K- reaction were measured at three beam energies 2.65, 2.70, and 2.83 GeV at the COSY-ANKE facility. The shape of the K+K- spectrum at low invariant masses largely reflects the importance of Kbar{K} final state interactions. It is shown that these data can be understood in terms of an elastic K+K- rescattering plus a contribution coming from the production of a K0bar{K}0 pair followed by a charge-exchange rescattering. Though the data are not yet sufficient to establish the size of the cusp at the K0bar{K}0 threshold, the low mass behaviour suggests that isospin-zero production is dominant.
Evidence has recently been presented for the existence of a dibaryon of mass 2380 MeV/c^2 and width 70 MeV/c^2, which decays strongly into the d pi0 pi0 channel [M. Bashkanov et al., Phys.Rev.Lett. 102 (2009) 052301; P. Adlarson et al., arXiv:1104.0123]. The decay rate of such a hypothesised dibaryon into the {pn}_{I=0} pi0 pi0 channel is estimated in a weakly model-dependent way by using final state interaction theory. It is shown that, if the resonance exists, it should then show up as strongly in this channel as in d pi0 pi0. The sum of the two decay modes would saturate most of the inelasticity predicted in the relevant partial waves in the 2380 MeV/c^2 region.
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