Search for Charged Lepton Flavor Violation in Narrow ${\rm Y}$ Decays

Charged-lepton flavor-violating processes are unobservable in the standard model, but they are predicted to be enhanced in several extensions to the standard model, including supersymmetry and models with leptoquarks or compositeness. We present a search for such processes in a sample of $99\times {10}^6{\rm Y}(2S)$ decays and $117\times {10}^6{\rm Y}(3S)$ decays collected with the BABAR detector. We place upper limits on the branching fractions $\mathcal{B}\mathbf{(}{\rm Y}(nS)\to e^{\pm }{\tau }^{\mp }\mathbf{)}$ and $\mathcal{B}\mathbf{(}{\rm Y}(nS)\to {\mu }^{\pm }{\tau }^{\mp }\mathbf{)}$ ($n=2,3$) at the ${10}^{-6}$ level and use these results to place lower limits of order 1 TeV on the mass scale of charged-lepton flavor-violating effective operators.

Figure 1

Maximum likelihood fit results for the leptonic $e\tau$ channel in ${\rm Y}(3S)$ data. In the inset, the red dotted line represents the signal PDF, the green dashed line represents the sum of all background PDFs, and the solid blue line represents the sum of these components. The inset shows a close-up of the region $0.95<x<1.02$. The top plot shows the normalized residuals $(\mathrm{\text{data}}-\mathrm{\text{fit}})/{\sigma }_{\mathrm{data}}$ (pull).

Figure 2

Excluded regions of effective field theory parameter spaces of mass scale ${\Lambda }_{\ell \tau }$ versus coupling constant ${\alpha }_{\ell \tau }$. The dotted blue line is derived from ${\rm Y}(2S)$ results only, the dashed red line is derived from ${\rm Y}(3S)$ results only, and the solid black line indicates the combined results. The yellow shaded regions are excluded at 90% C.L.