Impact of new invisible particles on $B\to K^{(\ast )}{E}_{\mathrm{miss}}$ observables

Abstract

Motivated by a recent Belle~II measurement that suggests an excess in the rare decay $B\to K{E}_{\mathrm{miss}}$, and building upon our recent differential decay rate likelihood analysis of the existing experimental information, we investigate possible new physics (NP) scenarios in which light invisible states participate in flavour-changing $b\to s$ transitions. In particular, we consider the total and differential $B\to K^{\ast }{E}_{\mathrm{miss}}$ decay rates and $K^{\ast }$ polarisation effects in each NP scenario preferred by the $B\to K{E}_{\mathrm{miss}}$ measurement. We show that future measurements of these $B\to K^{\ast }{E}_{\mathrm{miss}}$ observables will offer decisive discrimination among the different NP explanations. Our results highlight the strong complementarity of the rare semi-invisible $b$-hadron decay observables, and underline the importance of analysing their momentum transfer spectra when probing extensions of the Standard Model that feature new light degrees of freedom.

Comments: 14 pages, 10 figures