The hadronic vacuum polarization and automatic O(a) improvement for twisted mass fermions

TL;DR

This paper investigates the impact of short-distance terms on the O(a) improvement in twisted mass lattice QCD's vacuum polarization calculations, demonstrating that automatic O(a) improvement persists when tuned to maximal twist.

Contribution

It shows that even with short-distance contributions, automatic O(a) improvement is maintained in twisted mass fermions when properly tuned.

Findings

Short-distance terms do not spoil O(a) improvement at maximal twist.

Automatic O(a) improvement is robust against certain short-distance effects.

Tuning to maximal twist is crucial for preserving improvement properties.

Abstract

The vacuum polarization tensor and the corresponding vacuum polarization function are the basis for calculations of numerous observables in lattice QCD. Examples are the hadronic contributions to lepton anomalous magnetic moments, the running of the electroweak and strong couplings and quark masses. Quantities which are derived from the vacuum polarization tensor often involve a summation of current correlators over all distances in position space leading thus to the appearance of short-distance terms. The mechanism of O(a) improvement in the presence of such short-distance terms is not directly covered by the usual arguments of on-shell improvement of the action and the operators for a given quantity. If such short-distance contributions appear, the property of O(a) improvement needs to be reconsidered. We discuss the effects of these short-distance terms on the vacuum polarization function for twisted mass lattice QCD and find that even in the presence of such terms automatic O(a) improvement is retained if the theory is tuned to maximal twist.