Search

We report the results of a pion-electron scattering experiment to measure the charge radius of the pion. The experiment was performed in a 50 GeV/c negative, unseparated beam at the IHEP accelerator, Serpukhov, and has been briefly reported in an earlier publication [1]. A magnetic spectrometer instrumented with wire spark chambers was used to record the incident pion trajectory and the angles and momenta of the scattered particles. Events are reconstructed by detailed trackfinding programs, and a set of kinematic and geometric cuts define the elastic sample. Electrons are identified both by kinematic criteria and pulse height information from total absorption lead glass Cerenkov counters. The final elastic sample consisted of 40 000 πe events in the region of four-momentum transfer squared 0.013 (GeV/c)2 ⩽ q2 ⩽ 0.036 (GeV/c)2. A full error matrix fit to the form factors of the pion gave the r.m.s. charge radius of the pion: 〈r π 2 〉 1 2 = (0.78 −0.10 +0.09 ) fm .

The results of a study of the decay of the Λ8Li and Λ12B hypernuclei are presented. The Λ 8 Li →π − + 8 Be ∗ →π − + 4 He + 4 He and Λ 12 B →π − + 12 C ∗ →π − + 4 He + 4 He + 4 He modes have been investigated in detail. New information on the decay mechanism of the 16.11 MeV state of 12C has been obtained.

Further examples of the π− mesonic decay of Λ 7 Be , Λ 10 B and Λ 11 B hypernuclei are reported. The binding energy values obtained in this work are combined with previous results.

From the observation of 11 examples of the π − mesonic decay of Λ 13 C the binding energy of this hypernucleus is found to be 11.39 ± 0.15 MeV.

The results of a search for final-state interactions between the decay products of some light hypernuclei are presented. The following π− mesonic decays have been considered: Λ 3 H → π − + 1 H , Λ 4 H → π − + 1 H + 3 H , Λ 4 He → π − + 1 H + 3 He , Λ 5 He → π − + 1 H + 4 He , Λ 7 Li → π − + 3 He + 4 He , Λ 7 Li → π − + 1 H + 2 H + 4 He , Λ 7 Li → π − + 1 H + 6 Li , Λ 8 Li → π − + 4 He + 4 He , Λ 9 Li → π − + 4 He + 4 He + n , Λ 9 Be → π − + 1 H + 4 He + 4 He and Λ 12 B → π − + 4 He + 4 He + 4 He .

The recent reported evidence for the existence of the Λ6Li hypernucleus is questioned. No example of the (π−1H1H4He) decay mode of this hypernucleus was found in a sample of 7000 mesonically decaying hypernuclei.

The results of an experimental study of the binding energies of light hypernuclei are presented, based on a sample of 2008 uniquely identified π − mesonic decays. The problems of the calibration procedure and the selection criteria are discussed. Various aspects of the Λ -N interaction are reviewed in the light of these new results.

Twenty examples of π+ mesonic decays of light hypernuclei have been found together with 7000 π− mesonic decays. The ratio R4 of π+ mesonic decays for the gL4 He hypernucleus has been computed from the results of this work and those of a previous experiment [1] and is found to lie between the values 5.4 ± 1.5%1.7% and 6.9 ± 1.8%2.1%. The π+ mesonic decays of other species of hypernuclei are briefly discussed.

Some 2780 ( π − 1 H 4 He ) and 15(π − 2 H 3 He ) decays of the Λ5He hypernucleus have been collected and analyzed. The energy spectra and the angular distribution of the decay products have been studied. An improved formulation of the theoretical model describing the ( π − 1 H 4 He ) decay has been made, allowing a detailed comparison between theory and experiment.

The results of a compilation of the binding energy values of light hypernuclei ( A ⩽ 16) obtained by the European K − Collaboration and the EFINS-North-western Collaboration are presented. The events were analysed by the same computer programme and uniform stringent selection criteria were applied.