Exclusive Single Pion Electroproduction off the Proton: Results from CLAS

Abstract

Exclusive meson electroproduction off protons is a powerful tool to probe the effective degrees of freedom in excited nucleon states at the varying distance scale where the transition from the contributions of both quark core and meson-baryon cloud to the quark core dominance. During the past decade, the CLAS collaboration has executed a broad experimental program to study the excited states of the proton using polarized electron beam and both polarized and unpolarized proton targets. The measurements covered a broad kinematic range in the invariant mass W and photon virtuality Q 2 with nearly full coverage in polar and azimuthal angles in the hadronic CM system. As results, several low-lying nucleon resonance states in particular from pion threshold to W < 1.6 GeV have been explored. These include ∆( 1232) 3 2 + , N (1440) 1 2 + , N (1520) 3 2 − , and N (1535) 1 2 − states. In addition, we recently published the differential cross sections and helicity amplitudes of the reaction γ * p → nπ + at higher W (1.6 to 2.0 GeV) which are the N (1675) 5 2 − , N (1680) 5 2 + , and N (1710) 1 2 +states. These excited states with isospin 1/2 and with masses near 1.7 GeV can be accessed in single nπ + production as there are no isospin 3/2 states present in this mass range with the same spin-parity assignments. I will briefly discuss these states from CLAS results of the single charged pion electroproduction data.