Probing axion and muon-philic new physics with muon beam dump

Abstract: High-energy muon beam dump experiments are powerful probes of new physics models beyond the Standard Model, particularly those involving muon-philic interactions. In this study, we place 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 muon-philic dark sector mediated by a massless $U(1){L\mu-L_\tau}$ gauge boson. A key signature of these models is the significant missing energy from the production of new particles that either escape the downstream detectors or decay between them. We find that the current NA64$\mu$ data do not yet probe new parameter region on axion-photon coupling. For muon-philic axions, the current NA64$\mu$ data can exclude 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 a muon-philic dark sector, the current data provide stringent constraints that surpass existing ones by nearly one order of magnitude. The data from 2023 NA64$\mu$ run, once available, will be capable of excluding new axion-photon coupling parameter space and examine the axion explanation of muon $g-2$ deviation, with more sensitivity advancement expected in near future runs.