V. Mull

Strange vector currents and the OZI-rule

The authors investigate the role of correlated {pi}{rho} exchange in the extraction of matrix elements of the strange vector current in the proton. They show that a realistic isoscalar spectral function including this effect leads to sizably reduced strange vector form factors based on the dispersion--theoretical analysis of the nucleons electromagnetic form factors.

Can one discriminate between meson-exchange and quark-gluon transition mechanisms in the pp→ΛΛ process?

Abstract It is shown that a measurement of the depolarization parameter Dnn in the p p → Λ Λ reaction, for p momenta around 1.5 GeV/c, can discriminate between meson-exchange and simple constituent-quark models for the p p → Λ Λ transition.

Pion-pion scattering in nuclear matter

Abstract Based on a realistic meson-exchange model for ππ scattering in free space we study medium modifications for low partial waves in cold nuclear matter. With increasing density a strong accumulation of strength near threshold and eventually a series of bound states is found in the s-wave. The ϱ resonance in the p-wave is shifted to higher energy which, in turn, influences the s-wave through t -channel exchange. For the d-wave f 2 meson a significant narrowing of the width is found at higher densities. We also estimate the in-medium K + K − production cross section which may play a role in ultrarelativistic heavy-ion collisions.

Role of meson-meson correlation effects in the NN → ϱπ annihilation process

Abstract Meson-meson correlation effects are investigated in the N N → ϱπ annihilation process using a realistic meson-exchange model for the ϱπ interaction determined previously, together with a conventional baryon-exchange transition model and a consistent N N interaction. For N N S- states , they have a drastic effect and bring the relative ( 1 S 0 3 S 1 ) branching ratio up to the experimental value, thus offering an explanation for the long-standing so-called “ϱπ” puzzle. For N N P- states , their effect is of minor importance, and discrepancies remain for those ratios involving annihilation from the N N( 3 P J ) state to ϱπ(l′ = 2).

The p̄p → φφ reaction as a two-step process involving antihyperon-hyperon intermediate states

Abstract A model for the p p → φφ reaction is derived in the conventional hadronic framework based on two-step processes with antihyperon-hyperon (ḡLΛ, ḡSΣ) intermediate states. Quite naturally, it is able to account for the empirical situation.

The role of final-state interactions in the p̄p→π+π− annihilation process

Abstract We investigate the role of final-state interactions in the p p→π + π − annihilation process, based on a realistic meson-exchange model for the ππ interaction. In contrast to claims made sometimes in the literature, we find sizeable effects, also in J ≠ 0 partial waves, which strongly improve the results for spin observables in the p p→π + π − reaction. Off-shell effects turn out to be of importance.

The NN scattering data and the nature of the NN repulsion

Abstract It is shown that, due to gross uncertainties in the description of the annihilation interaction dominating the N N scattering process, N N data cannot determine at present specific features of the short-ranged elastic N N interaction.

pp annihilation into π+π−π0 in a hadron-exchange model

Abstract Proton-antiproton annihilation into three pions is studied in a hadron-exchange model. An uncorrelated three-pion channel is added to the two-meson channels considered in previous calculations. Initial-state interactions are taken into account in a distorted-wave Born approximation, using the most complete version of the Julich N N model. The uncorrelated three-pion channel adds a 10% contribution to the p p → π + π − π 0 total cross section. The relative branching ratios for annihilation into three uncorrelated pions from states of equal orbital angular momentum, but different total angular momentum and spin, are also calculated. They compare semi-quantitatively with the corresponding quantities calculated from the phase-space components of phenomenological amplitudes fitted to experimental data for stopped antiprotons. It is concluded that hadron exchange provides a viable model for annihilation into three non-resonant pions.

φ meson couplings to the nucleon and strange vector currents

We investigate within a meson-exchange model the OZI allowed coupling of the φ meson to the nucleon with the inclusion of kaon loops and hyperon excitations. All parameters of the model have previously been determined from a variety of hadronic reactions. A strong cancellation of the various contributions is observed which results in a small φNN coupling. We also show that a realistic isoscalar spectral function including the correlated πρ exchange leads to sizeably reduced strange vector form factors based on the dispersion-theoretical analysis of the nucleons’ electromagnetic form factors explaining our previous result.