Exclusive Single Pion Electroproduction off the Proton: Results from CLAS

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 Δ ( 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 → n π + at higher W (1.6–2.0 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 n π + 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.