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 \(W < 1.6\,\hbox {GeV}\) have been explored. These include \(\varDelta (1232)\frac{3}{2}^+\), \(N(1440)\frac{1}{2}^+\), \(N(1520)\frac{3}{2}^-\), and \(N(1535)\frac{1}{2}^-\) states. In addition, we recently published the differential cross sections and helicity amplitudes of the reaction \(\gamma ^*p\rightarrow n\pi ^+\) at higher W (1.6–2.0 GeV) which are the \(N(1675)\frac{5}{2}^-\), \(N(1680)\frac{5}{2}^+\), and \(N(1710)\frac{1}{2}^+\) states. These excited states with isospin 1/2 and with masses near 1.7 GeV can be accessed in single \(n\pi ^+\) 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.