A precision measurement of the $p$($e,e^{\prime }p$)$\ddot{I}{€}^0$ reaction at threshold

Abstract

New results are reported from a measurement of $\ddot{I}{€}^0$ electroproduction near threshold using the $p(e,e^{\prime }p)\ddot{I}{€}^0$ reaction. The experiment was designed to determine precisely the energy dependence of $s-$ and $p-$wave electromagnetic multipoles as a stringent test of the predictions of Chiral Perturbation Theory (ChPT). The data were taken with an electron beam energy of 1192 MeV using a two-spectrometer setup in Hall A at Jefferson Lab. For the first time, complete coverage of the $\ddot{I}{•}_{\ddot{I}}^{\ast }€$ and $\hat{I}{¸}_{\ddot{I}}^{\ast }€$ angles in the $p\ddot{I}{€}^0$ center-of-mass was obtained for invariant energies above threshold from 0.5 MeV up to 15 MeV. The 4-momentum transfer $Q^2$ coverage ranges from 0.05 to 0.155 (GeV/c)${}^2$ in fine steps. A simple phenomenological analysis of our data shows strong disagreement with $p-$wave predictions from ChPT for $Q^2>0.07$ (GeV/c)${}^2$, while the $s-$wave predictions are in reasonable agreement.

5 pages, 6 figures