CLAS physics database search results Measurement E151M2657 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.65 282 -0.2626 0.2655 0.0305 0.85 102 -0.0979 0.2865 0.0301 0.55 282 0.2053 0.3771 0.0303