CLAS physics database search results Measurement E151M3527 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.85 258 0.3574 0.2581 0.0308 0.85 270 0.0168 0.2523 0.03 0.85 294 0.0904 0.2867 0.0301