Lepton Universality Test in Upsilon(1S) decays at BaBar

TL;DR

This study measures the ratio of tau to muon decay modes of Upsilon(1S) to test lepton universality and search for signs of a light pseudoscalar Higgs boson, using data from BaBar's Upsilon(3S) decays.

Contribution

It provides the first precise measurement of the lepton universality ratio in Upsilon(1S) decays and explores potential deviations indicating new physics beyond the Standard Model.

Findings

Measured R_taumu ratio consistent with Standard Model expectations

No significant deviation observed that would suggest a light pseudoscalar Higgs boson

Results constrain NMSSM parameter space

Abstract

Using a sample of 122 million Upsilon(3S) decays collected with the BaBar detector at the PEP-II asymmetric energy collider at the SLAC National Accelerator Laboratory, we measure the ratio R_taumu=BR(Upsilon(1S)->tau^+tau^-)/BR(Upsilon(1S)->mu^+mu^-); the measurement is intended as a test of lepton universality and as a possible search for a light pseudoscalar Higgs boson in Next to Minimal Supersymmetric Standard Model (NMSSM) scenarios. Such a boson could appear in a deviation of the ratio R_taumu from the Standard Model expectation, that is 1, except for small lepton mass corrections. The analysis exploits the decays Upsilon(3S) -> Upsilon(1S) pi^+pi^-, Upsilon(1S) -> l^+l^-, where l = mu, tau.