High Precision Measurements of $θ_{\odot}$ in Solar and Reactor Neutrino Experiments
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We discuss the possibilities of high precision measurement of the solar neutrino mixing angle $θ_\odot \equiv θ_{12}$ in solar and reactor neutrino experiments. The improvements in the determination of $\sin^2θ_{12}$, which can be achieved with the expected increase of statistics and reduction of systematic errors in the currently operating solar and KamLAND experiments, are summarised. The potential of LowNu $ν-e$ elastic scattering experiment, designed to measure the $pp$ solar neutrino flux, for high precision determination of $\sin^2θ_{12}$, is investigated in detail. The accuracy in the measurement of $\sin^2θ_{12}$, which can be achieved in a reactor experiment with a baseline $L \sim (50-70)$ km, corresponding to a Survival Probability MINimum (SPMIN), is thoroughly studied. We include the effect of the uncertainty in the value of $\sin^2θ_{13}$ in the analyses. A LowNu measurement of the $pp$ neutrino flux with a 1% error would allow to determine $\sin^2θ_{12}$ with an error of 14% (17%) at 3$σ$ from a two-generation (three-generation) analysis. The same parameter $\sin^2θ_{12}$ can be measured with an uncertainty of 2% (6%) at 1$σ$ (3$σ$) in a reactor experiment with $L \sim60 $ km, statistics of $\sim$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 $\sin^2θ_{12}$ from 6% to 9% at 3$σ$. The inclusion of the $\sin^2θ_{13}$ uncertainty in the analysis changes the error on $\sin^2θ_{12}$ to 3% (9%). The effect of $\sin^2θ_{13}$ uncertainty on the $\sin^2θ_{12}$ measurement in both types of experiments is considerably smaller than naively expected.
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