High precision measurements of θ_☉ in the solar and reactor neutrino experiments

Abstract

We discuss the possibilities of high precision measurement of the solar neutrino mixing angle θ_☉ ≡ θ₁₂ in solar and reactor neutrino experiments. The improvements in the determination of sin²θ₁₂, which can be achieved with the expected increase of statistics and reduction of systematic errors in the currently operating solar and KamLAND experiments, are summarized. The potential of LowNu ν−e elastic scattering experiment, designed to measure the pp solar neutrino flux, for high precision determination of sin²θ₁₂, is investigated in detail. The accuracy in the measurement of sin²θ₁₂, which can be achieved in a reactor experiment with a baseline L∼(50–70)  km, corresponding to a Survival Probability Minimum (SPMIN) is thoroughly studied. We include the effect of the uncertainty in the value of sin²θ₁₃ in the analyses. A LowNu measurement of the pp neutrino flux with a 1% error would allow to determine sin²θ₁₂ with an error of 14% (17%) at 3σ from a two-generation (three-generation) analysis. The same parameter sin²θ₁₂ can be measured with an uncertainty of 2% (6%) at 1σ (3σ) in a reactor experiment with L∼60  km, statistics of ∼60 GWkTy and systematic error of 2%. For the same statistics, the increase of the systematic error from 2% to 5% leads to an increase in the uncertainty in sin²θ₁₂ from 6% to 9% at 3σ. The inclusion of the sin²θ₁₃ uncertainty in the analysis changes the error on sin²θ₁₂ to 3% (9%). The effect of sin²θ₁₃ uncertainty on the sin²θ₁₂ measurement in both types of experiments is considerably smaller than naively expected.