On hadron deformation: a model independent extraction of EMR from pion photoproduction data

TL;DR

This paper presents a model-independent extraction of the electromagnetic ratio EMR from pion photoproduction data at the Δ(1232) resonance, confirming baryon deformation and providing constraints for QCD models.

Contribution

It introduces a model-independent analysis scheme (AMIAS) to determine EMR, eliminating bias and validating the baryon deformation hypothesis.

Findings

EMR = -2.5 ± 0.4, confirming baryon deformation

Results agree with phenomenological and lattice QCD models

Provides stringent constraints on QCD-inspired hadronic models

Abstract

The multipole content of pion photoproduction at the $\Delta^+ (1232)$ resonance has been extracted from a data set dominated by recent Mainz Microtron (MAMI) precision measurements. The analysis has been carried out in the Athens Model Independent Analysis Scheme (AMIAS), thus eliminating any model bias. The benchmark quantity for nucleon deformation, $EMR = E2/M1 = E_{1+}^{3/2}/M_{1+}^{3/2}$, was determined to be $-2.5 \pm 0.4_{stat+syst}$, thus reconfirming in a model independent way that the conjecture of baryon deformation is valid. The derived multipole amplitudes provide stringent constraints on QCD simulations and QCD inspired models striving to describe hadronic structure. They are in good agreement with phenomenological models which explicitly incorporate pionic degrees of freedom and with lattice QCD calculations.