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

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 ``tag'' to enhance signal reconstruction. Typical tags are fully reconstructed, hadronic decays, whereas one tends to discard ``semileptonic'' modes, 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 $\ensuremath{\tau}$-lepton decay. Our strategy rests on the use of appropriate kinematic quantities, collectively denoted as ${M}_{2}$, that had never been applied in the context of meson decays. We introduce ${M}_{2}$ 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 $B\ensuremath{\rightarrow}K\ensuremath{\tau}\ensuremath{\mu}$, 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.