Study of spectator tagging in the reaction np → ppπwith a deuteron beam

The reaction \(dp \ensuremath \rightarrow ppp \pi^-\) has been studied in a kinematically complete experiment at a single beam momentum \(\ensuremath p_{d}=1.85\)GeV/c (T = 759MeV). All four ejectiles have been detected in the large-acceptance time-of-flight spectrometer COSY-TOF. We analyzed the data along the lines of the spectator model as a means to isolate the quasi-free \(\ensuremath np\rightarrow pp\pi^-\) reaction. The spectator proton was identified by its momentum and flight direction thus yielding access to the associated Fermi motion of the bound neutron. A comparison is carried out with Monte Carlo simulations based on two different parameterizations of the deuteron wave function. Up to a Fermi momentum of roughly 150MeV/c no significant deviations between experimental and simulated data of various observables were found from which we conclude that the deuteron can indeed be taken as a valid substitute for the neutron.