Separated cross-sections in $\pi^{0}_{}$ electroproduction at threshold at Q² = 0.05GeV²/c²

Abstract

The differential cross-sections σ₀ = σ_T + εσ_L , σ_LT , and σ_TT of $\pi^{0}_{}$ electroproduction 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 of Q ² = 0.05 GeV²/c² and a center-of-mass angle of θ = 90^° . By an additional out-of-plane measurement with polarized electrons σ_LT' was determined. This showed for the first time the presence of an imaginary part of the s -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.