Infrared Lines from Sterile-Neutrino Transition Magnetic Moments at JWST

Abstract

We investigate infrared line signatures from radiatively decaying sterile-neutrino dark matter using publicly available JWST/NIRSpec IFU blank-sky observations. The main signal considered is the sterile-to-sterile transition $N_1\to N_2\gamma$, induced by the transition magnetic dipole coefficient $d_{NN\gamma}$, with $m_1>m_2$. In contrast to ordinary two-photon decays of axion-like or Majoron-like particles, the observed photon energy is not fixed by the full dark matter mass, but by the small mass splitting $\Delta m=m_1-m_2$. Thus, a keV-scale sterile-neutrino dark matter state can generate an eV-scale infrared photon line in the JWST band. We construct a $\chi^2$-based line-search analysis using the NIRSpec IFU $\rm F170LP$-$\rm G235M$ blank-sky data toward $\rm GN\text{-}z11$, modelling the smooth continuum with a cubic spline and including the Milky Way halo decay flux. In the absence of a significant excess, we derive projected limits on $d_{NN\gamma}$ and on the decay width $\Gamma_{N_1\to N_2\gamma}$ for $0.1~{\rm eV}\lesssim\Delta m\lesssim1~{\rm eV}$. For a sterile component saturating the dark matter abundance, the strongest sensitivity reaches $d_{NN\gamma}\lesssim7\times10^{-14}~{\rm GeV}^{-1}$ and $\Gamma_{N_1\to N_2\gamma}\lesssim10^{-25}~{\rm s}^{-1}$. We also include a minimal anomalous-Majoron benchmark, $\omega\to\gamma\gamma$, obtaining JWST sensitivity to $\lambda_{\omega\gamma\gamma}\sim10^{-11}$-$10^{-9}~{\rm GeV}^{-1}$ for $m_\omega\sim0.6$-$1~{\rm eV}$.