Projected Sensitivity to Atmospheric Neutrino Oscillations using a 1725 m$^{3}$ Liquid-Nitrogen Detector at CJPL
In atmospheric neutrino analyses, the usable zenith-angle range is generally restricted to upward-going events to suppress the cosmic-ray muon background. Motivated by the exceptionally low underground muon background at the China Jinping Underground Laboratory (CJPL), we investigate whether extending the zenith-angle acceptance can improve the sensitivity of atmospheric neutrino oscillation measurements. An existing 1725 m$^{3}$ liquid-nitrogen volume is adopted as the basis of a simplified detector model, in which the accepted zenith-angle range is extended from $\cosθ_μ\in[-1,0]$ to $\cosθ_μ\in[-1,0.3]$, and the projected oscillation sensitivity is evaluated for a 10-year exposure. The atmospheric neutrino flux is estimated by interpolating standard flux predictions as a function of geomagnetic latitude. Neutrino interactions in the surrounding rock and the liquid-nitrogen volume are simulated for neutrino energies of $E\ge0.1$ GeV, followed by secondary-particle transport and geometry-based event selection. For the benchmark oscillation parameters $Δ$m$^{2}_{32}$ = $2.4\times10^{-3}\ \mathrm{eV}^{2}$ and $\sin^{2}θ_{23}=0.5$, the neutrino-induced muon flux from the surrounding rock is estimated to be $(3.65 \pm 1.00)\times10^{-13}$ cm$^{-2}$ s$^{-1}$ sr$^{-1}$, corresponding to a muon yield of $(0.13 \pm 0.034)$ day$^{-1}$ in the 1725 m$^{3}$ liquid-nitrogen volume. Three-flavor neutrino oscillations are incorporated into a Poisson-likelihood $χ^{2}$ analysis to evaluate the sensitivity to the oscillation parameters. Extending the usable zenith-angle range leads to a clear improvement in the projected oscillation sensitivity at the 90\% confidence level over a 10-year exposure.