Muon-Decay Parameters from COHERENT

TL;DR

The paper demonstrates that coherent elastic neutrino-nucleus scattering (CEνNS) measurements at spallation sources can constrain muon-decay physics, introducing the first direct constraints on muon-antineutrino Michel parameters from COHERENT data. It develops an EFT framework for muon decay with Wilson coefficients $g^X_{\epsilon\eta}$, links nonstandard production to flavor-dependent effective charges, and performs a numerical analysis of COHERENT data to bound the antineutrino parameters $P_{\bar{\nu}_L}$ and $w_{\bar{\nu}_L}$, as well as the vector coupling $|g^V_{LL}|^2$. The study also derives constraints on pion-decay pseudoscalar couplings and projects significant improvements with future CEνNS measurements (e.g., CENNS-750). Overall, the work broadens the applicability of CEνNS to production-level new physics and provides a path to probing lepton-flavor–violating interactions at SMEFT scales.

Abstract

We demonstrate that measurements of Coherent Elastic Neutrino-Nucleus Scattering (CE$ν$NS) at spallation sources are valuable probes of muon-decay physics. Using COHERENT data we derive the first direct constraint on the Michel parameters governing the $\barν_μ$ energy distribution. We also discuss future sensitivities, the implications for the Lorentz structure of the interactions mediating muon decay and the application to other neutrino-production mechanisms like pion decay.

Figures (3)

• Figure 1: Feynman diagram for $\mu^+$ decay and the subsequent CE$\nu$NS detection of the emitted muon antineutrino, providing access to its energy distribution.
• Figure 2: Allowed region (90% CL) for the $\bar{\nu}_\mu$ Michel parameters, $P_{\bar{\nu}_L}$ and $w_{\bar{\nu}_L}$, cf. Eq. \ref{['eq:DifferentialWidthAntineutrino']}. The use of $P_{\nu_L}=1$ and $w_{\nu_L}=0$ (SM values) or the intervals $P_{\nu_L} \,>\, 0.92$ and $w_{\nu_L} < 0.12$PhysRevLett.81.520, leads to negligible differences in the figure. The solid (dashed) line is the current (projected) bound. The region with unphysical values for $P_{\bar{\nu}_L}$ and $w_{\bar{\nu}_L}$ is grayed out.
• Figure 3: $\Delta\chi^2$ function for the $g^V_{LL}$ Wilson Coefficient, assuming only $g^V_{LL}$ and $g^S_{LL}$ are present. The solid (dashed) line is the current (projected) bound. The black line refers to the SM limit and the gray region to the excluded region, Eq. \ref{['eq:muon_decay_norm']}. We also include the lower $90\%$ CL bound from inverse muon decay (red) and KARMEN (green). Their upper bound is the SM value, i.e., the unity.