# Soft-gluon effective coupling and cusp anomalous dimension

## TL;DR

This paper extends the soft-gluon effective coupling in QCD beyond next-to-leading logarithms, establishing a relation with the cusp anomalous dimension and providing explicit expressions up to high orders.

## Contribution

It introduces two all-order proposals for the soft-gluon effective coupling linked to the cusp anomalous dimension, expanding the theoretical framework in QCD resummation.

## Key findings

- Derived explicit expressions for the couplings up to ${\cal O}(\as^2)$ in $d$ dimensions.
- Confirmed previous ${\cal O}(\as^3)$ results for one coupling.
- Established a relation between the couplings and the cusp anomalous dimension up to ${\cal O}(\as^4)$.

## Abstract

We consider the extension of the CMW soft-gluon effective coupling in the context of soft-gluon resummation for QCD hard-scattering observables beyond the next-to-leading logarithmic accuracy. We present two proposals of a soft-gluon effective coupling that extend the CMW coupling to all perturbative orders in the \ms\ coupling $\as$. Although both effective couplings are well-defined in the physical four-dimensional space time, we examine their behaviour in $d=4 -2\ep$ space time dimensions. We uncover an all-order perturbative relation with the cusp anomalous dimension: the (four dimensional) cusp anomalous dimension is equal to the $d$-dimensional soft-gluon effective coupling at the conformal point $\ep=\beta(\as)$, where the $d$-dimensional QCD $\beta$-function, $\beta(\as) - \ep$, vanishes. We present the explicit expressions of the two soft-gluon couplings up to ${\cal O}(\as^2)$ in $d$ dimensions. In the four-dimensional case we compute the two soft couplings up to ${\cal O}(\as^3)$. For one of the two couplings, we confirm the ${\cal O}(\as^3)$ result previously presented by other authors. For the other coupling, we obtain the explicit relation with the cusp anomalous dimension up to ${\cal O}(\as^4)$. We comment on Casimir scaling at ${\cal O}(\as^4)$.