Transition Form Factor from CLAS

The excitation of nucleon resonances in electromagnetic interaction has long been studied. The study of resonances helps us to understand the long- and short- range structures of the nucleon and its excited states in terms of quark confinement. While the existing data of the low-lying resonances are consistent with the well-studied SU(6) ⊗ O(3) constituent quark model classification, many open questions still remain. Exclusive electro-production is one of the best ways to investigate nucleon resonances. The exclusive electro-production process was measured in the photon virtuality range Q2 = 1.7 – 4.5 GeV2 and the invariant mass range for the nπ+ system of W = 1.15 – 1.7 GeV using the CEBAF Large Acceptance Spectrometer. For the first time, these kinematics are probed in exclusive π+ production from protons with nearly full coverage in the azimuthal and polar angles of the nπ+ center-of-mass system. The nπ+ channel has particular sensitivity to the isospin excited nucleon states, and together with the pπ0 final state will serve to determine the transition form factors of a large number of resonances. The largest discrepancy between these results and present modes was seen in the σLT/ structure function. Thanks to a large volume of data (31,295 cross section and 4,184 asymmetry data points), a reduced set of structure functions and Legendre polynomial moments are presented which are obtained in model-independent fits to the differential cross sections. In this paper, I will discuss the transition form factors of the nucleon resonances in terms of helicity amplitudes.