Tau decays and dark-sector searches at BaBar

We present some recent measurements performed using 514 fb−1 of e+e− collisions collected with the BaBar detector at the PEP-II asymmetric collider at SLAC. First we present a study of the decay τ± → K±KSντ , we measure the branching fraction B(τ± → K±KSντ ), the mass spectrum of the K±KS system and obtain the spectral function. Our results are in agreement with and improve previous measurements. We then report a search for a dark boson A′ produced in the reaction e+ e− → γ A′ in invisible final states A′ → χ χ . We find no evidence for such processes and set 90% confidence level upper limits on the coupling strength of A′ as a function of the A′ mass m′A. In particular, our limits exclude the values of the A’ coupling suggested by the dark-photon interpretation of the muon (g − 2) anomaly. Finally we report the search for a light non-Standard Model gauge boson Z′ coupling only to the second and third lepton families. Our results significantly improve current limits and further constrain the remaining region of the allowed parameter space.