Towards First Detection of the Solar MSW Transition With JUNO

Abstract

Matter-induced neutrino flavor mixing (the Mikheyev-Smirnov-Wolfenstein, or MSW, effect) is a central prediction of the neutrino mixing framework, but it has not been conclusively observed. Direct observation of the energy-dependent MSW transition in the solar electron-neutrino survival probability would solve this, but backgrounds have been prohibitive. We show that our new technique for suppressing muon-induced spallation backgrounds will allow JUNO to measure the MSW transition at $>$4$σ$ significance in 10 years. This would strongly support upcoming multi-\$1B next-generation long-baseline experiments and their goals in cementing the neutrino mixing framework.

Figures (4)

• Figure 1: Comparison of current BOREXINO:2018ohrSuper-Kamiokande:2023jbtSNO:2011hxd and projected measurements (using our background-rejection techniques for JUNO) of the solar MSW transition. The black line shows the MSW prediction, while the gray dashed lines show example allowed NSI models. The bins shown for JUNO are a projection, assuming the Standard Model, of how they could go beyond the quadratic fits used by SK and SNO. With our spallation cuts, JUNO can measure the MSW transition well.
• Figure 2: The $^8$B signal spectrum at JUNO using our neutron-tagged cuts compared to using JUNO's cuts JUNO:2020hqc, both compared to the remaining spallation background spectrum. With our spallation cuts, twice more signal would be retained.
• Figure 3: Comparison of JUNO's sensitivity to the solar MSW transition with our cuts and without, as well as to the combined SK+SNO result Super-Kamiokande:2023jbt. With our spallation cuts, greater significance would be reached, and more quickly.
• Figure 4: DUNE sensitivity to the neutrino mass ordering under different assumptions: Standard Model only, NSI within a currently allowed range, and NSI constrained by JUNO’s measurement of the MSW transition. JUNO's measurement of matter mixing effects will support DUNE's success.