Light new physics in B→K(*)νν¯ ?

Abstract

The study of the rare decays B→K(*)νν¯ offers a window into the dynamics operating at the electroweak scale, allowing studies of the Standard Model and searches for heavy new physics. However, the analysis of these decays is also potentially sensitive to the on shell production of new light bosons X through the process B→K(*)X. In particular, Belle II has recently measured B+→K+νν¯, finding a 2.8σ excess under the assumption of heavy new physics. Since this excess is rather localized in the kaon energy, a fit that includes the decay mode B+→K+X to the kinematic distributions prefers mX≈2  GeV with branching fraction Br[B→KX]=(8.8±2.5)×10−6 and a significance of ≈3.6σ. However, no excess was found in the measurements of B→K(*)νν¯, and a global analysis of the Belle II and data leads to Br[B→KX]=(5.1±2.1)×10−6 with a reduced significance of ≈2.4σ. We then study various simplified dark-flavored models and present a possible UV completion based on a gauged B3−L3 symmetry, highlighting the discovery potential of dedicated searches for B→K(*)X at Belle II. Published by the American Physical Society 2024