CLAS physics database search results Measurement E151M1471 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.85 114 -0.7291 0.7125 0.053 0.65 282 1.8534 1.8534 0.1119 0.85 270 0.2348 0.8447 0.033