Spectral Analysis of Excited Nucleons in Lattice QCD with Maximum Entropy Method

TL;DR

This paper uses lattice QCD simulations and the maximum entropy method to analyze the mass spectra of excited baryons, focusing on the level ordering of the Roper resonance and N*(1535), revealing results that challenge quark model predictions.

Contribution

It introduces a novel combination of boundary conditions and the maximum entropy method to extract spectral functions of excited baryons in lattice QCD.

Findings

Masses of N' and N* states are close across various quark masses.

Results contrast with traditional quark model predictions.

The role of Wilson doublers in spectral functions is examined.

Abstract

We study the mass spectra of excited baryons with the use of the lattice QCD simulations. We focus our attention on the problem of the level ordering between the positive-parity excited state N'(1440) (the Roper resonance) and the negative-parity excited state N^*(1535). Nearly perfect parity projection is accomplished by combining the quark propagators with periodic and anti-periodic boundary conditions in the temporal direction. Then we extract the spectral functions from the lattice data by utilizing the maximum entropy method. We observe that the masses of the N' and N^* states are close for wide range of the quark masses (M_pi=0.61-1.22 GeV), which is in contrast to the phenomenological prediction of the quark models. The role of the Wilson doublers in the baryonic spectral functions is also studied.