# Tau decays and dark-sector searches at BaBar

## TL;DR

This paper reports measurements of tau decay modes, searches for dark sector particles like dark photons, and sets new limits on their properties, advancing understanding of potential new physics beyond the Standard Model.

## Contribution

It provides new measurements of tau decay spectra and sets improved constraints on dark photon models using BaBar data.

## Key findings

- Measured tau -> K K_S nu branching fraction and spectral function.
- Set upper limits on dark photon coupling strength, excluding some models.
- Significantly improved constraints on dark photon parameter space.

## Abstract

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 tau -> K K_S nu, we measure the branching fraction B(tau -> K K_S nu), the mass spectrum of the K K_S 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- -> gamma A' in invisible final states A' -> chi anti-chi . 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'. 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 A' 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.