The study of the negative pion production in neutron-proton collisions at beam momenta below 1.8 GeV/c

A detailed investigation of the reaction np ? rightarrow pp p^- pi^{{-}}_{} has been carried out using the data obtained with the continuous neutron beam produced by charge exchange scattering of protons off the deuterium target. Partial-wave event-by-event analysis based on the maximum likelihood method was applied to determine contributions of different partial waves to the pion production process. A combined analysis of the np ? rightarrow pp p^- pi^{{-}}_{} and pp ? rightarrow pp p⁰ pi^{{0}}_{} data measured in the same energy region allows us to determine the contribution of isoscalar partial waves (I = 0 in the momentum range from 1.1 up to 1.8GeV/c . The decay of isoscalar partial waves into the (¹ S ₀)_pp p pi channel provides us with a good tool for the determination of the pp S -wave scalar scattering length in the final state which was found to be a _pp = - 7.5±0.3 fm.