Spin polarization of A hyperons in e+e- → A(A)over-bar at BESIII

The BESIII collaboration here reports the first observation of polarized A and A hyperons produced in two different processes: i) the resonant e+e AA, using a data sample of 1.31 x 109 J/ ti, events and ii) the non-resonant e+e 7* AA, using a 66.9 pb-1 data sample collected at \/7s = 2.396 GeV. In e+e J/ AA, the phase between the electric and the magnetic amplitude is measured for the first time to be 42.3' 0.6' 0.5'. The multi-dimensional analysis enables a model-independent measurement of the decay parameters for A (a_), A 257+ (a+) and A fi Tr (do). The obtained value a_ = 0.750 0.009 0.004 differs with 5a from the PDG value. This value, together with the measurement a+ = 0.758 0.010 0.007 allow for the most precise test of CP violation in A decays so far: Acp = (a_ + cx+)/(cx_ a+) of 0.006 0.012 0.007. The decay asymmetry Cxo = 0.692 0.016 0.006 is measured for the first time. The e+e AA reaction at 2.396 GeV enables a first complete measurement of the time-like electric and magnetic form factor of any baryon, of the modulus of the ratio R = IGEIGml and of the relative phase 0(13 = (DE (Dm. With the decay asymmetry parameters from the J/ data, the obtained values are R= 0.96 0.14 0.02 and,A(13 = 37- 12' 6'. In addition, the cross section has been measured with unprecedented precision to be a = 119.0 5.3 5.1 pb, which corresponds to an effective form factor of IQ = 0.123 0.003 0.003.