Probing singularities of Landau-gauge propagators with Pad\'e approximants

TL;DR

This study uses Padé approximants to analyze lattice data of SU(2) Landau-gauge gluon and ghost propagators, revealing complex poles and branch cuts that inform the understanding of their analytic structure in the infrared regime.

Contribution

It introduces a model-independent application of Padé approximants with rigorous error propagation to study propagator analytic structures, confirming complex poles and branch cuts.

Findings

Complex conjugate poles in gluon propagator

Zero on negative real axis of gluon propagator

Evidence of branch cut in ghost propagator

Abstract

Pad\'e approximants are employed in order to study the analytic structure of the four-dimensional SU(2) Landau-gauge gluon and ghost propagators in the infrared regime. The approximants, which are model independent, are used as fitting functions to lattice data for the propagators, carefully propagating uncertainties due to the fit procedure and taking into account all possible correlations. Applying this procedure systematically to the gluon-propagator data, we observe the presence of a pair of complex poles at $p^2_{\mathrm{pole}} = (-0.37 \pm 0.05_{\mathrm{stat}} \pm 0.08_{\mathrm{sys}}) \pm \, i\, (0.66 \pm 0.03_{\mathrm{stat}} \pm 0.02_{\mathrm{sys}}) \, \mathrm{GeV}^2$, where ``stat'' represents the statistical error and ``sys'' the systematic one. We also find a zero on the negative real axis of $p^2$, at $p^2_{\mathrm{zero}} = (-2.9 \pm 0.4_{\mathrm{stat}} \pm 0.9_{\mathrm{sys}}) \, \mathrm{GeV}^2$. We thus note that our procedure -- which is based on a model-independent approach and includes careful error propagation -- confirms the presence of a pair of complex poles in the gluon propagator, in agreement with previous works. For the ghost propagator, the Pad\'es indicate the existence of the single pole at $p^2 = 0$, as expected. We also find evidence of a branch cut on the negative real axis. Through the use of the so-called D-Log Pad\'e method, which is designed to approximate functions with cuts, we corroborate the existence of this cut for the ghost propagator.