Separated cross-sections in electroproduction at threshold at Q² = 0.05GeV²/c²

The differential cross-sections sigma(0) = sigma(T) + epsilon sigma(L) , sigma(LT) , and sigma(TT) of pi 0selectroproduction from the proton were measured from threshold up to an additional center-of-mass energy of 40MeV, at a value of the photon four-momentum transfer ofQ 2 = 0.05 GeV2/c and a center-of-mass angle of theta = 90 . By an additional out-of-plane measurement with polarized electrons sigma(LT') was determined. This showed for the first time the presence of an imaginary part of thes -wave above the pi threshold, which is usually interpreted as a unitary cusp. The predictions of the Heavy Baryon Chiral Perturbation Theory (HBChPT) are in disagreement with these data. On the other hand, the data are somewhat better predicted by the MAID phenomenological model and are in good agreement with the dynamical model DMT.