CLAS physics database search results Measurement E151M2140 EG4 Xiaochao Zheng 2016 cos(theta_pi+) phi(pi+) A_LL Stat. error Syst. error degrees 0.75 210 1.8755 2.0984 0.175 0.75 270 -1.9602 1.9602 0.131 0.85 210 -1.0194 0.8293 0.0728