CLAS physics database search results Measurement E151M1940 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.75 234 -1.2891 0.7914 0.0837 0.75 282 -2.2366 2.2366 0.1388 0.85 222 -0.6994 0.8432 0.0519