Leptonic B Meson Decays

B → μν at Belle II (Development)

B meson pairs are the dominant decay products of the Upsilon(4S) resonance, which is produced in large amounts in elctron positron collisions at the SuperKEKB collider in Japan, and their decays are measured by the Belle II experiment. Leptonic $B$ meson decays such as B to muon and neutrino are highly CKM- and helicity-suppressed. In a two-body decay like B to muon and neutrino, the muon momentum is exactly known in the rest frame of the signal-side B meson. By boosting the signal-side muon into that frame, a better signal resolution and improved sensitivity can thus be achieved compared to the center-of-mass frame. This requires a high-precision for the boost vector, which can be determined from the rest of the event that contains the decay products of the second B meson. At the same time, this information can be used to reconstruct the kinematics of the signal-side B meson. Boosted decision trees are trained to suppress background and increase signal purity.