The matrix element for $\mu$ capture (${\mu }^{-}+p\to n+\nu$) including not only the effects of vector, axial-vector, weak-magnetism, and induced-pseudoscalar couplings but also two "second-class" couplings has a small dependency on these hitherto undetected couplings. Capture in $\mu$-mesonic hydrogen from the $S$ states with both $F=1\mathrm{}$ and $F=0\mathrm{}$ is computed, and angular distributions of recoil neutrons and capture rates are given as functions of the six coupling constants. It is found that the second-class terms may contribute fully as much as weak-magnetism and induced-pseudoscalar terms.

• Received 26 October 1961

DOI:https://doi.org/10.1103/PhysRev.126.1567