Pion-nucleus Potential Research Articles

A survey of selected pion-nucleus reactions using phase-shift-equivalent potentials

Since pion-nucleus elastic scattering data can only constrain the asymptotic behavior of the scattered wave, it remains for inelastic scattering and reaction data to test the wave function in the nuclear interior. To this end, distorted-wave impulse approximation calculations have been performed for a variety of low-energy pion reactions using phase-shift-equivalent pion-nucleus potentials: inelastic excitation, photoproduction, and the ( π, p) reaction. Sensitivity of the reaction cross sections to variations in the equivalent potentials is examined, with an eye toward understanding what can be learned about pion-nucleus dynamics from a given set of reaction data.

Strong interaction effects in pionic atoms

Strong-interaction shifts and widths in pionic atoms of Al, Si, S, Ca, Fe, Cu, Zn, Ag, Cd, Pb, Bi and U have been measured. Various forms for an effective pion-nucleus potential have been fitted to the data for 2p levels and the predictions of these potentials compared with the results for 3d and 4f levels. The predictions have also been compared with available data for 1s states. The effects on pionic atoms of strongly bound pionic states, which it has recently been suggested might exist, are discussed.

Pionic x-rays and the neutron radius of 44Ca

Differences between strong interaction level shifts and widths for 2p states in pionic atoms of 44,40Ca have been measured. Analysis in terms of an effective pion-nucleus potential leads to a difference in neutron rms radii of r n(44)− r n(40) = 0.05 ± 0.05 fm.

Shifts and Widths of2pLevels in Pionic Atoms

Strong-interaction shifts and widths for $2p$ levels in pionic atoms of Al, Si, S, Ca, Fe, Cu, and Zn have been measured with generally an order of magnitude better accuracy than before. Parameters of an effective pion-nucleus potential have been determined.

Off-Shell Ambiguities and Phenomenological Pion-Nucleus Potentials

The pionic wave functions obtained by fitting phenomenological optical potentials to elastic-scattering data are found to be dependent on the assumed off-shell extrapolation of the $\ensuremath{\pi}N$ amplitude. Using data from low-lying excited states or employing finite range potentials does not eliminate the ambiguities.

Momentum-space study of pion-nucleus inelastic scattering

The inelastic scattering of intermediate energy pions from 12C is studied using a distorted wave impulse approximation (DWIA). A flexible momentum space formulation is presented which permits examination of a wide variety of non-local pion-nucleus potentials. Inelastic differential cross sections for the 2 + (4.43 MeV) and 3 −(9.64 MeV) states of 12C are generated from several phenomenological pion-nucleon collision matrices. Form factors for the inelastic transitions are obtained from the collective model and also from microscopic 1p-1h (Gillet-Vinh Mau) wave functions. The dramatic role of the (3,3) pion-nucleon resonance, along with an associated pion-nucleus absorption, is illustrated. The inelastic cross sections prove to be somewhat influenced by the off-shell pion-nucleon model, with the effect mostly confined to larger angles, and with changes that closely parallel those found earlier for elastic scattering. Back-angle inelastic cross sections are found to be large by a factor of about 10 compared to prior pion- 12C inelastic scattering calculations. This beneficial effect, relative to the available data, is attributed to a corresponding elastic scattering back-angle build-up which arises mainly from transforming the pion-nucleon interaction to the pion-nucleus center of mass frame. The inelastic scattering diffraction patterns are found to depend on the location and tail of the form factor, but much less so on its internal form, which is indicative of a surface direct reaction. Normalization of the collective model form factor to known B(E2) and B(E3) values yields general agreement with the magnitude of the forward angle inelastic 2 + and 3 − cross sections. These simple features of calculated pion- 12C inelastic scattering encourage further application of the DWIA and of direct reaction strong absorption methods to pion scattering in the resonance region.

The pion-nucleus optical potential and mesic atoms

It has been suggested that pionic atoms could be described by a local pion-nucleus potential rather than the more conventional momentum-dependent potential. These potentials give remarkably different answers for the energy shifts and level widths of such systems. It is pointed out that when the effects of short-range correlations are included in the two potentials there is in fact very little difference between them. The local potential requires a slightly more positive isoscalar πN scattering length than the non-local one.