CLAS physics database search results Measurement E151M1671 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.75 234 1.6228 0.6964 0.099 0.75 246 0.5291 0.6982 0.0789 0.85 246 -0.0017 0.6883 0.031