On the uncertainty estimates of the $\sigma$-pole determination by Pad\'e approximants

TL;DR

This paper evaluates the uncertainty in determining the $\sigma$-resonance pole using Padé approximants, revealing larger uncertainties compared to Roy equations due to the ill-posed nature of analytic continuation.

Contribution

It provides a systematic analysis of the uncertainties in $\sigma$-pole extraction via Padé approximants, comparing with Roy-type methods and highlighting the sensitivity to input parameterizations.

Findings

Roy-type integral representations yield consistent pole positions.

Padé approximants show larger spread in pole predictions.

Uncertainty in $\sigma$-pole determination is nearly doubled with Padé approximants.

Abstract

We discuss the determination of the $f_0(500)$ (or $\sigma$) resonance by analytic continuation through Pad\'e approximants of the $\pi\pi$-scattering amplitude from the physical region to the pole in the complex energy plane. The aim is to analyze the uncertainties of the method, having in view the fact that analytic continuation is an ill-posed problem in the sense of Hadamard. Using as input a class of admissible parameterizations of the scalar-isoscalar $\pi\pi$ partial wave, which satisfy with great accuracy the same set of dispersive constraints, we find that the Roy-type integral representations lead to almost identical pole positions for all of them, while the predictions of the Pad\'e approximants have a larger spread, being sensitive to features of the input parameterization that are not controlled by the dispersive constraints. Our conservative conclusion is that the $\sigma$-pole determination by Pad\'e approximants is consistent with the prediction of Roy-type equations, but has an uncertainty almost a factor two larger.