Measurement of the $p(e,e^{\prime }\ddot{I}{€}^{+})n$ reaction close to threshold and at low $Q^2$

Abstract

The cross section of the $p(e,e^{\prime }\ddot{I}{€}^{+})n$ reaction has been measured for five kinematic settings at an invariant mass of $W=1094$ MeV and for a four-momentum transfer of $Q^2=0.078$ (GeV/$c$)${}^2$. The measurement has been performed at MAMI using a new short-orbit spectrometer (SOS) of the A1 collaboration, intended for detection of low-energy pions. The transverse and longitudinal cross section terms were separated using the Rosenbluth method and the transverse-longitudinal interference term has been determined from the left-right asymmetry. The experimental cross section terms are compared with the calculations of three models: DMT2001, MAID2007 and $\ddot{I}‡$MAID. The results show that we do not yet understand the dynamics of the fundamental pion.

version accepted for publishing