CLAS physics database search results Measurement E151M1948 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 114 -0.6148 0.8502 0.0478 0.85 222 -1.8551 0.7442 0.1163