CLAS physics database search results Measurement E151M3161 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.75 222 0.1567 0.2799 0.0302 0.85 210 -0.1825 0.2528 0.0302