CLAS physics database search results Measurement E151M2893 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.75 234 0.9073 0.1276 0.0351 0.75 246 -0.0134 0.2236 0.0301 0.85 246 0.3162 0.2062 0.0307