CLAS physics database search results Measurement E151M2430 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 102 -1.3266 0.9998 0.1008 0.85 114 -0.1142 1.1266 0.0311