Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in pp Collisions at √s = 13 TeV

A search for collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged particles using the CMS detector at the CERN LHC. The analysis ffiffi uses data collected at a center-of-mass energy of √s = 13 TeV, corresponding to an integrated luminosity of 138 fb⁻¹. Jets are reconstructed with the anti-k_T algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity (equation presented). Two-particle correlations among the charged particles within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations (Δϕ* and Δη*) in a jet coordinate basis, where particles' η*, ϕ* are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to N^j_ch ≈ 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for (equation presented). For low-N^j_ch jets, the long-range elliptic anisotropic harmonic, v*₂, is observed to decrease with N^j_ch. This trend is well described by Monte Carlo event generators. However, a rising trend for v*₂ emerges at (equation presented), hinting at a possible onset of collective behavior, which is not reproduced by the models tested. This observation yields new insights into the dynamics of jet evolution in the vacuum.