CLAS physics database search results Measurement E151M1693 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.75 270 0.546 0.569 0.0447 0.95 102 0.1142 0.5456 0.0308 0.95 114 0.4193 0.5872 0.0393