Time-of-Flight Constraints on Neutrino Millicharge from Supernova Neutrinos in Galactic Magnetic Fields

Abstract

A millicharged neutrino propagating through magnetic fields experiences a small Lorentz-force deflection, which induces a geometric time delay. In the ultra-relativistic regime relevant for supernova neutrinos, this delay scales as $q_{\nu }^2{E}_{\nu }^{-2}$, where $q_{\nu }$ and $E_{\nu }$ denote the neutrino millicharge and energy, respectively, and thus shares the same leading energy dependence as the standard time-of-flight delay induced by neutrino mass. Motivated by this similarity, we propose a framework to reinterpret supernova time-of-flight limits on neutrino mass as constraints on neutrino millicharge. We express both effects in terms of a common $E_{\nu }^{-2}$ dispersion coefficient and compute the millicharge-induced contribution using a line-of-sight-dependent magnetic delay kernel, extending the original SN1987A uniform-field estimate. Applying this translation to existing SN1987A limits and to projected sensitivities for future Galactic core-collapse supernova observations, we obtain bounds ranging from the $\sim 10^{-17}e$ level for SN1987A to the low-$10^{-19}e$ regime for next-generation Galactic bursts, with optimistic combinations of detector sensitivity and Galactic sightline approaching $\sim 10^{-20}e$. We compare these results with other bounds in the literature and discuss how nonzero neutrino mass affects the interpretation.

Comments: 10+4 pages, 4 figures, 2 appendices. New subsection added comparing the results of JF12 to other GMF models from UF23