Probing third-generation New Physics with K→πνν¯ and B→K(⁎)νν¯

Abstract

The recent observation of the $K^{+}\to {\pi }^{+}\nu \overline{\nu }$ decay by NA62 is an important milestone in precision flavor physics. Together with evidence of $B^{+}\to K^{+}\nu \overline{\nu }$ reported by Belle-II, they are the only FCNC decays involving third-family leptons where a precision close to the SM expectation has been reached. We study the implications of these recent results in the context of a new physics scenario aligned to the third generation, with an approximate $U{\left(2\right)}^5$ flavor symmetry acting on the light families. We find that the slight excess observed in both channels supports the hypothesis of non-standard TeV dynamics of this type, as also hinted at by other B-meson decays, consistently with bounds from colliders and electroweak observables. We further discuss how future improvements in precision could affect this picture, highlighting the discovery potential in these di-neutrino modes.