A. Hoffart

What is the mechanism for three-nucleon absorption of pions in 3He?

Abstract Experimental data for 3 He(π + , pp)p at T π =120 MeV in a kinematical region dominated by three-nucleon absorption, where all three protons have obtained an appreciable amount of energy, are analyzed by means of variables specifically sensitive to different reaction mechanisms. The data are compared with predictions for two-step mechanisms and a genuine three-nucleon absorption mechanism. No evidence is found for such two-step processes as scattering or charge exchange preceding a two-nucleon absorption or as two-nucleon absorption followed by “Hard FSI”. The results, however, agree with a genuine three-nucleon absorption process, consistent with the recently measured phase-space-like behavior of this process.

Three-nucleon processes and the total pion absorption cross section in 3He

Abstract Pion absorption processes involving three nucleons have been investigated in 3 He with positive and negative pions of 64, 119, 162 and 206 MeV kinetic energy. The most important of these processes is characterized by a constant phase-space density and has comparable integrated cross sections for π + and π − with a Δ-resonance-like dependence on the pion energy. This process constitutes up to one third of the total absorption cross section. The total absorption cross section has been determined by adding up the measured cross sections for all the observed absorption channels.

Pion absorption in flight on 4He

Abstract We have measured double and triple coincidences following π + and π − absorption on 4 He at various pion energies. Differential and integrated cross sections are determined for all possible absorption channels. We study and compare the two-nucleon absorption on nucleon pairs with isospin I = 0 and I = 1. In the dNN final-state two absorption mechanisms are identified, one with only one spectator nucleon and one with phase-space behavior. The existence of a three-nucleon absorption component with a spectator nucleon is established and a four nucleon phase-space-like contribution is also identified and found to be small. The total absorption cross section is directly determined by adding the integrated cross sections of all measured reaction channels.

LARGE-SOLID-ANGLE STUDY OF PION ABSORPTION ON 3HE

Measurements have been made of [pi][sup +] absorption on [sup 3]He at [ital T][sub [pi]][sup +]=118, 162, and 239 MeV using the Large Acceptance Detector System. The nearly 4[pi] solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The total absorption cross section is reported. In addition, the total cross section is divided into components in which only two or all three nucleons play a significant role in the process. These are the first direct measurements of the total and three nucleon absorption cross sections.

Two-nucleon absorption of π+ and π− on 3He across the Δ-resonance region☆

Abstract The angular distributions of nucleons from a kinematically complete measurement of pions absorbed in 3He on nucleon pairs with isospin T = 0 and T = 1 at incident pion energies of 64, 119, 162 and 206 MeV are presented. The T = 0 data are quite similar to pion absorption data in deuterium but scale with the number of nucleon pairs. The T = 1 cross sections are an order of magnitude smaller and the angular distributions show a significant asymmetry. This asymmetry together with the measured energy dependence indicates interference effects between amplitudes with and without Δ isobars in the intermediate states.

A LARGE ACCEPTANCE DETECTOR SYSTEM (LADS) FOR STUDIES OF PION ABSORPTION

Abstract A Large Acceptance Detector System (LADS) has been designed and built at the Paul Scherrer Institute to study multiparticle final states following pion-nucleus absorption. It consists of a 28-sector cylinder of plastic scintillators of 1.6 m active length and 1.4 m diameter, two cylindrical wire chambers, and two 14-sector plastic scintillator end-caps which close each end. The nearly 4π solid angle coverage of this detector minimizes uncertainties associated with extrapolations over unmeasured regions of phase space. The design and the performance of the LADS detector are presented.

Contribution of initial state interactions to the three-nucleon absorption of pions by 3He

Abstract The distributions of protons after the absorption of positive pions by 3 He at 118, 162 and 239 MeV are presented. A decomposition of the three-nucleon cross section in terms of initial state interactions and other processes is given, based upon simple models. A clear signal of initial state interactions is visible in the data, diminishing at the lower energies.

Pion Absorption in 3He and 4He Nuclei

The absorption of positive and negative pions across the delta resonance was studied in the helium isotopes. New results on primary reaction mechanisms involving 2 and 3 nucleons were obtained.

New perspectives in multi-nucleon pion absorption on light nuclei

Multinucleon absorption cross sections have been measured in 3He and 4He with three unbound, energetic nucleons in the final state. The ratios of the different 3NA cross sections are difficult to understand in terms of only two-step processes. Similar difficulties appear in the decomposition of the 3NA yield into mechanisms. Although a part of this yield is identified to be due to two-step processes, a large fraction is well described by a simple phase space model. (AIP)