CLAS physics database search results Measurement E151M2828 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LU Stat. error Syst. error degrees 0.65 222 0.2451 0.3841 0.0304 0.75 222 -0.2065 0.3801 0.0303