Beam-Recoil Polarization Measurement of ${\ensuremath{\pi}}^{0}$ Electroproduction on the Proton in the Region of the Roper Resonance

Beam-Recoil Polarization Measurement of $π^{0}$ Electroproduction on the Proton in the Region of the Roper Resonance

S. Štajner et al. (A1 Collaboration)

Phys. Rev. Lett. 119, 022001 – Published 13 July 2017

Abstract

The helicity-dependent recoil proton polarizations $P_{x}^{'}$ and $P_{z}^{'}$ as well as the helicity-independent component $P_{y}$ have been measured in the $p (\vec{e}, e^{'} \vec{p}) π^{0}$ reaction at four-momentum transfer $Q^{2} ≃ 0.1 {GeV}^{2}$ , center-of-mass proton emission angle $θ_{p}^{*} ≃ 90 °$ , and invariant mass $W ≃ 1440 MeV$ . This first precise measurement of double-polarization observables in the energy domain of the Roper resonance $P_{11} (1440)$ by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of $Q^{2}$ , establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.