Role of chiral symmetry in the nucleon excitation spectrum

TL;DR

This study investigates whether chiral symmetry influences the low-lying nucleon excitation spectrum, specifically the Roper resonance, by comparing different fermion actions in lattice QCD simulations, and finds no significant role for chiral symmetry.

Contribution

It provides a systematic comparison of clover and overlap fermion actions to assess chiral symmetry's impact on the Roper resonance in lattice QCD.

Findings

Clover and overlap actions yield similar excitation energies.

No evidence found that chiral symmetry significantly affects the Roper resonance.

Results suggest fermion action chiral symmetry is not crucial for understanding the Roper resonance.

Abstract

The origin of the low-lying nature of the $N$*(1440), or Roper resonance, has been the subject of significant interest for many years, including several investigations using lattice QCD. The majority of lattice studies have not observed a low-lying excited state energy level in the region of the Roper resonance. However, it has been claimed that chiral symmetry could play an important role in our understanding of this resonance. The purpose of this study is to systematically examine the role of chiral symmetry in the low-lying nucleon spectrum by directly comparing the clover and overlap fermion actions. To ensure any differences in results are attributable to the choice of fermion action, simulations are performed on the same set of gauge field configurations at matched pion masses. Correlation matrix techniques are employed to determine the excitation energy of the first positive-parity excited state for each action. The clover and overlap actions show a remarkable level of agreement. We do not find any evidence that fermion action chiral symmetry plays a significant role in understanding the Roper resonance on the lattice.