CLAS physics database search results Measurement E151M1722 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.65 258 -0.3608 0.6302 0.0371 0.75 90 0.3579 0.6889 0.037 0.85 270 0.6269 0.5603 0.0484