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 \sim 60 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 .