CLAS physics database search results Measurement E151M2064 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 282 -0.8868 1.3414 0.0868 0.95 306 2.9901 3.5344 0.2764