Using supernova neutrinos to probe strange spin of proton with JUNO and THEIA

The strange quark contribution to proton's spin ($\Delta s$) is a fundamental quantity that is poorly determined from current experiments. Neutrino-proton elastic scattering (pES) is a promising channel to measure this quantity, and requires an intense source of low-energy neutrinos and a low-threshold detector with excellent resolution. In this paper, we propose that neutrinos from a galactic supernova and their interactions with protons in large-volume scintillation detectors can be utilized to determine $\Delta s$. The spectra of all flavors of supernova neutrinos can be independently determined using a combination of DUNE and Super-(Hyper-)Kamiokande. This allows us to predict pES event rates in JUNO and THEIA, and estimate $\Delta s$ by comparing with detected events. We find that the projected sensitivity for a supernova at 1 kpc (10 kpc), is approximately $\pm 0.01$ ($\pm 0.15$). Interestingly, the limits from a nearby supernova would be comparable to the results from lattice QCD, and better than polarized deep-inelastic scattering experiments. Using supernova neutrinos provides a true $Q^2\rightarrow 0$ measurement, and thus an axial-mass independent determination of $\Delta s$.