Exposing the parton-hadron transition within jets with energy-energy correlators in pp collisions at s = 5.02 TeV

This paper presents a fully corrected measurement of the energy-energy correlator (EEC) within jets in pp collisions. The EEC traces the energy flow as a highly energetic parton undergoes a QCD shower followed by the confinement of partons into hadrons, probing the correlation function of the energy flow inside jets. The EEC observable is measured as a function of the charged particle pair angular distance, R L , for 20 < p T ch jet < 80 GeV / c . In the perturbative region (large R L ), a good agreement between the data and a next-to-leading-log perturbative QCD calculation is observed. In the nonperturbative region (small R L ), the data exhibit a linear R L dependence. There is a transition region in between, characterized by a turnover in the EEC distribution, indicating where hadronization effects begin to dominate. The peak of this transition region is located at 2.43 ± 0.14 GeV / c / ⟨ p T ch jet ⟩ for jets of various energies, indicating a common energy scale for the hadronization process. State-of-the-art Monte Carlo event generators are compared with the measurements, and can be used to constrain the parton shower and hadronization mechanisms.