CLAS physics database search results Measurement E151M2415 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 90 -1.1914 1.0337 0.0914 0.95 210 -1.4285 0.9277 0.1078