Higgs Portal Interpretation of the Belle II $B^+ \to K^+ \nu \nu$ Measurement

TL;DR

This paper examines whether the recent Belle II measurement of $B^+ o K^+ u u$ can be explained by a minimal Higgs portal model involving a light scalar, and discusses implications for Higgs physics and dark sectors.

Contribution

It demonstrates that a light scalar in a Higgs portal model can explain the Belle II anomaly and explores the model's compatibility with Higgs decay constraints and cosmological considerations.

Findings

A scalar lighter than B mesons can accommodate Belle II results.

Current Higgs decay bounds exclude scalars heavier than ~10 GeV.

Future Higgs factories can test the favored parameter space.

Abstract

The Belle II experiment recently observed the decay $B^+ \to K^+ \nu \nu$ for the first time, with a measured value for the branching ratio of $ (2.3 \pm 0.7) \times 10^{-5}$. This result exhibits a $\sim 3\sigma$ deviation from the Standard Model (SM) prediction. The observed enhancement with respect to the Standard Model could indicate the presence of invisible light new physics. In this paper, we investigate whether this result can be accommodated in a minimal Higgs portal model, where the SM is extended by a singlet Higgs scalar that decays invisibly to dark sector states. We find that current and future bounds on invisible decays of the 125 GeV Higgs boson completely exclude a new scalar with a mass $\gtrsim 10$ GeV. On the other hand, the Belle II results can be successfully accommodated if the new scalar is lighter than $B$ mesons but heavier than kaons. We also investigate the cosmological implications of the new states and explore the possibility that they are part of an abelian Higgs extension of the SM. Future Higgs factories are expected to place stringent bounds on the invisible branching ratio of the 125 GeV Higgs boson, and will be able to definitively test the region of parameter space favored by the Belle II results.