Revised constraints and Belle II sensitivity for visible and invisible axion-like particles

TL;DR

This paper updates constraints on axion-like particles (ALPs) across a broad mass range and demonstrates that Belle II has significantly improved sensitivity, enabling exploration of ALP-related dark matter models through visible and invisible decay channels.

Contribution

It provides a comprehensive review of existing ALP constraints and a detailed calculation of Belle II's enhanced sensitivity, especially for long-lived and decay-mode-specific ALPs, advancing experimental prospects.

Findings

Belle II sensitivity surpasses previous estimates across key parameter space.

Belle II can probe ALP-mediated dark matter models with resonant freeze-out.

Single-photon searches at Belle II can test highly predictive ALP scenarios.

Abstract

Light pseudoscalars interacting pre-dominantly with Standard Model gauge bosons (so-called axion-like particles or ALPs) occur frequently in extensions of the Standard Model. In this work we review and update existing constraints on ALPs in the keV to GeV mass region from colliders, beam dump experiments and astrophysics. We furthermore provide a detailed calculation of the expected sensitivity of Belle II, which can search for visibly and invisibly decaying ALPs, as well as long-lived ALPs. The Belle II sensitivity is found to be substantially better than previously estimated, covering wide ranges of relevant parameter space. In particular, Belle II can explore an interesting class of dark matter models, in which ALPs mediate the interactions between the Standard Model and dark matter. In these models, the relic abundance can be set via resonant freeze-out, leading to a highly predictive scenario consistent with all existing constraints but testable with single-photon searches at Belle II in the near future.