Cosmic Ray Muon Polarization to Facilitate Atmospheric Neutrino Physics

Abstract

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 $θ_{23}$ with the uncertainty smaller than $5$° at 3$σ$ confidence level irrespective of the mass ordering.

Figures (8)

• Figure 1: An artistic representation of the measurement of cosmic-ray muon polarization by CRmuSR. The CRmuSR array (middle left) provides an opportunity to constrain the production processes of ATNs based on experimental measurements of the corresponding muons' polarization.
• Figure 2: The upper panel shows the atmospheric neutrino flux as a function of energy, while the lower panel presents the corresponding fractional uncertainty. The shaded bands illustrate the impact of varying assumptions on the uncertainty of the $K\texttt{-}\pi$ ratio, with the pion production uncertainty fixed at $5\%$. The blue band corresponds to an assumed $30\%$ uncertainty in $K\texttt{-}\pi$ ratio, and the orange band to a $10\%$ uncertainty.
• Figure 3: Precision measurement of the mixing angle $\theta_{23}$ with different flux and cross-section uncertainties at ATN experiment utilizing liquid scintillator (LS) with an exposure of 1500 $\text{kt}\cdot\text{yr}$, compared with Super-K measurements Super-Kamiokande:2019gzr, and combined results in normal (NO) and inverse (IO) mass ordering, respectively.
• Figure 4: Polarization of $\mu^+$ produced via two-body decays of pions and kaons as a function of the parent particle momentum. Owing to the larger kaon mass, the resulting muon polarization varies more mildly compared to that from pions. At high parent momentum, the muon polarization approaches $+1$, transitioning from the initial value of $-1$ in the rest frame.
• Figure 5: Normalized $\mathcal{P}_{\mu}$ distributions of cosmic-ray muons produced by different parent particles in MusAirS simulations. Red circles, green right-pointing triangles, and blue left-pointing triangles denote polarization for muons from pion decays, kaon, and $K_L^0$ decays, respectively.