Observation of enhanced long-range elliptic anisotropies inside high-multiplicity jets in pp collisions at the LHC

A search for QCD collective effects is performed with the CMS experiment via correlation measurements of charged constituents inside jets produced in proton-proton collisions at the LHC. The analysis uses data collected at a center-of-mass energy of $\sqrt{s}$ = 13 TeV, corresponding to an integrated luminosity of $138~\mathrm{fb}^{-1}$. For charged constituents within a reconstructed jet of cone radius 0.8, two-particle correlations as functions of relative azimuthal angle ($\Delta\phi^{\ast}$) and pseudorapidity ($\Delta\eta^{\ast}$) are performed in a novel ``jet frame,'' where constituent $\eta,\phi$ variables are redefined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to nearly 100. Anisotropy Fourier harmonics are extracted from long-range azimuthal correlation functions for $|\Delta\eta^{\ast}|>2$. For low-multiplicity jets, the long-range elliptic anisotropy harmonic, $v^{\text{j}}_2$, is observed to decrease with multiplicity. This trend is well described by Monte Carlo (MC) event generators. However, a rising trend of $v^{\text{j}}_2$ emerges at an in-jet charged-particle multiplicity above $\approx80$. This trend is not reproduced by MC models. This observation yields new insights into the dynamics of parton fragmentation processes in the vacuum. $$\textit{This document has been revised with respect to the version dated June 21, 2023.}$$