CLAS physics database search results Measurement E151M2879 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.85 258 0.18 0.2834 0.0302 0.55 246 0.1542 0.378 0.0302 0.85 246 -0.175 0.3422 0.0302