Measurement of the transverse–longitudinal cross sections in the p(e→,e′p)π0 reaction in the Δ region

Abstract Accurate measurements of the p( e → ,e′p) π 0 reaction were performed at Q 2 =0.127 (GeV/ c ) 2 in the Δ resonance energy region. The experiments at the MIT-Bates Linear Accelerator used an 820 MeV polarized electron beam with the out-of-plane magnetic spectrometer system (OOPS). In this Letter we report the first simultaneous determination of both the TL and TL′ (“fifth” or polarized) cross sections at low Q 2 where the pion cloud contribution is predicted to dominate the quadrupole amplitudes (E2 and C2). These are the real and imaginary parts of the transverse–longitudinal interference amplitudes and provide a sensitive determination of the Coulomb quadrupole amplitude and a test of reaction calculations. Comparisons with model calculations are presented. The empirical MAID calculation gives the best overall agreement with this accurate data. The parameters of this model for the values of the resonant multipoles are | M 1+ ( I =3/2)|=(40.9±0.3)×10 −3 / m π , CMR=C2/M1=−6.5±0.3%, EMR=E2/M1=−2.2±0.9%, where the errors are due to the experimental uncertainties.