Rho Exchange Research Articles

A potential model for hyperon-nucleon scattering

Low energy Λ N- and Σ N-scattering has been calculated in a potential model by solving the multichannel Schrödinger equation. Meson theoretic potentials are used which describe the exchange of nonets of pseudoscalar and vector mesons and uncorrelated two pion exchange. At short distances hard cores are employed. Coupling constants are taken from SU(3), SU(6), and the conserved current theory for vector mesons. Coulomb effects are included and different baryon masses within isomultiplets are considered. Charge symmetry breaking between the Λ p- and Λ n-channel due to one pion and one rho exchange has been taken into account. The model is checked in nucleon-nucleon scattering where it gives a good qualitative description of the NN-phaseshifts below the pion production threshold. A least-squares fit to the low energy Λp, Σ + p, and Σ − p data yields a very satisfactory result. The model leads then to the low energy parameters of Σ + p: a 8 c = −2.42 ± 0.30 fm, r 8 c = 3.41 ± 0.30 fm a t c = + 0.71 fm, r t c = −0.78 fm . The scattering lengths a and effective ranges r for Λp are a 8 p = −2.16 ± 0.26 fm, r 8 p = 2.03 ± 0.10 fm a t p = − 1.32 ± 0.07 fm, r t p = 2.31 ± 0.08 fm . and for Λn a 8 n = −2.67 ± 0.35 fm, r 8 n = 2.04 ± 0.10 fm a t n = − 1.02 ± 0.05 fm, r t p = 2.55 ± 0.10 fm . The possible existence of 2S 1 resonances in the neighborhood of the ΣN thresholds is discussed. Coupled channel effective range expansions are given around the Σ N-thresholds.

Dominant processes in the reaction πp → π−π−π+p at 3.9 GeV/c

New experimental results are reported for 7975 events of the type π−p → π−π−π+p at 3.9 GeV/c. A model based on pion exchange, rho exchange and diffraction dissociation is demonstrated to explain all the main features of the data. Only the relative amount of each of five dominant resonance production processes was varied in a maximum likelihood fit with no background assumed to the resonance amplitude in a given process. No form factors, absorption or off-mass-shell corrections were used. The model is consistent, by duality, with the multiperipheral models used at higher energies. Background resulting from the kinematic reflections of the competing processes are shown to produce profound effects. Diffraction dissociation is found to be equal in strength to pion exchange at this energy, and accounts for the A1(1070) enhancement in our data without a resonance pole. Evidence for the production of the resonant state N∗ (1460) by diffraction dissociation, and its cascade decay through ▵++π−, is presented.

W-Meson Production inπ−+p→W−+pInteractions

The cross section for production of $W$ bosons in the reaction ${\ensuremath{\pi}}^{\ensuremath{-}}+p\ensuremath{\rightarrow}{W}^{\ensuremath{-}}+p$ is calculated on the basis of single-particle exchange for $W$ masses in the region 2-5 BeV. Corrections for absorption in the initial channel are included. Pion exchange is found to dominate rho exchange and gives (4-12) ${|{F}_{\ensuremath{\pi}}({{m}_{W}}^{2})|}^{2}\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}34}$ ${\mathrm{cm}}^{2}$ as an estimate of the total production cross section. Here ${F}_{\ensuremath{\pi}}({{m}_{W}}^{2})$ is the pion electromagnetic form factor evaluated at the $W$-meson mass.

The absorption model and production of 2 mesons

The absorption model for high-energy peripheral production of resonances is applied to the production of spin-parity 2+ mesons in pion-nucleon interaction with the twofold purpose of testing the model against the experimental data and of discussing some approximations made within the model. By a detailed numerical investigation of the partial-wave expansion we show how the impact parameter approximation, often used in the application of the model, may significantly affect both the predicted cross-section and the decay distribution. moreover, the cross-section, but not so much the decay distribution, is sensitive to the detailed way in which each partial wave in the model is modified by the absorption. We trace these features back to the fact that the low partial waves, although strongly suppressed as compared to the unmodified Born approximation, still play a major role. The comparison with experiment for production of fo (pion exchange) shows that the slope of the cross-section is correctly predicted but that the absolute magnitude is too big by a factor of around two. For A2 (1310) production (rho exchange) the slope of the cross-section can be made to agree with experiment only after the introduction of a fairly strong form factor; the absolute ma gntude and the energy dependence of the cross-section are wrong. These results strengthen the conclusion that the absorption model is unable to reproduce the experimental data for production of high-spin resonances, especially if vector meson exchange has to be assumed. The spin-2 formalism, including decay widths and decay distributions, is presented in the Appendices.