Search for Lepton-Flavor-Violating <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>τ</mml:mi></mml:math> Decays to a Lepton and an Invisible Boson at Belle II
I. Adachi, K. Adamczyk, L. Aggarwal, H. Ahmed, H. Aihara, Н. Акопов, A. Aloisio, N. Anh Ky, D. M. Asner, T. Aushev, V. Aushev, H. Bae, S. Bahinipati, P. Bambade, Sw. Banerjee, J. Baudot, M. Bauer, A. Baur, A. Beaubien, Jürgen Becker, P. K. Behera, J. V. Bennett, E. Bernieri, F. U. Bernlochner, V. Bertacchi, M. Bertemes, E. Bertholet, M. Bessner, S. Bettarini, B. Bhuyan, F. Bianchi, T. Bilka, S. Bilokin, D. Biswas, D. Bodrov, A. E. Bolz, J. Borah, A. Bożek, M. Bračko, P. Branchini, T. E. Browder, A. Budano, S. Bussino, M. Campajola, L. Cao, G. Casarosa, C. Cecchi, M.-C. Chang, P. Chang, R. Cheaib, P. Cheema, V. Chekelian, Y. Q. Chen, B. G. Cheon, K. Chilikin, K. Chirapatpimol, H.-E. Cho, K. Cho, S.-J. Cho, S.-K. Choi, S. Choudhury, D. Cinabro, L. Corona, S. Cunliffe, S. Das, F. Dattola, E. De La Cruz–Burelo, S. A. De La Motte, G. De Nardo, M. De Nuccio, G. De Pietro, R. de Sangro, M. Destefanis, S. Dey, A. De Yta-Hernandez, R. Dhamija, A. Di Canto, F. Di Capua, J. Dingfelder, Z. Doležal, I. Domínguez Jiménez, T. V. Dong, M. Dorigo, K. Dort, D. Dossett, S. Dreyer, S. Dubey, G. Dujany, P. Ecker, M. Eliachevitch, D. Epifanov, P. Feichtinger, T. Ferber, D. Ferlewicz, T. Fillinger, C. Finck, G. Finocchiaro, K. T. Flood, A. Fodor, F. Forti
Abstract
We search for lepton-flavor-violating ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}\ensuremath{\alpha}$ and ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\alpha}$ decays, where $\ensuremath{\alpha}$ 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 $62.8\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$, 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 ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{e}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ and ${\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}{\ensuremath{\nu}}_{\ensuremath{\tau}}$ decays. We report 95% confidence-level upper limits on the branching-fraction ratio $\mathcal{B}({\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}\ensuremath{\alpha})/\mathcal{B}({\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{e}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{e}{\ensuremath{\nu}}_{\ensuremath{\tau}})$ in the range $(1.1--9.7)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ and on $\mathcal{B}({\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}\ensuremath{\alpha})/\mathcal{B}({\ensuremath{\tau}}^{\ensuremath{-}}\ensuremath{\rightarrow}{\ensuremath{\mu}}^{\ensuremath{-}}{\overline{\ensuremath{\nu}}}_{\ensuremath{\mu}}{\ensuremath{\nu}}_{\ensuremath{\tau}})$ in the range $(0.7--12.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ for $\ensuremath{\alpha}$ masses between 0 and $1.6\text{ }\text{ }\mathrm{GeV}/{c}^{2}$. These results provide the most stringent bounds on invisible boson production from $\ensuremath{\tau}$ decays.