Refined Low-Energy Supernova Constraints on Lepton Flavor Violating Axions

TL;DR

This work derives refined LESN constraints on LFV-ALPs with electron–muon couplings by incorporating four production channels and a detailed mantle-energy accounting. It computes both core-to-mantle energy transfer (E_c) and mantle energy drain (E_m) using full 10 s Garching SN profiles and explicit ALP absorption along trajectories, yielding a net deposition E_d = E_c − E_m that must satisfy E_d ≤ 0.1 B. The analysis reveals a mass-dependent hierarchy of production channels: muon decay dominates at m_a ≲ 30 MeV, semi-Compton scattering governs 30 MeV ≲ m_a ≲ 110 MeV, and electron–muon coalescence dominates for m_a ≳ 110 MeV, with lepton bremsstrahlung subdominant. Including mantle energy drain and accurate absorption leads to LESNe providing the strongest constraints for m_a ≳ 110 MeV, improving prior SN bounds on LFV-ALPs and underscoring LESNe’s sensitivity to lepton-flavor-violating new physics, with bounds reaching g_{aeμ} ∼ 2 × 10^{−11} at m_a ≈ 200 MeV in the updated analysis.

Abstract

The supernova (SN) core, characterized by its extreme temperature and density, serves as a unique laboratory for new-physics searches. Low-energy supernovae (LESNe) provide particularly powerful probes, as their low explosion energies place stringent limits on any additional energy deposition in the mantle by new particles. We present refined LESN constraints on lepton-flavor-violating (LFV) axions and axion-like particles (ALPs) with electron-muon couplings. We consider four production channels in the SN: muon decay, lepton bremsstrahlung, electron-muon coalescence, and semi-Compton scattering, the last of which is investigated here for the first time in the context of LFV-ALPs. We find that muon decay dominates in the low-mass regime, electron-muon coalescence in the high-mass regime, and semi-Compton scattering in the intermediate-mass range. To derive accurate limits, we compute both the energy transfer from the SN core to the mantle and the energy loss due to ALP production in the mantle, which can be substantial for both large and small couplings -- the latter case, to our knowledge, not previously noted in the literature. We find that LESNe provide the most stringent constraints on the parameter space for ALP masses above $\sim 110$ MeV. These refined results strengthen previous SN bounds and highlight the exceptional sensitivity of LESNe to LFV new physics.

Figures (11)

• Figure 1: Schematic plot of the production position $r$, the propagation distance $s$, and the angle $\beta$ inside the SN core.
• Figure 2: Left: Spatial evolution of the Garching profiles for the SFHo-18.8 SN model garching-profile at 1 s post-bounce. Right: Temporal evolution of the same model at a fixed radial distance of $r = 5$ km, spanning 10 seconds post-bounce. The displayed profiles include the temperature $T$, the electron chemical potential $\mu_e$, the muon chemical potential $\mu_\mu$, the negative of the proton interaction potential $U_p$, the in-medium proton mass $m_p^*$, the proton chemical potential $\mu_p$ which includes $m_p^\ast$Li:2025beu, the gravitational lapse factor, and the in-medium electron mass $m_e$ calculated using Eq. \ref{['eq:e-mass']}.
• Figure 3: Production processes of LFV-ALP in the SN: (a) muon decay, (b) lepton bremsstrahlung, and (c) $e$-$\mu$ coalescence.
• Figure 4: The Feynman diagrams for the semi-Compton scattering $\ell + \gamma\rightarrow \ell' + a$.
• Figure 5: LFV-ALP production rates per unit volume at $t_\mathrm{pb}=1$ s and $r=5$ km with $g_{ae\mu}=10^{-9}$ from various channels: (1) muon decay $\mu^-\rightarrow e^-a$ (solid orange), (2) lepton bremsstrahlung $e^-p\rightarrow\mu^-pa$ (solid blue) and $\mu^-p\rightarrow e^-pa$ (dashed blue), (3) electron-muon coalescence $e^-\mu^+\rightarrow a$ (solid black) and $e^+\mu^-\rightarrow a$ (dashed black), and (4) semi-Compton scattering $\gamma e^-\rightarrow\mu^-a$ (solid red) and $\gamma\mu^-\rightarrow e^-a$ (dashed red). We use the Garching group's muonic model SFHo-18.8. The $\mu^-p\rightarrow e^-pa$ channel is shown for masses above $\sim 100$ MeV (multiplied with a factor of $10^4$) since we require $m_e+m_a>m_\mu$ for this channel to avoid double counting with muon decay.