High Precision Measurements of θ⊙ in Solar and Reactor Neutrino Experiments

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 sin θ12, 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 θ12, is investigated in detail. The accuracy in the measurement of sin θ12, which can be achieved in a reactor experiment with a baseline L ∼ 60 km, corresponding to a Survival Probability MINimum (SPMIN), is thoroughly studied. We include the effect of the uncertainty in the value of sin θ13 in the analyses. A LowNu measurement of the pp neutrino flux with a 1% error would allow to determine sin θ12 with an error of 14% (17%) at 3σ from a two-generation (three-generation) analysis. The same parameter sin θ12 can be measured with an uncertainty of 2% (6%) at 1σ (3σ) in a reactor experiment with L ∼ 60, statistics of ∼60 GWkTy and systematic error of 2%. The inclusion of the sin θ13 uncertainty in the analysis changes the error on sin 2 θ12 to 3% (9%). The effect of sin θ13 uncertainty on the sin 2 θ12 measurement in both type of experiments is considerably smaller than naively expected.