Isolating the Roper Resonance in Lattice QCD

TL;DR

This paper reports the first successful lattice QCD calculation of the Roper resonance at light quark masses using a variational analysis with smeared operators, advancing understanding of nucleon excited states.

Contribution

It introduces a variational method with smeared operators to isolate the Roper resonance in lattice QCD at light quark masses, a novel achievement.

Findings

First identification of the Roper resonance at light quark masses in lattice QCD.

The Roper state approaches the physical mass as pion mass decreases.

Demonstrates the effectiveness of the variational approach with smeared operators.

Abstract

We present results for the first positive parity excited state of the nucleon, namely, the Roper resonance ($N^{{{1/2}}^{+}}$=1440 MeV) from a variational analysis technique. The analysis is performed for pion masses as low as 224 MeV in quenched QCD with the FLIC fermion action. A wide variety of smeared-smeared correlation functions are used to construct correlation matrices. This is done in order to find a suitable basis of operators for the variational analysis such that eigenstates of the QCD Hamiltonian may be isolated. A lower lying Roper state is observed that approaches the physical Roper state. To the best of our knowledge, the first time this state has been identified at light quark masses using a variational approach.