Search for Lepton-Flavor-Violating $\ddot{I}„$ Decays to a Lepton and an Invisible Boson at Belle II

Abstract

We search for lepton-flavor-violating $\ddot{I}{„}^{-}\to e^{-}\hat{I}\pm$ and $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\hat{I}\pm$ decays, where $\hat{I}\pm$ is an invisible spin-0 boson. The search uses electron-positron collisions at $10.58$ GeV center-of-mass energy with an integrated luminosity of $62.8$ fb${}^{-1}$, produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known $\ddot{I}{„}^{-}\to e^{-}\bar{\hat{I}}{½}_e\hat{I}{½}_{\ddot{I}}„$ and $\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\bar{\hat{I}}{½}_{\hat{I}}¼\hat{I}{½}_{\ddot{I}}„$ decays. We report 95\% confidence-level upper limits on the branching-fraction ratio $\mathcal{B}(\ddot{I}{„}^{-}\to e^{-}\hat{I}\pm )/\mathcal{B}(\ddot{I}{„}^{-}\to e^{-}\bar{\hat{I}}{½}_e\hat{I}{½}_{\ddot{I}}„)$ in the range $(1.1-9.7)\times 10^{-3}$ and on $\mathcal{B}(\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\hat{I}\pm )/\mathcal{B}(\ddot{I}{„}^{-}\to \hat{I}{¼}^{-}\bar{\hat{I}}{½}_{\hat{I}}¼\hat{I}{½}_{\ddot{I}}„)$ in the range $(0.7-12.2)\times 10^{-3}$ for $\hat{I}\pm$ masses between 0 and 1.6 GeV/c${}^2$. These results provide the most stringent bounds on invisible boson production from $\ddot{I}„$ decays.

7 pages, 4 figures