Bounds on neutrino mixing with exotic singlet neutrinos

Abstract

We examine the effects of mixing induced non-diagonal light-heavy neutrino weak neutral currents on the amplitude for the process \(v_a \overline v _a \to ZZ\) (with a=e, μ or τ). By imposing constraint that the amplitude should not exceed the perturbative unitarity limit at high energy \(\left( {\sqrt s = \Lambda } \right)\), we obtain bounds on light-heavy neutrino mixing parameter sin2\(\theta _L^{v_a } \) where \(\theta _L^{v_a } \) is the mixing angle. In the case of one heavy neutrino (mass mξ) or mass degenerate heavy neutrinos, for Λ=1 TeV, no bound is obtained for mξ<0.50 TeV. However, sin2\(\theta _L^{v_a } \)≤3.8 × 10−6 for mξ=5 TeV and sin \(\theta _L^{v_a } \)≤6.0 × 10−8 for mξ=10 TeV. For Λ=∞, no constraint is obtained for mξ<0.99 TeV and sin2\(\theta _L^{v_a } \)≤3.8 × 10−2 (for mξ=5 TeV) and sin2\(\theta _L^{v_a } \)≤9.6 × 10−3 (for mξ=10 TeV).