CLAS physics database search results Measurement E151M2305 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 258 -0.0593 0.9724 0.0303 0.85 270 -0.5769 0.8651 0.0515 0.85 294 -0.2382 0.9462 0.0346