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 (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 Nchj≈100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for |Δη*|>2. For low-Nchj jets, the long-range elliptic anisotropic harmonic, v2*, is observed to decrease with Nchj. This trend is well described by Monte Carlo event generators. However, a rising trend for v2* emerges at Nchj≳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.
© 2024 CERN, for the CMS Collaboration
2024
CERN