Neutron magnetic polarisability with Landau mode operators

TL;DR

This paper introduces novel Landau mode-based quark operators to improve the calculation of neutron magnetic polarizability in lattice QCD, accounting for Landau level effects in a magnetic field.

Contribution

The authors develop and apply Landau mode eigenmode-projected quark operators to enhance the precision of nucleon energy shift measurements in magnetic fields.

Findings

Neutron magnetic polarizability calculated as 2.05(25)(19) x 10^{-4} fm^3.

Landau mode operators improve ground state isolation in magnetic field simulations.

Results agree with chiral effective-field theory predictions.

Abstract

The application of a uniform background magnetic field makes standard quark operators utilising gauge-covariant Gaussian smearing inefficient at isolating the ground state nucleon at nontrivial field strengths. In the absence of QCD interactions, Landau modes govern the quark energy levels. There is evidence that residual Landau mode effects remain when the strong interaction is turned on. Here we introduce novel quark operators constructed from the two-dimensional $U(1)$ Laplacian eigenmodes that describe the Landau levels of a charged particle on a periodic finite lattice. These eigenmode-projected quark operators provide enhanced precision for calculating nucleon energy shifts in a magnetic field. Using asymmetric source and sink operators, we are able to encapsulate the predominant effects of both the QCD and QED interactions in the interpolating fields for the neutron. The neutron magnetic polarizability is calculated using these techniques on the $32^3 \times 64$ dynamical QCD lattices provided by the PACS-CS Collaboration. In conjunction with a chiral effective-field theory analysis, we obtain a neutron magnetic polarizability of $\beta^n = 2.05(25)(19) \times 10^{-4}$ fm$^3$, where the numbers in parentheses describe statistical and systematic uncertainties.