MC study for the lepton flavor violating tau decay into a lepton and an undetectable particle

Abstract We search for the charged lepton flavor violating (cLFV) two-body decay, τ → l X using data samples collected with the Belle detector at the KEKB asymmetric-energy e + e − collider, where l is muon or electron and X is a missing particle. One of candidates for an X particle is, for example, a light Z ′ boson in the L μ − L τ model. This decay mode has been searched for in the ARGUS experiment in 1995. No evidence of the signal has been found and they set upper limits for B ( τ → l X ) / B ( τ → l ν ν ) at several values of X mass. With the Belle data sample, we can perform a much more sensitive search. In this paper we report the estimation of the sensitivities and upper limits of the branching fraction for τ → μ X depending on the X mass with a 1  ab − 1 data sample.