Supernova constraints on lepton flavor violating ALPs

TL;DR

This work investigates how core-collapse supernova cooling constrains lepton-flavor-violating axion-like particles that couple to electrons and muons. By evaluating ALP production through four channels—muon decay, lepton bremsstrahlung, electron-muon coalescence, and semi-Compton—and accounting for ALP absorption with realistic SN profiles, the authors identify the electron-muon coalescence and semi-Compton processes as the dominant production mechanisms in distinct mass regimes. They find that these channels yield new SN cooling constraints in the $m_a \in (m_a)$ range $m_a \in (105,280)$ MeV, reaching couplings as small as $g_{ae\mu} \sim 4\times 10^{-10}$ near $m_a \simeq 200$ MeV, and that absorption effects are crucial for accuracy. These SN-derived bounds complement laboratory limits, especially where rare muon decays are kinematically inaccessible, and highlight the mantle contribution and potential energy deposition implications for future SN observations.

Abstract

Supernovae offer a unique hot and dense environment to probe new physics beyond the Standard Model. We investigate supernova cooling constraints on lepton-flavor-violating (LFV) axions and axion-like particles (ALPs) that couple to electrons and muons. For LFV-ALP production in supernovae, muon decay and lepton bremsstrahlung have been considered previously. In this work, we identify the electron-muon coalescence channel as an efficient new production mechanism in the high-mass regime. We also include the semi-Compton scattering process, which has recently been shown to provide sizable contributions for electron-coupled ALPs. We find that muon decay dominates in the low-mass regime, electron-muon coalescence becomes the leading channel at high masses, and semi-Compton scattering provides the dominant contribution in the intermediate mass range. We find that the electron-muon coalescence process yields the strongest constraints in the mass range of $\sim (115,280)$ MeV, probing the ALP-electron-muon coupling down to $\sim 4\times 10^{-10}$ for an ALP mass of $\sim200$ MeV.

Figures (5)

• Figure 1: The profiles of temperature $T$, electron chemical potential $\mu_e$, muon chemical potential $\mu_\mu$, proton chemical potential $\mu_p$, effective proton mass $m_p^*$, and the negative of the proton interaction potential $U_p$, as well as the gravitational lapse, for the Garching muonic SN model SFHo-18.8 at 1 second postbounce garching-profile. The effective electron mass is calculated with $T$ and $\mu_e$ using Eq. \ref{['eq:e-mass']}. Note that the proton chemical potential $\mu_p$ given in garching-profile excludes the rest mass; here, we include the rest mass in $\mu_p$.
• Figure 2: LFV-ALP production processes in the SN. Upper: muon decay (left) and lepton bremsstrahlung (middle and right). Lower: $e$-$\mu$ coalescence (left) and semi-Compton (middle and right).
• Figure 3: Total ALP production rate up to the gain radius as a function of the ALP mass, for different production channels: (1) muon decay (orange), (2) lepton bremsstrahlung: $e+p\to \mu+p+a$ (blue solid) and $\mu+p\to e+p+a$ (blue dashed), (3) $e$-$\mu$ coalescence: $e^-+\mu^+\to a$ (black solid) and $\mu^-+e^+ \to a$ (black dashed), (4) semi-Compton: $\gamma+e\to \mu+a$ (red solid) and $\gamma+\mu\to e+a$ (red dashed). Here we fix $g_{ae\mu} = 10^{-10}$.
• Figure 4: The SN cooling constraints on LFV-ALPs (red), using the Garching muonic SN model SFHo-18.8 garching-profile. We consider four LFV-ALP production processes: (i) muon decay, (ii) lepton bremsstrahlung, (iii) $e$-$\mu$ coalescence, and (iv) semi-Compton. Also shown is the SN cooling limit with the muon decay process only (black curve) Calibbi:2020jvd. The gray shaded region shows the constraints from rare muon decay experiments: Derenzo Derenzo:1969za, Bilger et al. Bilger:1998rp, Jodidio et al. Jodidio:1986mz, TWIST TWIST:2014ymv, and PIENU PIENU:2020loi.
• Figure 5: Absorption processes of LFV-ALP in the SN. Upper: $e$-$a$ coalescence (left) and inverse bremsstrahlung (middle and right). Lower: ALP decay (left) and inverse Compton (middle and right).