Heavy QCD Axion in $b\to s$ transition: Enhanced Limits and Projections

TL;DR

This paper investigates a heavy QCD axion with mass around a GeV, analyzing its detection prospects via B-meson decays, performing novel 2-loop calculations, and projecting future experimental sensitivities.

Contribution

It provides the first 2-loop calculations for heavy QCD axion production in B decays and enhances existing experimental bounds, offering new insights into axion detection strategies.

Findings

Existing limits are improved by at least an order of magnitude.

Bounds are robust against unknown UV physics.

Future Belle II data could probe axion decay constants of a few TeV.

Abstract

We study a "heavy" QCD axion whose coupling to the standard model is dominated by $a G \widetilde{G}$ but with $m_a \gg m_\pi f_\pi / f_a$. This is well motivated as it can solve the strong CP problem while evading the axion quality problem. It also poses interesting challenges for its experimental search due to its suppressed couplings to photons and leptons. Such axion with mass around a GeV is kinematically inaccessible or poorly constrained by most experimental probes except B-factories. We study $B \to K a$ transitions as a powerful probe of the heavy QCD axion by performing necessary 2-loop calculations for the first time, together with some improvement on the existing analysis strategy. We find some of the existing limits are enhanced by at least an order of magnitude. We also demonstrate that the bounds are robust against unknown UV physics. For forthcoming data sets of the Belle II experiment, we provide a projection that $f_a$ of a few TeV is within its future reach, which is relevant to the quality problem.