Energy-energy correlators and determination of Collins-Soper kernel

Energy-energy correlators (EECs) are event-shape observables that can be used to study QCD by examining angular correlations between produced particles. As the EEC is weighted by the energies of the produced particles, it is an infrared-safe observable that is less sensitive to physics at the nonperturbative scales. The energy weight also allows one to exploit the momentum-sum rule of fragmentation functions, making the EEC less sensitive to the nonperturbative hadronization in the final state and allowing one to zoom in on other aspects of the collision process.

In the back-to-back limit, we can use the transverse-momentum-dependent (TMD) factorization to describe the EEC to high orders in perturbation theory. One crucial ingredient of the TMD description is the universal Collins-Soper (CS) kernel, which contains a nonperturbative contribution that has to be either fitted to the data or determined from lattice QCD calculations. In this talk, I will describe our recent extraction of the nonperturbative CS kernel from the EEC data in e⁺e^- collisions [1]. Our results agree with other methods for determining the non-perturbative CS kernel and provide further evidence of its univeral nature.

[1] Zhong-Bo Kang, JP, Congyue Zhang, 2410.21435 [hep-ph]