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 $F170LP$-$G235M$ blank-sky data toward $\mathrm{GN}\text{-}z11$, 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 $0.1\mathrm{eV}\lesssim \Delta m\lesssim 1\mathrm{eV}$. For a sterile component saturating the dark matter abundance, the strongest sensitivity reaches $d_{NN\gamma }\lesssim 7\times 10^{-14}{\mathrm{GeV}}^{-1}$ and ${\Gamma }_{N_1\to N_2\gamma }\lesssim 10^{-25}{\mathrm{s}}^{-1}$. We also include a minimal anomalous-Majoron benchmark, $\omega \to \gamma \gamma$, obtaining JWST sensitivity to ${\lambda }_{\omega \gamma \gamma }\sim 10^{-11}$-$10^{-9}{\mathrm{GeV}}^{-1}$ for $m_{\omega }\sim 0.6$-$1\mathrm{eV}$.

12 pages, 6 figures