First observation of the decay tau- ---> K*- eta tau-neutrino

The decay {tau}{sup -} {yields} K{sup -} {eta}{nu}{sub {tau}} has been observed with the CLEO II detector. The {eta} meson is reconstructed using two decay channels, {eta} {yields} {gamma}{gamma} and {pi}{sup +}{pi}{sup -}{pi}{sup 0}. The measured branching fraction is B({tau}{sup -} {yields} K{sup -}{eta}{nu}{sub {tau}}) = (2.6 {+-} 0.5 {+-} 0.5) x 10{sup -4}, somewhat higher than the theoretical estimates. An improved upper limit for the second-class-current decay {tau}{sup -} {yields} {pi}{sup -}{eta}{nu}{sub {tau}} is set, B({tau}{sup -} {yields} {pi}{sup -}{eta}{nu}{sub {tau}}) < 1.4 x 10{sup -4} at 95% CL, consistent with the theoretical expectations.