CLAS physics database search results Measurement E151M436 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_UL Data errors Systematic errors degrees 0.75 222 -0.1579 0.6147 0.0316 0.55 222 0.6412 1.2732 0.0489 0.75 210 0.8811 1.2491 0.061