Bounds on neutrino mixing with exotic singlet neutrinos

We examine the effects of mixing induced non-diagonal light-heavy neutrino weak neutral currents on the amplitude for the process v(a)(v) over bar (a) --> ZZ (with a = e, mu or tau). By imposing constraint that the amplitude should not exceed the perturbative unitarity limit at high energy (roots = Lambda), we obtain bounds on light-heavy neutrino mixing parameter sin(2) theta(L)(va) where theta(L)(va) is the mixing angle. In the case of one heavy neutrino (mass m(xi)) or mass degenerate heavy neutrinos, for Lambda = 1 TeV, no bound is obtained for m(xi) < 0.50 TeV. However, sin(2) theta(L)(va) less than or equal to 3.8 x 10(-6) for m(xi) = 5 TeV and sin(2) theta(L)(va) less than or equal to 6.0 x 10(-8) for m(xi) = 10 TeV. For Lambda = infinity, no constraint is obtained for mxi < 0.99 TeV and sin(2) theta(L)(va) less than or equal to 3.8 x 10(-2) (for m(xi) = 5 TeV) and sin(2) theta(L)(va) less than or equal to 9.6 x 10(-3) (for m(xi) = 10 TeV).