The reaction np$\rightarrow$ pp $\pi^-$ from threshold up to 570 MeV

Abstract.

The reaction np$\rightarrow$ pp$\pi^-}$ has been studied in a kinematically complete measurement with a large acceptance time-of-flight spectrometer for incident neutron energies between threshold and 570 MeV. The proton-proton invariant mass distributions show a strong enhancement due to the pp($^1{\rm S}_0$) final state interaction. A large anisotropy was found in the pion angular distributions in contrast to the reaction pp$\rightarrow$pp$\pi^0$. At small energies, a large forward/backward asymmetry has been observed. From the measured integrated cross section $\sigma({{\rm n}{\rm p} \rightarrow{\rm p}{\rm p} \pi^-})$, the isoscalar cross section $\sigma_{01}$ has been extracted. Its energy dependence indicates that mainly partial waves with Sp final states contribute.