Constraining the Axion Portal with B -> K l+ l-

TL;DR

The paper analyzes constraints on axion‑like states in the axion portal framework by exploiting flavor changing neutral current decays b→s a. The authors derive a finite b→s a amplitude from a PQ‑symmetric 2HDM, connect it to B→K a rates via standard form factors, and confront the predictions with B→Kℓℓ data to place multi‑TeV lower bounds on the axion decay constant f_a, especially at small tanβ. These bounds have significant implications for axion portal dark matter scenarios and NMSSM–like models with a light pseudoscalar, and they can be strengthened by dedicated analyses at BaBar, Belle, LHCb, and future super‑B factories. The work highlights the power of flavor observables to probe new light states and demonstrates complementary search channels to direct axion probes.

Abstract

We investigate the bounds on axionlike states from flavor-changing neutral current b->s decays, assuming the axion couples to the standard model through mixing with the Higgs sector. Such GeV-scale axions have received renewed attention in connection with observed cosmic ray excesses. We find that existing B->K l+ l- data impose stringent bounds on the axion decay constant in the multi-TeV range, relevant for constraining the "axion portal" model of dark matter. Such bounds also constrain light Higgs scenarios in the next-to-minimal supersymmetric standard model. These bounds can be improved by dedicated searches in B-factory data and at LHCb.

Figures (4)

• Figure 1: Bounds on $f_a$ as a function of $\tan\beta$ and $m_H$ for $n=1$ in Eq. (\ref{['thetadef']}), for $m_a^2\ll m_B^2$. For each displayed value of $f_a$ there are two contour lines, and the region between them is allowed for $f_a$ below the shown value. The bound disappears along the dashed curve, and gets generically weaker for larger $\tan\beta$.
• Figure 2: The shaded regions of $f_a \tan^2 \beta$ are excluded in the large $\tan \beta$ limit. To indicate the region of validity of the large $\tan \beta$ approximation, the dashed (dotted) curve shows the bound for $\tan \beta = 3$ ($\tan \beta = 1$).
• Figure 3: Bounds on $\sin^2\theta\, \text{Br}(a\to\mu^+\mu^-)$ as a function of $\tan\beta$ and $m_H$. Similar to Fig. \ref{['fig:fabound']}, the successively darker regions going away from the dashed curve are allowed for $\sin^2\theta\, \text{Br}(a\to\mu^+\mu^-)$ above the indicated values. When $m_a$ is not small compared to $m_B$, these bounds should be modified by Eq. (\ref{['abovetau']}), but this is a small effect.
• Figure 4: Bounds on $\sin^2\theta\, \text{Br}(a\to\mu^+\mu^-)/\tan^2 \beta$ in the large $\tan \beta$ limit. The shaded region is excluded, and the dashed (dotted) curve shows $\tan \beta = 3$ ($\tan \beta = 1$).