P. Salvisberg

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.

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 position-sensitive detector system with subnanosecond timing and its utilization in the measurement of pion absorption

A complex position-sensitive, time-of-flight detector was designed at the SIN to be used in the kinematically complete measurements of pion absorption in light nuclei, for the detection of both neutrons and charged particles. Design parameters and TOF properties resulting from the computer calculations and suitable test measurements are given for the system consisting of 200 × 10 × 5 cm3 NE 102A scintillator bars, XP 2230 photomultipliers and discriminator. Position dependence of the timing parameters of the 2 m long NE 102A bar viewed from the ends by photomultipliers has been clearly revealed in our model and experimentally verified.

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.

Energy-dependent partial wave analysis of the reaction π–pp(1S0)→ pn

We have performed an energy-dependent partial wave analysis of the reaction π– pp → pn where the two initial-state protons are in a 1 S 0 state. In contrast to a previous single-energy analysis, we find our partial wave amplitudes to be consistent with theoretical calculations involving just meson and baryon degrees of freedom.

Direct three-nucleon pion-absorption processes in 3He

A new reaction mechanism has been discovered in pion absorption on 3 He in which all three nucleons of the target are involved. From detailed measurements it has been shown that this type of reaction has a constant matrix element over the entire phase space of the 3N final state. This feature strongly supports a direct three-nucleon absorption process. Sequential processes like knock-out of one nucleon with subsequent absorption of the pion on the other two nucleons can be excluded. The fraction of the new reaction mechanism was found to be approximately 20% of the total pion absorption cross section in 3He. Possible relations of the new process with a three-body force are briefly mentioned.