Heavy QCD Axion at Belle II: Displaced and Prompt Signals

TL;DR

This paper proposes new search strategies for heavy QCD axions at Belle II, focusing on displaced and prompt signals, to explore previously inaccessible parameter regions and solve the strong CP problem.

Contribution

It introduces novel experimental search methods for heavy QCD axions at Belle II, including displaced vertex detection and photon decay channels, expanding the search parameter space.

Findings

Displaced $a\to 3\pi$ decay provides background-free signals.

Searches can probe decay constants from 100 GeV to 10 TeV.

Photon decay channel covers unexplored mass range 150-500 MeV.

Abstract

The QCD axion is a well-motivated addition to the standard model to solve the strong $C\!P$ problem. If the axion acquires mass dominantly from a hidden sector, it can be as heavy as $O(1)$ GeV, and the decay constant can be as low as $O(100)$ GeV without running into the axion quality problem. We propose new search strategies for such heavy QCD axions at the Belle II experiment, where the axions are expected to be produced via $B\to K a$. We find that a subsequent decay $a\to 3\pi$ with a displaced vertex leads to a unique signal with essentially no background, and that a dedicated search can explore the range $O(1$-$10)$ TeV of decay-constant values. We also show that $a\to \gamma\gamma$ can cover a significant portion of currently unexplored region of $150 \lesssim m_a \lesssim 500$ MeV.