New possibilities for studying nucleon resonances in the production of pi+pi(-) pairs by polarized electrons on an unpolarized proton

摘要

The difference of the cross sections for double charged-pion production in the scattering of opposite-helicity electrons on an unpolarized proton is evaluated within the phenomenological model developed previously. The electromagnetic nucleon-resonance form factors and the parameters of nonresonance processes were taken from a fit to the latest data of the CLAS Collaboration on double charged-pion electroproduction. The effect of the longitudinal excitations of the P-11 (1440) and D-13 (1520) states on the difference of the helicity components of the cross section is studied. The sensitivity of this observable to the nucleon-resonance contribution opens the possibility of employing it both in extracting the Coulomb form factors and in seeking new baryon states. Signals from one of these states were possibly observed in the latest data of the CLAS Collaboration. (C) 2004 MAIK "Nauka/Interperiodica".