Observation of enhanced long-range elliptic anisotropies inside high-multiplicity jets in pp collisions at √s =13TeV

Abstract

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 √s 13TeV, 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 GeVand pseudorapidity jηjetj < 1.6. 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 Nj ch ≈ 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for jΔη j > 2. For low-Nj ch jets, the long-range elliptic anisotropic harmonic, v2, is observed to decrease with Nj ch. This trend is well described by Monte Carlo event generators. However, a rising trend for v2 emerges at Nj ch ≳ 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.