CLAS physics database search results Measurement E151M2404 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 234 0.6252 1.0818 0.0544 0.95 210 -0.5747 1.0715 0.0513