Fermi-Gamow-Teller mixing in the allowed isospin-hindered positron decay of /sup 56/Co

A new high-precision measurement of the asymmetry coefficient, ${A}_{\ensuremath{\beta}}$, for the positron decay of polarized $^{56}\mathrm{Co}$ is reported: ${A}_{\ensuremath{\beta}}=+0.359\ifmmode\pm\else\textpm\fi{}0.009$. If one assumes the $V\ensuremath{-}A$ form of the weak interaction and time-reversal invariance, the Fermi-Gamow-Teller mixing ratio, $y=\frac{{C}_{V}{M}_{F}}{{C}_{A}{M}_{\mathrm{GT}}}$, is then -0.091\ifmmode\pm\else\textpm\fi{}0.005. Since $y$ is nonzero and isospin is not conserved, $^{56}\mathrm{Co}$ ${\ensuremath{\beta}}^{+}$ decay is a good candidate for sensitive time-reversal invariance tests of the weak interaction.