CLAS physics database search results Measurement E151M435 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_UL Data errors Systematic errors degrees 0.85 258 0.6906 0.7398 0.0522 0.55 246 1.8941 1.5044 0.1179 0.85 246 -0.5912 0.7534 0.0466