# Search for Low-mass Dark-Sector Gauge Boson with the BABAR Detector

## TL;DR

This study searches for a hypothetical dark gauge boson ($Z'$) in muon pair events at BABAR, setting new limits on its coupling and mass, but finds no evidence of its existence.

## Contribution

First search for a muonic dark-force gauge boson at BABAR, providing improved constraints on its coupling to second and third lepton families.

## Key findings

- No significant $Z'$ signal observed.
- New limits set on dark-sector coupling constants.
- Parameter space for $Z'$ mass and coupling is further constrained.

## Abstract

We report searches for a new muonic dark force mediated by a gauge boson ($Z'$) coupling only to the second and third lepton families. The existence of the $Z'$ boson is probed in $e^+e^- \to \mu^+\mu^- Z'$, $Z' \to \mu^+ \mu^-$ events, with an analysis based on the full data sample collected with the BABAR detector at the PEP-II $e^+e^-$ collider. No significant signal is observed. Limits on dark-sector coupling constants are derived; these improve upon current bounds, and further constrain the allowed parameter space.