High precision measurements of theta(circle dot) in the solar and reactor neutrino experiments

We discuss the possibilities of high precision measurement of the solar neutrino mixing angle $\theta_\odot \equiv \theta_{12}$ in solar and reactor neutrino experiments. The improvements in the determination of $\sin^2\theta_{12}$, 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 $\nu-e$ elastic scattering experiment, designed to measure the $pp$ solar neutrino flux, for high precision determination of $\sin^2\theta_{12}$, is investigated in detail. The accuracy in the measurement of $\sin^2\theta_{12}$, 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 $\sin^2\theta_{13}$ in the analyses. A LowNu measurement of the $pp$ neutrino flux with a 1\% error would allow to determine $\sin^2\theta_{12}$ with an error of 14\% (17\%) at 3$\sigma$ from a two-generation (three-generation) analysis. The same parameter $\sin^2\theta_{12}$ can be measured with an uncertainty of 2\% (6\%) at 1$\sigma$ (3$\sigma$) 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 $\sin^2\theta_{12}$ from 6\% to 9\% at 3$\sigma$. The inclusion of the $\sin^2\theta_{13}$ uncertainty in the analysis changes the error on $\sin^2\theta_{12}$ to 3\% (9\%). The effect of $\sin^2\theta_{13}$ uncertainty on the $\sin^2\theta_{12}$ measurement in both types of experiments is considerably smaller than naively expected.