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

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 =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 |𝜂jet|<1.6. Two-particle correlations among the charged particles within the jets are studied as functions of the particles’ azimuthal angle and pseudorapidity separations (Δ⁢𝜙* and Δ⁢𝜂*) 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 𝑁jch ≈100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for |Δ⁢𝜂*| >2. For low-𝑁jch jets, the long-range elliptic anisotropic harmonic, 𝑣*2, is observed to decrease with 𝑁jch. This trend is well described by Monte Carlo event generators. However, a rising trend for 𝑣*2 emerges at 𝑁jch ≳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.