Collinear functions for QCD resummations

TL;DR

This paper develops and analyzes collinear functions derived from splitting kernels in QCD, addressing their role in resummation of hard-scattering observables and exploring their dependence on auxiliary vectors and process-specific effects.

Contribution

It introduces a systematic way to define and compute collinear functions for QCD resummation, including their dependence on auxiliary vectors and higher-order corrections.

Findings

Time-like auxiliary vectors avoid rapidity divergences.

Computed collinear functions at ${\cal O}(\alpha_{\rm S})$ and ${\cal O}(\alpha_{\rm S}^2)$.

Identified process dependence of collinear functions beyond ${\cal O}(\alpha_{\rm S}^2)$.

Abstract

The singular behaviour of QCD squared amplitudes in the collinear limit is factorized and controlled by splitting kernels with a process-independent structure. We use these kernels to define collinear functions that can be used in QCD resummation formulae of hard-scattering observables. Different collinear functions are obtained by integrating the splitting kernels over different phase-space regions that depend on the hard-scattering observables of interest. The collinear functions depend on an auxiliary vector $n^\mu$ that can be either light-like $(n^2=0)$ or time-like $(n^2 > 0)$. In the case of transverse-momentum dependent (TMD) collinear functions, we show that the use of a time-like auxiliary vector avoids the rapidity divergences, which are instead present if $n^2=0$. The perturbative computation of the collinear functions lead to infrared (IR) divergences that can be properly factorized with respect to IR finite functions that embody the logarithmically-enhanced collinear contributions to hard-scattering cross sections. We evaluate various collinear functions and their $n^\mu$ dependence at ${\cal O}(\alpha_{\rm S})$. We compute the azimuthal-correlation component of the TMD collinear functions at ${\cal O}(\alpha_{\rm S}^2)$, and we present the results of the ${\cal O}(\alpha_{\rm S}^2)$ contribution of linearly-polarized gluons to transverse-momentum resummation formulae. Beyond ${\cal O}(\alpha_{\rm S}^2)$ the collinear functions of initial-state colliding partons are process dependent, as a consequence of the violation of strict collinear factorization of QCD squared amplitudes.