CLAS physics database search results Measurement E151M1946 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.75 210 0.48 0.8507 0.0418 0.75 270 -0.4737 0.8796 0.0415 0.85 222 0.4781 0.8548 0.0417