Restrictions for asymmetry and polarizations of recoil in muon capture

Using the helicity formalism, muon capture by targets of spin-zero is discussed. Owing to the definite neutrino helicity, three independent observables define a complete experiment. The precise relation between asymmetry alpha and longitudinal polarization P/sub L/ of recoil, alpha =1+2jP/sub L/, comes only from rotational invariance. When time- reversal invariance is inserted, there is an additional restriction between the average polarization P/sub av/ and the longitudinal polarization P/sub L/. (7 refs).