Signatures of Light New Particles in $B\to K^{(*)} E_{\rm miss}$

TL;DR

This paper explores new physics explanations for the Belle II $B o K E_{miss}$ anomaly, proposing light-invisible particles as alternatives to neutrinos, and identifies preferred scenarios and parameter regions consistent with experimental data.

Contribution

It introduces novel light-invisible particle scenarios in $B$ decays, reconstructs likelihoods from experimental data, and identifies preferred couplings and masses for these new particles.

Findings

Current data favors two-body decays with a single massive scalar or vector.

Three-body decays with pairs of scalars or fermions are also compatible.

Preferred parameter regions for couplings and masses are identified.

Abstract

The recent Belle II observation of $B \to K E_{\rm miss}$ challenges theoretical interpretations in terms of Standard Model neutrino final states. Instead, we consider new physics scenarios where up to two new light-invisible particles of spin 0 up to 3/2 are present in the final state. We identify viable scenarios by reconstructing the (binned) likelihoods of the relevant $B \to K^{(*)} E_{\rm miss}$ and also $B_s \to E_{\rm miss}$ experimental analyses and present preferred regions of couplings and masses. In particular, we find that the current data prefers two-body decay kinematics involving the emission of a single massive scalar or a vector particle, or alternatively, three-body decays involving pairs of massive scalars or spin 1/2 fermions. When applicable, we compare our findings with existing literature and briefly discuss some model-building implications.