Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in <i>pp</i> Collisions at √<i>s</i>=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 uses data collected at a center-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 138 fb(-1). Jets are reconstructed with the anti-kT algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity vertical bar eta(jet)vertical bar < 1.6. Two-particle correlations among the charged particles within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations (Delta phi* and Delta eta*) in a jet coordinate basis, where particles' eta*, phi* are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet chargedparticle multiplicity up to N-ch(J) approximate to 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for vertical bar Delta etavertical bar > 2. For low-N-ch(j) jets, the long-range elliptic anisotropic harmonic, v(2), is observed to decrease with N-ch(j). This trend is well described by Monte Carlo event generators. However, a rising trend for v(2)* emerges at N-ch(j) greater than or similar to 80, 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.