Neutral $B$-anomalies from an $\mathit{on\text{-}shell}$ scalar exchange

TL;DR

This paper proposes that on-shell scalar exchange can explain neutral B-anomalies, predicts measurable effects in specific observables, and compares scalar singlet models to axion-like particles, expanding the viable parameter space.

Contribution

It introduces a scalar singlet exchange model to explain B-anomalies, showing it fits data within 1σ and predicts distinctive experimental signatures.

Findings

Solutions fit anomalies within 1σ for specific mass ranges.

Predicts sizable effects in null tests like the flat term F_H.

Negligible corrections to B_s → e^+ e^- branching ratio.

Abstract

The neutral $B$-anomalies are analysed in terms of the tree-level exchange of a (pseudo)scalar gauge singlet $a$. Solutions to both $R_{K^{(*)}}$ central bin anomalies are found within $1\sigma$ for ${m_{a}^2 \in [{1.1},\,{6}]\,\text{GeV}^2}$, while the low $q^2$-bin anomaly can also be accounted for with masses close to the bin threshold. The impact of these solutions on other ${b\to s e^+ e^-}$ observables is discussed in detail. Due to the $\mathit{on\text{-}shell}$ enhancement, sizable effects are expected in null tests of the SM, such as the flat term, $F_H$, of the $B\to K e^+ e^-$ angular distribution. At the same time, the observable sensitive to the $K^\ast$ polarisation, $F_L\, (B\to K^* e^+ e^-)$, and the lepton forward-backward asymmetry, ${A_{FB}\, (B\to K^* e^+ e^-)}$, can be suppressed with respect to their SM values. Corrections from the new physics to $\mathcal{B}(B_s \to e^+ e^-)$ are, on the other hand, negligible. Along with the previous observables, improved measurements of the cross section ${\sigma (e^+ e^- \to a (e^+ e^-) \gamma)}$ could potentially probe the relevant parameter space of the model. A comparison between our results and those stemming from an axion-like particle exchange is also discussed, showing that the exchange of a general scalar singlet offers a noticeably wider parameter space.