Test of Lepton Universality in ${\rm Y}(1S)$ Decays at BABAR

The ratio $R_{\tau \mu }\mathbf{(}{\rm Y}(1S)\mathbf{)}={\Gamma }_{{\rm Y}(1S)\to {\tau }^{+}{\tau }^{-}}/{\Gamma }_{{\rm Y}(1S)\to {\mu }^{+}{\mu }^{-}}$ 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, $R_{\tau \mu }\mathbf{(}{\rm Y}(1S)\mathbf{)}=1.005\pm 0.013(\mathrm{stat})\pm 0.022(\mathrm{syst})$, shows no deviation from the expected SM value, while improving the precision with respect to previous measurements.

Figure 1

1D fit projections for $M_{{\mu }^{+}{\mu }^{-}}$ (top) and for $M_{{\pi }^{+}{\pi }^{-}}^{\mathrm{reco}}$ (middle) in the $D_{\mu }$ sample, and for $M_{{\pi }^{+}{\pi }^{-}}^{\mathrm{reco}}$ (bottom) in the $D_{\tau }$ sample. In each plot the dashed line represents the background shape, while the solid line is the sum of signal and background contributions to the fit, and the points are the data.