CLAS physics database search results Measurement E151M2139 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 90 -0.9365 0.7638 0.0679 0.85 210 0.5102 0.8822 0.0447