The $\pi^+$--emission puzzle in $^4_\Lambda$He decay

We re-examine the puzzling $\pi^+$ emission from the weak decay of $^4_\Lambda$He and propose an explanation in terms of a three-body decay of the virtual $\Sigma^+$. Such a resolution of the $\pi^+$ decay puzzle is consistent with the calculated $\Sigma^+$ probability in light $\Lambda$ hypernuclei as well as the experimentally observed $\pi^+$ energy spectrum and $s$--wave angular distribution.