New approach to semileptonic tags in B -meson semi-invisible decays

Abstract

Semi-invisible decays of mesons are a prime strategy for searches of new particles or forbidden decays. At current and upcoming facilities beauty mesons are produced in pairs, and the second decay often provides a tagto enhance signal reconstruction. Typical tags are fully reconstructed, hadronic decays, whereas one tends to discard semileptonicmodes, clean and abundant, but with an elusive neutrino. We introduce a new strategy that makes semileptonic tags competitive with hadronic tags, potentially implying a substantial increase in the usable event statistics for any semi-invisible meson or τ-lepton decay. Our strategy rests on the use of appropriate kinematic quantities, collectively denoted as M2, that had never been applied in the context of meson decays. We introduce M2 definitions that leverage two key features of high-intensity meson-decay events: the known decaying-meson mass and its flight direction - accessible thanks to the exquisite vertexing capabilities available. We benchmark these definitions in the B→Kτμ, a null test of the Standard Model of great current interest. In an unrefined application we already observe a substantial improvement in sensitivity - halfway to the ideal case of using fully reconstructed semileptonic tags with true kinematics. © 2023 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the https://creativecommons.org/licenses/by/4.0/Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI. Funded by SCOAP3.