CLAS physics database search results Measurement E151M3514 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.85 258 -0.0395 0.2664 0.03 0.95 318 -0.3644 0.2591 0.0309