π–4He interactions at T=106 MeV

π – He 4 interactions at T π = 106 MeV have been studied by means of a self-shunted streamer chamber filled with helium at atmospheric pressure. This technique allowed reconstruction of the complete kinematics of the nuclear events under analysis, since tracks of slow p , α , tritium and 3He are readily measurable. The study revealed that the 4He nucleus behaves as a Planck radiator, emitting a T = 14 MeV Planck-like spectrum of high energy γ s, when hit by a π beam of T π = 106 MeV . A resonant behaviour in the π − n invariant mass spectrum has been observed, with M π − n = 1160 MeV and Γ = 20 MeV in the π − 4 He neutron knockout reaction: we consider this to be the first experimental evidence for the existence of the Δ − . The observed mass shift and width narrowing are compatible with the activation of an isobaric collective resonance in the 4He nucleus. The collaboration has also observed a resonant behaviour in the π − pp invariant mass spectrum in the π + ( Ag 105 , Ag 103 ) π − pp double-charge-exchange reaction, on nuclear photoemulsion, compatible with the activation of the J P = 0 − d ′ dibaryonic resonance with a strong p–p final state interaction. A new direct measured upper limit for the ν μ mass has been derived by measuring a complete decay π – μ – e event recorded at the CERN PS179 experiment ( p ̄ 20 Ne scattering): at a 90% confidence level, M μ ν ≤ 2.2 MeV .