In-medium nuclear interactions of low-energy hadrons

Abstract

Exotic atoms provide a unique laboratory for studying strong interactions and nuclear medium effects at zero kinetic energy. Experimental and theoretical developments of the last decade in the study of exotic atoms and some related low-energy reactions are reviewed. The exotic atoms considered are of π - , K - , p ¯ , Σ - , and also the so far unobserved Ξ - atoms. The analysis of these atomic systems consists of fitting density-dependent optical potentials V opt = t ( ρ ) ρ to comprehensive sets of data of strong-interaction level shifts, widths and yields across the periodic table. These provide information on the in-medium hadron–nucleon t matrix t ( ρ ) over a wide range of densities up to central nuclear densities. For pions, the review focuses on the extraction of the π N in-medium s-wave interaction from pionic atoms, which include also the deeply bound π - atomic states recently observed at GSI in isotopes of Sn and Pb. Also included are recent measurements at PSI of elastic scattering of π ± on Si, Ca, Ni and Zr at 21.5 MeV. The experimental results are analyzed in the context of chirally motivated π -nuclear potentials, and the evidence for partial restoration of chiral symmetry in dense nuclear matter is critically discussed. For antikaons, we review the evidence from K - atoms, and also from low-energy K - p scattering and reaction data for and against a deep K ¯ -nucleus potential of 150–200 MeV attraction at nuclear matter density. The case for relatively narrow deeply bound K - atomic states is made, essentially independent of the potential-depth issue. Recent experimental suggestions from KEK and DA Φ NE (Frascati) for signals of K ¯ -nuclear deeply bound states are reviewed, and dynamical models for calculating binding energies and widths of K ¯ - nuclear states are discussed. For kaons we review the evidence, from K + total and reaction cross section measurements at the AGS (BNL) on Li, C, Si and Ca at p lab = 500 – 700 MeV / c , for significant absorptivity of t KN ( ρ ) beyond that expected from t KN free within the impulse approximation. Attempts to explain the extra absorptivity for the relatively weak interaction of K mesons in terms of a hypothetical exotic S =+ 1 pentaquark Θ + strength are reviewed. For antiprotons the exceptionally broad data base due to the recent results of the PS209 collaboration at CERN are analyzed, together with results of radiochemical experiments. We discuss the dependence of the phenomenological p ¯ -nucleus interaction on the model adopted for the neutron density, showing how the neutron densities favored by our comprehensive analysis are compatible with densities from other sources, including our own analysis of pionic atoms. It is also shown how the strong absorptivity of the p ¯ -nucleus interaction, which leads to the prediction of saturation of widths in deeply bound p ¯ -atom states, also explains the observed saturation effects in low-energy p ¯ annihilation on nuclei. For Σ hyperons we review the evidence, from continuum Σ - hypernuclear ( π - , K + ) spectra obtained recently at KEK on C, Si, Ni, In and Bi, for substantial repulsion in the Σ -nucleus interaction, and the relationship to the inner repulsion established earlier from the density-dependence analysis of Σ - atoms and by analyses of past ( K - , π ± ) AGS experiments. Lastly, for Ξ hyperons we review prospects of measuring X-ray spectra in Ξ - atoms and thereby extracting meaningful information on the Ξ -nucleus interaction. The significance of the latter to the physics of Λ Λ hypernuclei and to extrapolation into multistrange hypernuclei are briefly reviewed.