Study of the muon decay-in-flight in the τ- → μ-(ν)over-barμνt decay to measure the Michel parameter ξ'

We present the first measurement of the Michel parameter.0 in the tau(-) -> mu(-)(nu) over bar (mu)nu t decay using the full data sample of 988 fb(-1) collected by the Belle detector operating at the KEKB asymmetric energy ethornecollider. The method is based on the reconstruction of the mu(-) -> e(-)(nu) over bar (e)nu(mu) decay-in-flight in the Belle central drift chamber and relies on the correlation between muon spin and its daughter electron momentum. We study the main sources of the background that can imitate the signal decay, such as kaon and pion decays-in-flight and charged particle scattering on the detector material. Highly efficient methods of their suppression are developed and applied to select 165 signal-candidate events. We obtain xi' = 0.22 +/- 0.94 +/- 0.42 where the first uncertainty is statistical, and the second one is systematic. The result is in agreement with the Standard Model prediction of xi' = 1.

Automated summary

We present the first measurement of the Michel parameter in the tau(-) -> mu(-)(nu) over bar (mu)nu t decay using the full data sample collected by the Belle detector operating at the KEKB asymmetric energy collider. The study focuses on reconstructing the mu(-) -> e(-)(nu) over bar (e)nu(mu) decay-in-flight and suppressing background sources to obtain the signal. The result, xi' = 0.22 +/- 0.94 +/- 0.42, is in agreement with the Standard Model prediction of xi' = 1.