Using supernova neutrinos to probe the strange quark contribution to the proton spin

The strange quark contribution to proton's spin (Δs) 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 Δs. 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 Δs by comparing with detected events. We find that the projected sensitivity for a supernova at 1 kpc (10 kpc), is approximately ± 0.01(± 0.15). We also consider the possibility of measuring Δs using neutronization burst, which is a robust prediction of supernova simulations. 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 Q² → 0 measurement, and thus an axial-mass independent determination of Δs.