CMS-PAS-HIN-21-013

CMS-PAS-HIN-21-013
Observation of enhanced long-range elliptic anisotropies inside high-multiplicity jets in pp collisions at the LHC
CMS Collaboration
21 June 2023

Abstract: A search for QCD collective effects is performed with the CMS experiment via correlation measurements of charged constituents inside jets produced in proton-proton collisions at the LHC. The analysis uses data collected at a center-of-mass energy of $ \sqrt{s} $ = 13 TeV, corresponding to an integrated luminosity of 138 fb$ ^{-1} $. For charged constituents within a reconstructed jet of cone radius 0.8, two-particle correlations as functions of relative azimuthal angle ($ \Delta\phi^{\ast} $) and pseudorapidity ($ \Delta\eta^{\ast} $) are performed in a novel ``jet frame,'' where constituent $ \eta$, $\phi $ variables are redefined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to nearly 100. Anisotropy Fourier harmonics are extracted from long-range azimuthal correlation functions for $ \|\Delta\eta^{\ast}\| > $ 2. For low-multiplicity jets, the long-range elliptic anisotropy harmonic, $ v^{\text{j}}_2 $, is observed to decrease with multiplicity. This trend is well described by Monte Carlo (MC) event generators. However, a rising trend of $ v^{\text{j}}_2 $ emerges at an in-jet charged-particle multiplicity above $ \approx $ 80. This trend is not reproduced by MC models. This observation yields new insights into the dynamics of parton fragmentation processes in the vacuum. This document has been revised with respect to the version dated June 21, 2023.
Links: CDS record (PDF) ; Physics Briefing ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

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