Exposing the parton-hadron transition within jets with energy-energy correlators in pp collisions at $\sqrt{\mathit{\text{s}}}=5.02$ 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, $R_{\mathrm{L}}$, for $20<p_{\mathrm{T}}^{\mathrm{c}\mathrm{h}\mathrm{j}\mathrm{e}\mathrm{t}}<80$ GeV/$c$. In the perturbative region (large $R_{\mathrm{L}}$), a good agreement between the data and a next-to-leading-log perturbative QCD calculation is observed. In the non-perturbative region (small $R_{\mathrm{L}}$), the data exhibits a linear $R_{\mathrm{L}}$ dependence. There is a transition region in between, characterized by a turnover in the EEC distribution, corresponding to the confinement process. The peak of this transition region is located at $2.39\pm 0.17$ GeV/$c\mathrm{/}⟨p_{\mathrm{T}}^{\mathrm{c}\mathrm{h}\mathrm{j}\mathrm{e}\mathrm{t}}⟩$ 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.