Separated cross sections in \pi^0 electroproduction at threshold at Q^2 = 0.05 GeV^2/c^2

TL;DR

This study measured separated cross sections in neutral pion electroproduction near threshold at low Q^2, revealing the cusp effect and testing theoretical models, with findings challenging some predictions and supporting others.

Contribution

First measurement of separated cross sections including the cusp effect in b0 electroproduction at threshold, providing critical data to test and refine theoretical models.

Findings

Observed cusp effect above b1^+ threshold in the imaginary part of the s-wave.

Heavy Baryon Chiral Perturbation Theory predictions do not agree with data.

Data align better with the DMT dynamical model than with MAID.

Abstract

The differential cross sections \sigma_0=\sigma_T+\epsilon \sigma_L, \sigma_{LT}, and \sigma_{TT} of \pi^0 electroproduction from the proton were measured from threshold up to an additional center of mass energy of 40 MeV, at a value of the photon four-momentum transfer of Q^2= 0.05 GeV^2/c^2 and a center of mass angle of \theta=90^\circ. By an additional out-of-plane measurement with polarized electrons \sigma_{LT'} was determined. This showed for the first time the cusp effect above the \pi^+ threshold in the imaginary part of the s-wave. The predictions of Heavy Baryon Chiral Perturbation Theory 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.