Using low energy atmospheric neutrinos for precision measurement of the mixing parameters

Abstract

Use of low energy atmospheric neutrinos is considered for precision measurement of neutrino mixing parameters. At around energy $E \simeq$ a few $\times$100 MeV and baseline $L$ of a few $\times$1000 km, CP phase effect is $\sim$10 times larger than that of the conventional LBL accelerator neutrino experiments. We report here a few progresses: (1) To analyze physics in the region, a new perturbative framework at around the solar-scale enhancement is developed. (2) To know the characteristic features of CP $\delta$ dependence of the atmospheric neutrinos at low energies, we plot the ratio of the $\nu_{e}$ and $\bar{\nu}_{e}$ fluxes $F(\delta)/F(\delta=0)$ for $\delta=\pm \frac{\pi}{2}$ and $\pi$ as a function of $E$ using the Honda {\it et al.} flux. Interestingly, it shows $\delta$ dependence of $\simeq$5-10\% level, with positive (negative) sign for $\nu_{e}$ ($\bar{\nu}_{e}$). (3) To reduce the flux systematic errors, measurement of muon energy distribution around $\sim$1 GeV at high altitude may be useful. Water Cherenkov or muon range detectors may be the options with advantage in the latter if it is magnetized in view of the $\delta$ dependence of the flux.