Multinucleon pion absorption in the 4He( pi +,ppp)n reaction.

Abstract

Three-proton emission cross sections for the $^{4}\mathrm{He}$(${\mathrm{\ensuremath{\pi}}}^{+}$,ppp)n reaction were measured at an incident pion kinetic energy of ${\mathit{T}}_{\mathrm{\ensuremath{\pi}}}^{+}$=165 MeV over a wide angular range in a kinematically complete experiment. Angular correlations, missing momentum distributions, and energy spectra are compared with three- and four-body phase-space Monte Carlo calculations. The results provide strong evidence that most of the three-proton coincidences result from three-nucleon absorption. From phase-space integration the total three-nucleon absorption cross section is estimated to be ${\mathrm{\ensuremath{\sigma}}}^{3\mathit{N}}$=4.8\ifmmode\pm\else\textpm\fi{}1.0 mb. The cross section involving four nucleons is small and is estimated to be ${\mathrm{\ensuremath{\sigma}}}^{4\mathit{N}}$2 mb. On the scale of the total absorption cross section in $^{4}\mathrm{He}$, multinucleon pion absorption seems to represent only a small fraction.