Bethe-Salpeter Amplitudes and Density Correlations for Mesons with Wilson Fermions

TL;DR

This paper investigates gauge invariant and gauge-dependent Bethe-Salpeter amplitudes for mesons in lattice QCD, demonstrating effects of smearing, Lorentz contraction, and polarization dependence, and discusses limitations in extracting charge radii.

Contribution

It compares various definitions of meson Bethe-Salpeter amplitudes, analyzes the impact of smearing, and explores polarization effects and wavefunction fits in lattice QCD.

Findings

Smearing broadens gauge invariant BS amplitudes.

Lorentz contraction observed at non-zero momentum.

Hydrogen-like wavefunctions fit the $ ho$ meson polarization data.

Abstract

We present an investigation of various gauge invariant definitions of the $q\bar q$ Bethe-Salpeter (BS) amplitude for mesons in lattice QCD, and compare them to the Coulomb and Landau gauge BS amplitudes. We show that the gauge invariant BS amplitude is considerably broadened by the use of ``fat'' gauge links (constructed by smearing the links of the original lattice). A qualitative demonstration of the Lorentz contraction of the pion wavefunction at non-zero momentum is given. We also calculate density-density correlations and discuss the limitations in extracting the charge radius of the pion from these observables. Lastly, the polarization dependence of the BS amplitude for the $\rho$ meson is exhibited, and we extract the relative admixture of $l=0$ and $l=2$ states showing that simple hydrogen-like wavefunctions provide a good fit.