Test of lepton universality in Upsilon(1S) decays at BaBar

TL;DR

This paper measures the ratio of tau to muon decay rates in Upsilon(1S) decays to test lepton universality and search for light pseudoscalar Higgs bosons, finding results consistent with the Standard Model.

Contribution

It provides a more precise measurement of the lepton universality ratio in Upsilon(1S) decays, constraining new physics beyond the Standard Model.

Findings

The measured ratio is 1.005 ± 0.013 (stat) ± 0.022 (syst).

No deviation from the Standard Model expectation of 1.

Improves the precision of previous measurements.

Abstract

The ratio $R_{\tau\mu}(\Upsilon(1S)) = \Gamma_{\Upsilon(1S)\to\tau^+\tau^-}/\Gamma_{\Upsilon(1S)\to\mu^+\mu^-}$ is measured using a sample of $(121.8\pm1.2)\times10^6$ $\Upsilon(3S)$ events recorded by the BABAR detector. This measurement is intended as a test of lepton universality and as a search for a possible light pseudoscalar Higgs boson. In the standard model (SM) this ratio is expected to be close to 1. Any significant deviations would violate lepton universality and could be introduced by the coupling to a light pseudoscalar Higgs boson. The analysis studies the decays $\Upsilon(3S)\to\Upsilon(1S)\pi^+\pi^-$, $\Upsilon(1S)\to l^+l^-$, where $l=\mu,\tau$. The result, $R_{\tau\mu}(\Upsilon(1S)) = 1.005 \pm 0.013(stat.) \pm 0.022(syst.)$, shows no deviation from the expected SM value, while improving the precision with respect to previous measurements.