CLAS physics database search results Measurement E151M500 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_UL Data errors Systematic errors degrees 0.65 258 0.9124 0.6005 0.0634 0.75 90 -0.0186 0.5951 0.03 0.85 270 -0.4452 0.4495 0.0405