Lattice QCD Determination of the Collins-Soper Kernel in the Continuum and Physical Mass Limits

TL;DR

This paper presents a first-principles lattice QCD calculation of the Collins-Soper kernel, providing a nonperturbative constraint on its behavior at large transverse separations, crucial for understanding nucleon structure and TMD evolution.

Contribution

The study achieves the first continuum and physical mass limit determination of the CS kernel from lattice QCD, incorporating systematic improvements and matching to phenomenological data.

Findings

CS kernel agrees with perturbative QCD at small $b_{\perp}$

Results extend reliably up to $b_{\perp} \sim 1$ fm

Provides the most precise nonperturbative constraint on the CS kernel's long-distance behavior

Abstract

The Collins-Soper (CS) kernel governs the rapidity evolution of transverse-momentum-dependent (TMD) parton distributions, a cornerstone for QCD factorization and linking nucleon structure data across scales. Its nonperturbative behavior at large transverse separations ($b_{\perp}$) remains weakly constrained due to phenomenological model dependencies. We present a first-principles determination of the CS kernel at the continuum limit and physical pion mass from lattice QCD in the large-momentum effective theory framework. Using (2+1)-flavor configurations (lattice spacings $a \in[0.052, 0.105]$ fm, and pion mass $m_{\pi} \approx ( 136, 230, 300, 320)$ MeV), we simulating the nonlocal equal-time correlation function and extract the quasi-TMD wave functions. Taking into account systematic improvements including hypercubic smearing, nonperturbative renormalization, and a $b_{\perp}$-unexpanded matching kernel, we obtain the CS kernel at the continuum, chiral, and infinite-momentum limits. Our results are determined up to $b_{\perp} \sim 1$ fm, with controllable uncertainties, and agree with perturbative QCD at small $b_{\perp}$ and global TMD phenomenological extractions. We conduct a global analysis integrated with phenomenological fits and demonstrate the impact of our results on such fits. This work yields the most precise nonperturbative constraint on the CS kernel's long-distance behavior from Lattice QCD, which not only bridges Lattice QCD, perturbation theory, and nucleon structure experiments for TMD studies, but also boosts the utility of our constraint for future global TMD analyses.