Implications of recent $\left(g-2\right)_μ$ measurements for MeV-GeV dark sector searches

TL;DR

The paper exploits the now-consistent muon $g-2$ measurements to constrain light BSM mediators in the MeV--GeV mass window. It computes ALP (pseudoscalar and scalar) contributions to $a_\mu$ via derivative couplings, including Yukawa-like and Barr-Zee diagrams, and translates them into exclusion regions in $(m_a, g_{a\gamma\gamma})$ and $(g_{a\mu\mu}, g_{a\gamma\gamma})$ using a UV cutoff $\Lambda=1$ TeV; it also analyzes dark-photon scenarios with vector and axial-vector couplings, highlighting the stronger low-mass constraints for the axial case. The results show that the new bounds are often competitive with or stronger than existing collider exclusions and that the magnetic moment measurements provide complementary, relatively model-independent probes, especially when combined with upcoming Belle II, BESIII, and fixed-target data. Overall, the study maps a robust, multi-port approach to probing MeV--GeV dark sectors, guiding future experimental searches and UV-completion considerations.

Abstract

Recent theoretical and experimental studies of the muon magnetic moment indicate the absence of the previously reported discrepancy, providing a vital opportunity to constrain potential BSM physics. In this work, we explore the MeV-GeV mass range, where existing exclusion limits remain relatively loose. We analyze both scalar and pseudoscalar as well as vector and axial vector mediators. We demonstrate that the new bounds are not only comparable to - but in several cases, significantly more stringent than - the constraints obtained from previous collider experiments, even when near-future projections are considered.

Figures (7)

• Figure 1: Yukawa-like correction to the anomalous magnetic moment.
• Figure 2: ALP-muon and scalar-muon coupling constraints under the assumption of a single-coupling contribution. $X$ denotes either ALP or scalar. The colored regions are excluded at $2\sigma$ level.
• Figure 3: Barr-Zee correction to the anomalous magnetic moment.
• Figure 4: Coupling constraints, projected onto the $\left(g_{a\mu\mu},g_{a\gamma\gamma}\right)$ plane (the colored regions are excluded at $2\sigma$ level). Red refers to $\left|g_{a\mu\mu}\right|=\left|g_{a\gamma\gamma}\right|$ scenario, blue stands for $\left|g_{a\mu\mu}\right|=-\left|g_{a\gamma\gamma}\right|$. The visible thin line in the first case represents the situation where the two contributions cancel each other out. Dashed lines are used for the model-dependent bounds. Dot-dashed line for Belle II is the projection based on the assumption of $50 \, \text{ab}^{-1}$ integrated luminosity.
• Figure 5: Coupling constraints, projected onto the $\left(m_a,g_{a\gamma\gamma}\right)$ plane (the colored regions are excluded at $2\sigma$ level). Red refers to $\left|g_{a\mu\mu}\right|=\left|g_{a\gamma\gamma}\right|$ scenario, blue stands for $\left|g_{a\mu\mu}\right|=-\left|g_{a\gamma\gamma}\right|$. The visible thin line in the first case represents the situation where the two contributions cancel each other out. For electron the difference between two possibilities is negligible in this domain, as the Barr-Zee diagram dominates. It also becomes noncompetitive for lower values of $g_{a\mu\mu}$ and thus now shown on the right panel. Dashed lines are used for the model-dependent bounds. Dot-dashed line for Belle II is the projection based on the assumption of $50 \, \text{ab}^{-1}$ integrated luminosity.