Decoding the B→Kνν excess at Belle II: Kinematics, operators, and masses

Abstract

An excess in the branching fraction for B+→K+νν recently measured at Belle II may be a hint of new physics. We perform thorough likelihood analyses for different new physics scenarios such as B→KX with a new invisible particle X, or B→Kχχ through a scalar, vector, or tensor current with χ being a new invisible particle or a neutrino. We find that vector-current three-body decay with mX≃0.6 GeV—which may be dark matter—is most favored, while two-body decay with mX≃2  GeV is also competitive. The best-fit branching fractions for the scalar and tensor cases are a few times larger than for the two-body and vector cases. Past measurements provide further discrimination, although the best-fit parameters stay similar. Published by the American Physical Society 2024