ALP production in Lepton Flavour Violating meson, tau and gauge boson decays
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2026
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| author | Ardu, Marco Calibbi, Lorenzo Fedele, Marco Mescia, Federico |
| author_facet | Ardu, Marco Calibbi, Lorenzo Fedele, Marco Mescia, Federico |
| contents | In this paper we study axion-like particles (ALPs) with lepton-flavour-violating (LFV) couplings in the mass regime above the muon threshold, $m_a>m_μ$, where the strong bound from the exotic muon decay $μ\to ea$ no longer apply and the decay channel $a\to eμ$ becomes kinematically accessible. In this region, the ALP typically decays promptly, motivating new search strategies based on its production in decays involving virtual muons. We analyse charged-meson and $W$ decays, neutral-current processes such as $Z$ and quarkonium decays, and, when couplings to the third generation are present, LFV $τ$ decays. The subsequent decay $a\to eμ$ leads to striking LFV signatures with negligible Standard Model backgrounds. Combining these production modes with current low-energy constraints, we assess the sensitivity of future high-energy $e^+e^-$ colliders, flavour factories such as Belle II and STCF, fixed-target experiments such as NA62, and proton beam-dump facilities such as SHiP. Overall, our results identify LFV ALP production in meson, gauge-boson, quarkonium and $τ$ decays (with displaced vertices) as a promising and largely unexplored avenue to test ALP interactions with charged leptons above the muon mass threshold. |
| format | Preprint |
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arxiv_https___arxiv_org_abs_2604_11889 |
| institution | arXiv |
| publishDate | 2026 |
| record_format | arxiv |
| spellingShingle | ALP production in Lepton Flavour Violating meson, tau and gauge boson decays Ardu, Marco Calibbi, Lorenzo Fedele, Marco Mescia, Federico High Energy Physics - Phenomenology High Energy Physics - Experiment In this paper we study axion-like particles (ALPs) with lepton-flavour-violating (LFV) couplings in the mass regime above the muon threshold, $m_a>m_μ$, where the strong bound from the exotic muon decay $μ\to ea$ no longer apply and the decay channel $a\to eμ$ becomes kinematically accessible. In this region, the ALP typically decays promptly, motivating new search strategies based on its production in decays involving virtual muons. We analyse charged-meson and $W$ decays, neutral-current processes such as $Z$ and quarkonium decays, and, when couplings to the third generation are present, LFV $τ$ decays. The subsequent decay $a\to eμ$ leads to striking LFV signatures with negligible Standard Model backgrounds. Combining these production modes with current low-energy constraints, we assess the sensitivity of future high-energy $e^+e^-$ colliders, flavour factories such as Belle II and STCF, fixed-target experiments such as NA62, and proton beam-dump facilities such as SHiP. Overall, our results identify LFV ALP production in meson, gauge-boson, quarkonium and $τ$ decays (with displaced vertices) as a promising and largely unexplored avenue to test ALP interactions with charged leptons above the muon mass threshold. |
| title | ALP production in Lepton Flavour Violating meson, tau and gauge boson decays |
| topic | High Energy Physics - Phenomenology High Energy Physics - Experiment |
| url | https://arxiv.org/abs/2604.11889 |