[Paper Review] The nucleon resonance structure from exclusive $π^+π^-p$ photo-/electroproduction off protons

Volker Burkert, V. Mokeev+1 more|arXiv (Cornell University)|Jan 25, 2019

Quantum Chromodynamics and Particle Interactions2 references4 citations

TL;DR

This paper presents the first extraction of nucleon resonance electrocouplings from exclusive $\pi^+\pi^-p$ electroproduction data using CLAS detector measurements at Jefferson Lab, revealing a complex interplay between the quark core and meson-baryon cloud in excited nucleon states up to 2.0 GeV. The results, derived from $Q^2 < 5.0$ GeV$^2$ data, provide critical constraints on the $Q^2$-evolution of $\gamma_v p N^*$ amplitudes, offering new insights into strong QCD dynamics and dressed quark structure in baryons.

ABSTRACT

The results on the photo- and electroexcitation amplitudes of most nucleon resonances in the mass range up to 2.0 GeV determined from the CLAS experimental data on exclusive $π^+π^-p$ photo-/electroproduction off protons in collaboration between the Jefferson Lab and Moscow State University are presented. The first and only available results on electroexcitation amplitudes from CLAS in a wide range of photon virtualities $Q^2$ $

Motivation & Objective

• To extract photo- and electroexcitation amplitudes of nucleon resonances ($N^*$) in the mass range up to 2.0 GeV from exclusive $\pi^+\pi^-p$ electroproduction data.
• To probe the interplay between the three-quark core and meson-baryon cloud in $N^*$ states via $Q^2$-dependent electrocouplings.
• To provide experimental constraints on the $Q^2$-evolution of $\gamma_v p N^*$ amplitudes for testing quark models and Dyson-Schwinger equations in strong QCD.
• To lay the foundation for future high-precision studies of $N^*$ structure at low and high $Q^2$ using the upgraded CLAS12 detector.
• To search for hybrid baryon states and 'missing' resonances in the 2.0–2.5 GeV range through $Q^2$-dependent electrocoupling analysis.

Proposed method

• The Jefferson Lab-Moscow State University (JM) reaction model is used to describe $\pi^+\pi^-p$ electroproduction, incorporating all significant reaction mechanisms in the final state.
• The model includes resonant contributions from known $N^*$ states, non-resonant $\pi\Delta$ and $\rho p$ intermediate states, and $\pi$-exchange diagrams.
• Resonance parameters, including $\gamma_v p N^*$ electrocouplings and $\pi\Delta$, $\rho p$ hadronic decay widths, are extracted via a simultaneous fit to nine one-fold differential cross sections binned in $W$ and $Q^2$.
• The analysis uses CLAS data with full phase space coverage, enabling robust separation of resonant and non-resonant contributions across $W < 2.0$ GeV and $Q^2 < 5.0$ GeV$^2$.
• The $Q^2$-independent resonance mass and width parameters are assumed to test the model’s consistency and validate the presence of new states in a nearly model-independent way.
• Future studies with CLAS12 will extend the $Q^2$ coverage to 12 GeV$^2$, enabling access to the quark-core-dominated regime of $N^*$ states.

Experimental results

Research questions

• RQ1 How do the $\gamma_v p N^*$ electrocouplings of nucleon resonances evolve with photon virtuality $Q^2$ in the range 0.25–5.0 GeV$^2$?
• RQ2 What is the relative contribution of the quark core versus meson-baryon cloud to the structure of excited nucleon states in $\pi^+\pi^-p$ electroproduction?
• RQ3 Can the $Q^2$-independent resonance parameters extracted from data at $Q^2 < 5.0$ GeV$^2$ validate the existence of new or missing baryon states?
• RQ4 How do the shapes of resonant and non-resonant contributions differ in one-fold differential cross sections across various $W$ and $Q^2$ bins?
• RQ5 What is the potential for identifying hybrid baryon states through $Q^2$-dependent electrocoupling evolution in the 2.0–2.5 GeV mass range?

Key findings

• The first available results on $\gamma_v p N^*$ electrocouplings from $\pi^+\pi^-p$ electroproduction at $Q^2 < 5.0$ GeV$^2$ have been extracted, revealing a complex interplay between quark core and meson-baryon cloud in $N^*$ states.
• Resonant contributions remain significant across the entire $W < 2.0$ GeV and $Q^2 < 5.0$ GeV$^2$ range, with increasing relative strength in the second and third resonance regions at higher $Q^2$.
• The shapes of resonant and non-resonant contributions in one-fold differential cross sections exhibit distinct features, enabling robust separation and parameter extraction.
• Correlations between contributions in different cross sections show unique patterns, supporting the reliability of the extracted electrocouplings and hadronic widths.
• The successful description of data using $Q^2$-independent resonance parameters provides a nearly model-independent validation of new or missing $N^*$ states.
• Future CLAS12 measurements will extend $Q^2$ coverage to 12 GeV$^2$, enabling direct access to the quark-core-dominated regime and the dressed quark mass function up to 1.5 GeV momentum scale.