Probing axion and flavored new physics with the NA64$\mu$ experiment

TL;DR

This paper evaluates the NA64μ experiment's ability to detect new physics involving axions and dark sectors, setting new constraints on certain models and highlighting the potential of muon beam dump experiments for future discoveries.

Contribution

It demonstrates the potential of the NA64μ experiment to constrain axion and dark sector models, providing the first such constraints from recent data and projecting future sensitivities.

Findings

Current data exclude new axion-muon coupling parameter space.

Dark sector constraints surpass existing bounds by nearly an order of magnitude.

Future data will further probe axion-photon coupling and flavor-dependent new physics.

Abstract

High-energy muon beam dump experiments are powerful probes of new physics models beyond the Standard Model, particularly those involving flavor-dependent interactions. We demonstrate the potential of muon beam dump by placing strong constraints on three new physics models utilizing data from the recent NA64$\mu$ experiment: (1) axions coupling to photons, (2) axions coupling to muons, and (3) a dark sector mediated by a massless $U(1)_{L_\mu-L_\tau}$ gauge boson. The new particles can be identified from the significant missing energy as the invisible channel, or the distinct energy deposition signature as the visible channel. We find that the current NA64$\mu$ data do not yet probe new parameter region on axion-photon coupling, while excluding new parameter space for the axion-muon coupling $g_{a\mu\mu}\gtrsim4\times10^{-3}$~GeV$^{-1}$ and the axion mass $m_a\lesssim 0.2$~GeV. For the dark sector, the current data provide stringent constraints that surpass existing ones by nearly one order of magnitude. The data from the 2023 NA64$\mu$ run, once available, will be capable of excluding new axion-photon coupling parameter space and probing the flavor structure of new physics, with more sensitivity advancement expected in near future runs.