Measurement of the generalized form factors near threshold via $\gamma^* p \to n\pi^+$ at high $Q^2$

TL;DR

This study measures the generalized form factors near the pion production threshold at high momentum transfer, providing new insights into nucleon structure through experimental data and theoretical analysis.

Contribution

First extraction of pion-nucleon multipoles near threshold at high Q^2, linking experimental results with light-cone sum rule and axial form factor calculations.

Findings

Dominance of s-wave transverse multipole $E_{0+}$ near threshold

Consistent results from LCSR and multipole fit methods

Almost Q^2 independent behavior of measured form factors

Abstract

We report the first extraction of the pion-nucleon multipoles near the production threshold for the $n\pi^+$ channel at relatively high momentum transfer ($Q^2$ up to 4.2 $\rm{GeV^2}$). The dominance of the s-wave transverse multipole ($E_{0+}$), expected in this region, allowed us to access the generalized form factor $G_1$ within the light-cone sum rule (LCSR) framework as well as the axial form factor $G_A$. The data analyzed in this work were collected by the nearly $4\pi$ CEBAF Large Acceptance Spectrometer (CLAS) using a 5.754 $\rm{GeV}$ electron beam on a proton target. The differential cross section and the $\pi-N$-multipole $E_{0+}/G_D$ were measured using two different methods, the LCSR and a direct multipole fit. The results from the two methods are found to be consistent and almost $Q^2$ independent.