Probing axion and flavored new physics with the NA64$μ$ experiment

TL;DR

This work shows that high-energy muon beam dumps like NA64μ can probe flavor-dependent beyond-Standard-Model physics through missing-energy and energy-deposition signatures. It analyzes three benchmarks—axion–photon coupling, axion–muon coupling, and a flavor-dependent dark sector mediated by a massless Z' of U(1)_{L_mu-L_tau}—using Weizsäcker–Williams-based production and two signal channels (visible and invisible). Current NA64μ data place new constraints (e.g., g_a_mu_mu ≳ 4×10^-3 GeV^-1 for m_a ≲ 0.2 GeV and improved bounds on (g_Z' g_chi)^2 for m_chi ≲ 1 GeV) while the axion–photon channel remains competitive primarily through future higher muon luminosities. Looking ahead, planned runs with MOT up to 10^14 promise substantial improvements and offer a direct window into the flavor structure of new physics, including potential tau-related signatures, with sensitivity scaling depending on the specific model.

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$μ$ experiment: (1) axions coupling to photons, (2) axions coupling to muons, and (3) a dark sector mediated by a massless $U(1)_{L_μ-L_τ}$ 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$μ$ data do not yet probe new parameter region on axion-photon coupling, while excluding new parameter space for the axion-muon coupling $g_{aμμ}\gtrsim4\times10^{-3}$~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$μ$ 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.

Figures (4)

• Figure 1: Feynman diagrams for the production of dark particles in the NA64$\mu$ experiment: (a) axion production through photon-photon fusion, (b) axion production through muon bremsstrahlung, and (c) a dark sector where the dark particle $\chi$ is produced through a massless mediator $Z'$.
• Figure 2: 90% C.L. limits on the axion-photon coupling with the current NA64$\mu$ data (blue shaded region for the visible channel and orange shaded region for the invisible channel), and the projected sensitivity with $1.5\times10^{11}$, $10^{12}$ and $10^{14}$ muon on target (dashed blue lines for the visible channel and orange lines for invisible). Existing constraints from collider and beam dump experiments are shown as grey shaded regions, including LEP Jaeckel:2015jla, OPAL Knapen:2016moh, PrimEx Aloni:2019ruo, BESIII BESIII:2022rzzBESIII:2024hdv, E141 Dobrich:2017gcm, NA64 electron beam dump NA64:2020qwq, BaBar Dolan:2017osp, NuCal Blumlein:1990ay, E137 Bjorken:1988as and CHARM CHARM:1985anb.
• Figure 3: Same as Fig. \ref{['fig:axion-photon']} but for axion-muon coupling using the invisible channel. Existing constraints are shown as grey shaded regions, including BaBar BaBar:2016sci and SN1987A Croon:2020lrfBollig:2020xdrCaputo:2021ruxCaputo:2022rca.
• Figure 4: Same as Fig. \ref{['fig:axion-photon']} but for a flavor-dependent dark sector using the invisible channel. We recast the existing limits of millicharged particles in the grey shade regions, including Colliders Davidson:2000hf, MilliQan Ball:2020dnx, ArgoNeuT ArgoNeuT:2019ckq, MiniBooNE Magill:2018tbb, LSND Magill:2018tbb, SLAC mQ Prinz:1998ua, BEBC Marocco:2020dqu and SENSEI SENSEI:2023gie.