CLAS physics database search results Measurement E151M1752 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.75 234 -0.1678 0.7577 0.0317 0.75 246 0.5104 0.6518 0.0431 0.85 246 -1.5004 0.6549 0.0957