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

Abstract

The ratio $R_{\tau \mu }({\rm Y}(1S))={\Gamma }_{{\rm Y}(1S)\to {\tau }^{+}{\tau }^{-}}/{\Gamma }_{{\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}}$ is measured using a sample of $(121.8\pm 1.2)\times 10^6$ ${\rm Y}(3S)$ 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 ${\rm Y}(3S)\to {\rm Y}(1S){\pi }^{+}{\pi }^{-}$, ${\rm Y}(1S)\to l^{+}{l}^{-}$, where $l=\mu ,\tau$. The result, $R_{\tau \mu }({\rm Y}(1S))=1.005\pm 0.013(stat.)\pm 0.022(syst.)$, shows no deviation from the expected SM value, while improving the precision with respect to previous measurements.

7 pages, 3 figures

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• The BABAR Detector
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• Studies of Stability and Robustness for Artificial Neural Networks and Boosted Decision Trees

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