Cosmic Ray Muon Polarization to Facilitate Atmospheric Neutrino Physics
Atmospheric neutrinos (ATNs) offer a paradigm for understanding neutrino properties, while it is critical to quantify uncertainties in flux modeling. Since ATNs are produced simultaneously with cosmic ray muons, precision measurements of cosmic ray muons, including arrival direction, energy spectra, and spin polarization, will help reduce ATN production uncertainties and facilitate atmospheric neutrino physics. This letter proposes using an array strategy to measure the spin polarization of cosmic ray muons, thereby strengthening the emergent synergies between cosmic ray and atmospheric neutrino physics. Constraints on long-standing atmospheric neutrino flux uncertainties at the percentage level in a few-GeV energy range are achievable within one year using a $O(10)~\text{m}^2$ array of Cosmic-Ray muon Spin polarization detectoRs (CRmuSRs). With the resulting reduction in flux uncertainties, oscillation analysis of atmospheric neutrinos in a liquid scintillator detector with an exposure of 1500 $\text{kt}\cdot\text{yr}$ will break the octant degeneracy and achieve the precision measurement of $\theta_{23}$ with the uncertainty smaller than $5${\deg} at 3$\sigma$ confidence level irrespective of the mass ordering.