Non perturbative physics from NSPT: renormalons, the gluon condensate and all that

TL;DR

This paper uses Numerical Stochastic Perturbation Theory to accurately determine the gluon condensate in lattice QCD, addressing longstanding issues related to renormalons and asymptotic series in the operator product expansion.

Contribution

It presents the first QCD computation of the gluon condensate with massless fermions using NSPT, including a novel subtraction method for power divergences.

Findings

Successful subtraction of power divergences in the plaquette

First QCD calculation of the gluon condensate with massless fermions

Demonstration of NSPT's effectiveness in high-order perturbative computations

Abstract

Numerical Stochastic Perturbation Theory (NSPT) enables very high order computations in Lattice Gauge Theories. We report on the determination of the gluon condensate from lattice QCD measurements of the basic plaquette. This is a long standing problem, which was eventually solved a few years ago in pure gauge. In this context NSPT is crucial: it is actually the only tool enabling the subtraction of the power divergent contribution associated to the identity operator in the OPE for the plaquette. This subtraction is actually a delicate issue, since the perturbative expansion of the plaquette is on general ground expected to be an asymptotic one, due to renormalons. This in turn results in ambiguities and the separation of scales in the OPE does not correspond to a separation of perturbative and non-perturbative contributions. All in all, one needs to absorb the ambiguities attached to the perturbative series into the definition of the condensate itself, i.e. one needs a prescription. A possible one amounts to summing the perturbative series up to its minimal term, which means computing up to orders which only NSPT can aim at. Our computation is the first one in QCD, with massless staggered fermions. In order to remove the zero-mode of the gauge field, twisted boundary conditions are adopted for the latter, consistently coupled to fermions in the fundamental representation supplemented with smell degrees of freedom.