CLAS physics database search results Measurement E151M1688 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.95 138 -0.0697 0.5594 0.0303 0.95 150 -1.4909 1.4909 0.0952