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HK: Hadronen und Kerne
HK 5: Hadronic Probes I, Protons, Antiprotons, Pions
HK 5.5: Group Report
Monday, March 16, 1998, 15:45–16:15, C
Latest Results in Pion Absorption — •Albert Lehmann — Paul Scherrer Institute, CH-5232 Villigen PSI; currently at University of Tübingen, Auf der Morgenstelle 14, 72076 Tübingen
The absorption of a pion by nucleons is one of the basic reactions in strong interaction. Pion absorption on two nucleons (2NA) is known to be the main channel at energies around the Δ(1232)-resonance, but a significant fraction of the total absorption cross section leads to more than two energetic final state nucleons [1]. The first investigations of this three-nucleon absorption (3NA) process did not reveal any kinematic signatures, which would attribute the 3NA yield to two-step processes like ISI+2NA or 2NA+FSI (ISI: Initial State Interaction; FSI: Final State Interaction). This led to discussions of whether the 3NA yield might reflect a yet unknown nuclear response.
For a detailed investigation of the underlying dynamics of the 3NA process the Large Acceptance Detector System (LADS) was built at the Paul Scherrer Institute (PSI). Its large solid angle coverage (≈ 98% of 4π) and low particle threshold (e.g. Tpthr < 20 MeV) make a large fraction of the phase space accessible.
The final LADS results, in particular on the light nuclei 3He and 4He, will be summarized and compared to former pion (and photon) absorption data. The LADS data conclusively confirm the existence of a significant 3NA component. The high statistics data samples allow the unambiguous identification of kinematic signatures which show that initial state interactions (ISI+2NA) [2] are important in pion absorption. However, semi-classical cascade models of the two-step processes ISI+2NA and 2NA+FSI can only account for about one-third of the 3NA cross sections [3][4]. The remaining strength reveals structures in the differential distributions that are not yet explained. Whether those originate from interferences of the elementary pion-nucleon and nucleon-nucleon partial waves, is currently subject of Faddeev-based calculations (e.g. [5]). Altogether, the LADS data provide a significant step forward in understanding the dynamics of pion absorption on nuclei.
[1] G. Backenstoss et al., Phys. Rev. Lett. 55 (1985) 2782
[2] G. Backenstoss et al., Phys. Lett. B379 (1996) 60
[3] A. Lehmann et al., Phys. Rev. C55 (1997) 2931
[4] A. Lehmann et al., Phys. Rev. C56 (1997) 1872
[5] H. Kamada et al., Phys. Rev. C55 (1997) 2563