Understanding the first measurement of $\mathcal{B}(B\to K \nu \bar{\nu})$

TL;DR

The paper discusses the first measurement of the B to K neutrino-antineutrino decay, its potential implications for new physics beyond the Standard Model, and how it correlates with other decay modes and SMEFT interpretations.

Contribution

It highlights the correlation between B to K nu nu and B to K* nu nu decays and explores SMEFT scenarios to interpret the experimental results.

Findings

Measurement of B to K nu nu decay is nearly 3σ above Standard Model predictions.

A tau-only coupling scenario in SMEFT can fit the data but struggles with R_D(*) ratios.

Additional SMEFT operators are needed to reconcile all experimental constraints.

Abstract

Recently, Belle II reported on the first measurement of $\mathcal{B}(B^\pm\to K^\pm \nu\bar{\nu})$ which appears to be almost $3\sigma$ larger than predicted in the Standard Model. We point out the important correlation with $\mathcal{B}(B\to K^{\ast} \nu\bar{\nu})$ so that the measurement of that decay mode could help restraining the possible options for building the model of New Physics. We then try to interpret this new experimental result in terms of physics beyond the Standard Model by using SMEFT and find that a scenario with coupling only to $\tau$ can accommodate the current experimental constraints but fails in getting a desired $R_{D^{(\ast )}}^\mathrm{exp}/R_{D^{(\ast )}}^\mathrm{SM}$, unless one turns the other SMEFT operators that are not related to $b\to s\ell\ell$ or/and $b\to s\nu\nu$.