The complete singlet contribution to the massless quark form factor at three loops in QCD

TL;DR

This paper computes the complete singlet contribution to the massless quark form factor at three loops in QCD, including exact top quark mass dependence, and discusses its implications for low energy physics and renormalization.

Contribution

It provides the first full numerical computation of the singlet contribution with exact top mass dependence at three loops in QCD.

Findings

Numerical results for singlet contributions across the full kinematic range.

Extraction of the 3-loop Wilson coefficient in a non-\overline{MS} scheme.

Assessment of the approximation's accuracy in the low energy region.

Abstract

It is well known that the effect of top quark loop corrections in the axial part of quark form factors (FF) does not decouple in the large top mass or low energy limit due to the presence of the axial-anomaly type diagrams. The top-loop induced singlet-type contribution should be included in addition to the purely massless result for quark FFs when applied to physics in the low energy region, both for the non-decoupling mass logarithms and for an appropriate renormalization scale dependence. In this work, we have numerically computed the so-called singlet contribution to quark FFs with the exact top quark mass dependence over the full kinematic range. We discuss in detail the renormalization formulae of the individual subsets of the singlet contribution to an axial quark FF with a particular flavor, as well as the renormalization group equations that govern their individual scale dependence. Finally we have extracted the 3-loop Wilson coefficient in the low energy effective Lagrangian, renormalized in a non-$\overline{\mathrm{MS}}$ scheme and constructed to encode the leading large mass approximation of our exact results for singlet quark FFs. We have also examined the accuracy of the approximation in the low energy region.