Beam-Recoil Polarization Measurement of π[superscript 0] Electroproduction on the Proton in the Region of the Roper Resonance

The helicity-dependent recoil proton polarizations P′x and P′z as well as the elicity-independent component Py have been measured in the p(→e,e′→p)π0 reaction at four-momentum transfer Q2≃0.1 GeV2, 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 P11(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q2, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.