César Bernardes Observation of enhanced long-range elliptic anisotropies inside high-multiplicity jets in pp collisions at the LHC CMS-PAS-HIN-21-013 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 √s = 13 TeV, corresponding to an integrated luminosity of 138 fb−1. For charged constituents within a reconstructed jet of cone radius 0.8, two-particle correlations as functions of relative azimuthal angle (Δϕ∗) and pseudorapidity (Δη∗) are performed in a novel ``jet frame,'' where constituent η,ϕ 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 |Δη∗|>2. For low-multiplicity jets, the long-range elliptic anisotropy harmonic, vj2, is observed to decrease with multiplicity. This trend is well described by Monte Carlo (MC) event generators. However, a rising trend of vj2 emerges at an in-jet charged-particle multiplicity above ≈80. This trend is not reproduced by MC models. This observation yields new insights into the dynamics of parton fragmentation processes in the vacuum.