CLAS physics database search results Measurement E151M1282 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.65 234 -0.9571 0.6624 0.112 0.65 246 -0.0154 0.5699 0.03 0.75 246 -0.2002 0.7163 0.0375