CLAS physics database search results Measurement E151M530 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_UL Data errors Systematic errors degrees 0.75 234 -0.3187 0.6112 0.0358 0.75 246 -0.0176 0.5608 0.03 0.85 246 0.2032 0.6613 0.0325