τ- μFlavor Violation as a Probe of the Scale of New Physics

Abstract

Motivated by the recent strong experimental evidence of large ν_mu-ν_tau neutrino mixing, we explore current bounds on the analogous mixing in the charged lepton sector. We present a general formalism for dimension-6 fermionic effective operators involving tau-mu mixing with typical Lorentz structure (\bar{mu} Γtau)(\bar{q}^a Γ{q}^b), and discuss their relationship to the standard model gauge symmetry and the underlying flavor dynamics. We derive the low-energy constraints on the new physics scale associated with each operator, mostly from current experimental bounds on rare decay processes of tau, hadrons or heavy quarks. For operators involving at least one light quark (u,d,s), these constraints typically give a bound on the new physics scale of a few TeV or higher. Those operators with two heavy quarks turn out to be more weakly constrained at the present, giving bounds of a few hundred GeV. A few scalar and pseudo-scalar operators are free from all current experimental constraints.