CLAS physics database search results Measurement E151M3124 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.75 246 -0.2585 0.2723 0.0304 0.85 234 -0.1357 0.2418 0.0301