CLAS physics database search results Measurement E151M2883 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.55 210 -0.1154 0.3128 0.0301 0.45 42 -0.1482 0.2663 0.0301