CLAS physics database search results Measurement E151M309 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_UL Data errors Systematic errors degrees 0.65 234 -0.0574 0.5823 0.0302 0.75 222 0.7885 0.6433 0.0573 0.75 246 0.5383 0.6404 0.0448