Single $\mathrm{\ensuremath{\pi^{-}}}$ production in np collisions for excess energies up to 90 MeV

Abstract.

The quasifree reaction $np\rightarrow pp\ensuremath{\pi^{-}}$ was studied in a kinematically complete experiment by bombarding a liquid-hydrogen target with a deuteron beam of momentum 1.85GeV/c and analyzing the data along the lines of the spectator model. In addition to the three charged ejectiles the spectator proton was also detected in the large-acceptance time-of-flight spectrometer COSY-TOF. It was identified by its momentum and flight direction thus yielding access to the Fermi motion of the bound neutron and to the effective neutron 4-momentum vector $\mathbb {P}$ _n which differed from event to event. A range of almost 90MeV excess energy above threshold was covered. Energy-dependent angular distributions, invariant-mass spectra as well as fully covered Dalitz plots were deduced. Sizeable pp FSI effects were found as were contributions of p and d partial waves. In comparison with existing literature data the results provide a sensitive test of the spectator model. The behavior of the elementary cross-section σ₀₁ close to threshold is discussed in view of new cross-section data.