Revisiting $B\to K^{(\ast)} \nu \bar{\nu}$ decays in the Standard Model and beyond

TL;DR

This paper reevaluates Standard Model predictions for B to K(*) neutrino-antineutrino decays, explores their combination with B to K(*) lepton-lepton decays to probe New Physics, and discusses experimental strategies for testing form factors.

Contribution

It introduces a novel approach to combine decay rates for better extraction of effective Wilson coefficients and sensitivity to New Physics beyond the Standard Model.

Findings

The ratio of decay modes can extract $C_9^\mathrm{eff}$ reliably.

Combined decay measurements enhance sensitivity to New Physics.

Separate high and low $q^2$ measurements test form factor assumptions.

Abstract

In this letter we revisit the Standard Model predictions for $\mathcal{B}(B\to K^{(\ast)}\nu\bar{\nu})$ and discuss the opportunities that open up when combining its partial decay rate with that of $B\to K^{(\ast)}\ell\ell$. In the Standard Model a suitable ratio of these two modes can be used to extract $C_9^\mathrm{eff}$, which is essential for a reliable phenomenological analysis of the $B\to K^{(\ast)}\ell\ell$ angular observables. The same ratio also proves to be more sensitive to the presence of New Physics in many plausible extensions of the Standard Model. We also suggest that the separate measurement of $\mathcal{B}(B\to K\nu\bar{\nu})$ for high and for low $q^2$'s can be helpful for testing the assumed shape of the vector form factor, because the lattice QCD data are obtained at high $q^2$'s, whereas the low $q^2$ region is obtained through an extrapolation.