Beam-Recoil Polarization Measurement of pi(0) Electroproduction on the Proton in the Region of the Roper Resonance

摘要

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((e) over right arrow, e' (p) over right arrow)pi(0) reaction at four-momentum transfer Q(2) similar or equal to 0.1GeV(2), center-of-mass proton emission angle. theta(p)* similar or equal to 90 degrees, and invariant mass W similar or equal to 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.