CLAS physics database search results Measurement E151M3232 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.85 282 0.1181 0.2581 0.0301 0.85 306 -0.0933 0.2664 0.0301