Motivated by recent experimental indications of large nu_mu - nu_tao 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 structures and discuss their relationship to the standard model gauge symmetry. We find 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 present, giving bounds of a few hundred GeV. A few scalar and pseudo-scalar operators are not subject to any current experimental constraints.

Authors: Deirdre Black, Tao Han, Hong-Jian He, Marc Sher

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