Radiative Pion Capture inHe4

Abstract

The spectrum of high-energy $\ensuremath{\gamma}$ rays following the absorption of negative pions in $^{4}\mathrm{He}$ has been investigated. The observed shape of the spectrum and the measured total capture rate [(0.704\ifmmode\pm\else\textpm\fi{}0.066) \ifmmode\times\else\texttimes\fi{}${10}^{15}$ ${\mathrm{sec}}^{\ensuremath{-}1}$] agree well with theoretical predictions, assuming that the pions proceed through excitation of states in $^{4}\mathrm{H}$ with ${J}^{\ensuremath{\pi}}={2}^{\ensuremath{-}}$ at 3.4, ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ at 5.1, and ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ at 7.4 MeV relative to the $^{3}\mathrm{H}+n$ threshold, and taking an operator $\ensuremath{\approx}{\ensuremath{\tau}}^{(\ensuremath{-})}\ensuremath{\sigma}{e}^{i\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}\ifmmode\cdot\else\textperiodcentered\fi{}\stackrel{\ensuremath{\rightarrow}}{\mathrm{r}}}$ for the calculation of the nuclear-matrix elements.