τ−μflavor violation as a probe of the scale of new physics

Abstract

Motivated by the recent strong experimental evidence of large ${\ensuremath{\nu}}_{\ensuremath{\mu}}\ensuremath{-}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ 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 $\ensuremath{\tau}\ensuremath{-}\ensuremath{\mu}$ mixing with a typical Lorentz structure $(\overline{\ensuremath{\mu}}\ensuremath{\Gamma}\ensuremath{\tau})({q}^{\ensuremath{\alpha}}\ensuremath{\Gamma}{q}^{\ensuremath{\beta}}),$ 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 $\ensuremath{\tau},$ 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 pseudoscalar operators are free from all current experimental constraints.