CLAS physics database search results Measurement E151M3337 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.85 78 0.4572 0.231 0.0314 0.85 258 0.2031 0.2558 0.0303 0.65 126 -0.1078 0.2692 0.0301