Low-Energy Supernova Constraints on Lepton Flavor Violating Axions

TL;DR

This paper investigates low-energy supernova (LESN) bounds on lepton-flavor-violating axions/ALPs that couple to electrons and muons, focusing on masses above the threshold $m_a > m_\mu + m_e$. By requiring that energy deposition in the SN mantle from ALP production and decay remains below the LESN explosion energy ($E_m \le 0.1 B$), the authors compute constraints using the electron-muon coalescence production channel within the SFHo-18.8 LESN SN model and include in-medium corrections to the electron mass. They find that LESN bounds constrain $g_{ae\mu}$ down to $\mathcal{O}(10^{-11})$ for $m_a$ in the range $\sim 110$--$550$ MeV, extending beyond existing SN cooling limits. This work demonstrates that LESNe probe a previously unexplored region of LFV ALP parameter space with potential implications for astrophysical and particle-physics constraints on ALP models.

Abstract

The extreme conditions within the supernova core, a high-temperature and high-density environment, create an ideal laboratory for the search for new physics beyond the Standard Model. Of particular interest are low-energy supernovae, characterized by their low explosion energies, which place strong constraints on the new-physics energy transfer from the core to the mantle. We compute low-energy supernova constraints on lepton-flavor-violating axions and axion-like particles that couple to both electrons and muons. For axion mass above the muon mass, the electron-muon coalescence and the axion decay are dominant production and reabsorption processes, respectively. We find that the low-energy supernovae provide the most stringent constraints on the axions in the mass range of $\sim (110,550)$ MeV, probing the coupling constant down to $g_{aeμ} \simeq {\cal O}(10^{-11})$.

Figures (6)

• Figure 1: The electron-muon coalescence processes for the ALP production: $e^- + \mu^+ \rightarrow a$ (left) and $\mu^- + e^+ \rightarrow a$ (right).
• Figure 2: The Garching profiles for the SFHo-18.8 SN model at 1 second postbounce garching-profile: temperature $T$, the electron chemical potential $\mu_e$, the muon chemical potential $\mu_\mu$, the gravitational lapse factor, and the in-medium electron mass $m_e$ computed via Eq. \ref{['eq:e-mass']}.
• Figure 3: The ALP decay processes: $a \rightarrow e^- + \mu^+$ (left) and $a \rightarrow \mu^- + e^+$ (right).
• Figure 4: Left panel: ALP production rate as a function of the radius $r$ in $e^- + \mu^+ \rightarrow a$ (blue dashed) and $\mu^- + e^+ \rightarrow a$ (red solid). Right panel: ALP absorption rate (normalized to its vacuum value, $\Gamma_0$) as function of radius $r$ for $a \rightarrow e^- + \mu^+$ (blue dashed) and $a \rightarrow \mu^- + e^+$ (red solid). For both panels, we use $E_a=250$ MeV, $m_a=200$ MeV, and $g_{ae\mu}=1.4 \times 10^{-11}$. $R_{\rm NS}=20$ km is the neutron star core radius (black dotted).
• Figure 5: Left panel: Electron chemical potential $\mu_e$ (red) and muon chemical potential $\mu_\mu$ (blue) from the SFHo-18.8 model profiles garching-profile. Right panel: Number densities of $e^-$ (red-solid), $\mu^-$ (blue-solid), $e^+$ (red-dashed), and $\mu^+$ (blue-dashed) calculated using Eq. \ref{['eq:number-density']}.