Roper Resonance in 2+1 Flavor QCD

TL;DR

This study uses lattice QCD with 2+1 flavors to investigate the nucleon's low-lying states, especially the Roper resonance, revealing its approach to physical mass and differences from quenched QCD, offering new insights into QCD dynamics.

Contribution

First lattice QCD analysis of the Roper resonance in 2+1 flavor QCD, showing its mass behavior and chiral curvature near physical quark masses.

Findings

Roper resonance approaches physical mass with light quark masses.

Significant chiral curvature observed in the Roper state.

Results differ markedly from quenched QCD simulations.

Abstract

The low-lying even-parity states of the nucleon are explored in lattice QCD using the PACS-CS collaboration 2+1-flavor dynamical-QCD gauge-field configurations made available through the International Lattice Datagrid (ILDG). The established correlation-matrix approach is used, in which various fermion source and sink smearings are utilized to provide an effective basis of interpolating fields to span the space of low-lying energy eigenstates. Of particular interest is the nature of the first excited state of the nucleon, the $N{1/2}^{+}$ Roper resonance of $P_{11}$ pion-nucleon scattering. The Roper state of the present analysis approaches the physical mass, displaying significant chiral curvature at the lightest quark mass. These full QCD results, providing the world's first insight into the nucleon mass spectrum in the light-quark regime, are significantly different from those of quenched QCD and provide interesting insights into the dynamics of QCD.