Monopole and instanton effects in QCD

TL;DR

This paper investigates how magnetic monopoles and instantons influence observable properties in QCD by introducing monopole pairs into the vacuum and analyzing their effects on the Dirac spectrum, chiral condensate, and meson properties.

Contribution

It demonstrates the specific impacts of monopoles and instantons on QCD observables using lattice gauge theory and the overlap Dirac operator, revealing their roles in spectrum modification and hadron properties.

Findings

Monopole pairs create instantons without altering the QCD vacuum structure.

Instanton density increase raises pseudoscalar decay constants.

Instantons decrease the magnitude of the chiral condensate and affect pion properties.

Abstract

We aim to show the effects of the magnetic monopoles and instantons in quantum chromodynamics (QCD) on observables; therefore, we introduce a monopole and anti-monopole pair in the QCD vacuum of a quenched SU(3) by applying the monopole creation operator to the vacuum. We calculate the eigenvalues and eigenvectors of the overlap Dirac operator that preserves the exact chiral symmetry in lattice gauge theory using these QCD vacua. We then investigate the effects of magnetic monopoles and instantons. First, we confirm the monopole effects as follows: (i) The monopole creation operator makes the monopoles and anti-monopoles in the QCD vacuum. (ii) A monopole and anti-monopole pair creates an instanton or anti-instanton without changing the structure of the QCD vacuum. (iii) The monopole and anti-monopole pairs change only the scale of the spectrum distribution without affecting the spectra of the Dirac operator by comparing the spectra with random matrix theory. Next, we find the instanton effects by increasing the number density of the instantons and anti-instantons as follows: (iv) The decay constants of the pseudoscalar increase. (v) The values of the chiral condensate, which are defined as negative numbers, decrease. (vi) The light quarks and the pseudoscalar mesons become heavy. The catalytic effect on the charged pion is estimated using the numerical results of the pion decay constant and the pion mass. (vii) The decay width of the charged pion becomes wider than the experimental result, and the lifetime of the charged pion becomes shorter than the experimental result. These are the effects of the monopoles and instantons in QCD.