CLAS physics database search results Measurement E151M2916 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.85 90 -0.3724 0.2617 0.0309 0.85 102 0.3154 0.2756 0.0307