CLAS physics database search results Measurement E151M2036 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 270 -0.3271 1.1642 0.0425 0.75 270 0.6482 0.7507 0.0517 0.85 258 0.5978 0.8334 0.0491