Searching for axion-like particles from tau exotic decays at the Super Tau-Charm Facility and its far detectors

TL;DR

The paper investigates leptophilic axion-like particles (ALPs) produced in exotic tau decays at the future Super Tau-Charm Facility (STCF). It develops an effective field theory framework for ALP–lepton interactions, examining two electroweak structures (EW preserving and EW violating) and two decay scenarios (tauphilic and LFU), and analyzes both prompt and far-detector strategies to maximize reach. Using Monte Carlo simulations, it derives sensitivities across ALP masses and couplings, showing that tauphilic ALPs can be probed in a high-mass region around $m_a\sim 1$ GeV with lifetimes enabling far-detector detection, while LFU ALPs yield much shorter lifetimes, enhancing main-detector searches in the $m_a \sim 100$–$200$ MeV range. The results demonstrate the STCF’s potential to explore previously unprobed regions of leptophilic ALP parameter space and motivate the development of a far detector to complement the main detector.

Abstract

Leptophilic axion-like particles (ALPs) extend the Standard Model (SM) with brand-new interactions between the ALP and leptons. In this work, we focus on studying exotic $τ$ decays to explore such ALPs. Both the tauphilic and lepton flavor universal (LFU) scenarios, with electroweak preserving and violating benchmarks, have been investigated at a future $τ$-factory, namely the Super Tau-Charm Facility (STCF) under development in China. Both prompt and far detection are proposed, targeting on leptonic $τ$ decays, $τ^- \to \ell^-\barν_\ell ν_τa$. For prompt detection, the $τ^+τ^-$ threshold $\sqrt s=3.56$ GeV and higher central energy $\sqrt s=4.2$ GeV are taken into account. In addition, a cylinder-like far detector has been proposed to complement prompt detection. We demonstrate the huge potential to detect leptophilic ALPs at the STCF. Concretely, for tauphilic scenario, the STCF is mostly sensitive in the currently unprobed region with $m_a\sim 1000$ MeV. However, in the LFU scenario, the dilepton channel tremendously shortens the ALP lifetime, and eventually the STCF only allows a precise measurement in the new regime with $100~\text{MeV}\lesssim m_a\lesssim 200~\text{MeV}$.

Figures (5)

• Figure 1: The Feynman diagrams of ALP productions via leptonic $\tau$ decays. The red dot represents the vertex involving an ALP.
• Figure 2: The branching ratios for ALP productions via leptonic $\tau$ decays. The EWP and EWV scenarios are shown for the $\mu$-mode decay as dashed and solid curves, respectively. The corresponding $e$-mode decay yields nearly identical results (within $\sim 5\%$), and is omitted for clarity.
• Figure 3: Left: The lifetime of leptophilic ALP, with red and blue lines representing tauphilic and LFU benchmarks, respectively. Right: When LFU is assumed, the branching ratios of the ALP decays are presented in red, blue and orange lines, showing $a\to \gamma\gamma$, $a\to e^+e^-$ and $a\to \mu^+\mu^-$, respectively.
• Figure 4: Left: In the EWP scenario, the projections at $95\%$ confidence level are achieved at the main and far detectors. The red curve stands for far detection. The blue and orange curves represent projections for main detector with $\sqrt s=3.56$ GeV and $\sqrt s=4.2$ GeV, respectively. Dashed and solid curves correspond to the main detection outside and inside the ECAL, respectively. Existing bounds from other facilities are also shown for comparison, including NA64 NA64:2024klw, LEP Jaeckel:2015jla, E137 Bjorken:1988asEberhart:2025lyu, and CHARM CHARM:1985anb. Right: Comparison between the EWP and EWV scenarios for main and far detections.
• Figure 5: The projections at $95\%$ confidence level for ALPs with LFU. The far detection, main detection at $\sqrt s = 3.56$ GeV and main detection at $\sqrt s = 4.2$ GeV are shown in red, blue and orange curves, respectively. The blue dashed curve corresponds to the projections achieved with $l_1=0$ at $\sqrt s=3.56$ GeV. For comparison, the limits achieved at other experiments are presented as shaded regions, including NA64 NA64:2024klw, SINDRUM SINDRUM:1986klz, Charm CHARM:1985anb, E137 Bjorken:1988asEberhart:2025lyu, BaBar BaBar:2020jma, Belle Belle:2022gbl and the latest muon $(g-2)_\mu$ measurement Muong-2:2025xyk with the SM prediction Aliberti:2025beg.