Study of η-production reaction π⁻p → ηn in the near-threshold region

The differential cross-sections (DCS) of the η-production reaction π − p → ηn are measured at momenta of incident π − mesons (700, 710 and 720MeV/c) close to the threshold of this reaction. The experiment is carried out at the pion channel of the PNPI synchrocyclotron (with the momentum spread of the pion beam equal to 1.5%) using the Neutral Meson Spectrometer (NMS) designed and created at the Meson Physics Laboratory for detecting two photons from the decay η → 2γ. This spectrometer consisting of two electromagnetic total absorption calorimeter (24 CsI(Na) crystals in each calorimeter) allowed to obtain the DCS in the whole angular range from 0° to 180° in the c.m.s. It is shown that at 700MeV/c the DCS angular dependence is isotropic, indicating on the S-wave character of the η-production process while at higher momenta a significant contribution of the D-wave has led to the anisotropic angular dependence having a bowl-like shape.