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CMS-PAS-HIN-24-024
Unveiling the dynamics of long-range correlations in high-multiplicity jets through substructure engineering in pp collisions at CMS
Abstract: Recent CMS data have revealed long-range correlations at high charged-particle multiplicity ($ N_\text{ch}^\mathrm{j} $) within jets produced in proton-proton (pp) collisions, suggesting collective behavior in systems much smaller than those typical of heavy ion collisions. In the polar coordinate system about the reconstructed jet axis, two-particle azimuthal correlations show an unexpected rise in elliptic anisotropy ($ v^{*}_2 $) at large pseudorapidity separations ($ \Delta\eta^{*} > $ 2) as a function of $ N_\text{ch}^\mathrm{j} $, a trend not reproduced by event generators like PYTHIA or SHERPA. In this paper, we present detailed measurements of long-range correlations using LHC Run 2 data at $ \sqrt{s} = $ 13 TeV and compare the results with model predictions. The transverse momentum and $ \Delta\eta^{*} $dependence of $ v^{*}_2 $ is shown across a wide range in $ N_\text{ch}^\mathrm{j} $. Furthermore, the role of jet substructure, particularly in jets exhibiting two-prong features, is examined to unveil a potential connection between the $ v^{*}_2 $ enhancement and the initial-state jet geometry. A surprising increase in $ v^{*}_2 $ emerges exclusively in these two-prong jets at high $ N_\text{ch}^\mathrm{j} $.
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Figure 1:
Schematic representation of jets in the ``jet frame,'' a coordinate system where the $ z $-axis is aligned with the jet momentum direction. Left: illustration of a jet exhibiting two sub-jets with unequal momentum magnitudes. Right: illustration of a jet with two sub-jets of approximately equal momentum magnitudes, also referred to as a two-prong jet.

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Figure 2:
The 2D two-particle angular correlation functions in the jet frame for trigger particles with 0.7 $ < j_{\mathrm{T}}^{\text{trg}} < $ 1.3 GeV and associated particles with 0.3 $ < j_{\mathrm{T}}^{\text{assoc.}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets. The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 2-a:
The 2D two-particle angular correlation functions in the jet frame for trigger particles with 0.7 $ < j_{\mathrm{T}}^{\text{trg}} < $ 1.3 GeV and associated particles with 0.3 $ < j_{\mathrm{T}}^{\text{assoc.}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets. The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 2-b:
The 2D two-particle angular correlation functions in the jet frame for trigger particles with 0.7 $ < j_{\mathrm{T}}^{\text{trg}} < $ 1.3 GeV and associated particles with 0.3 $ < j_{\mathrm{T}}^{\text{assoc.}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets. The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 3:
The elliptic anisotropy coefficient $ v^{\ast}_2 $, obtained from two-particle correlations, as a function of the minimum $ \Delta\eta^{\ast} $ limit for eight $ N_\text{ch}^\mathrm{j} $ intervals. Results correspond to anti-$ k_{\mathrm{T}} \mathrm{R}= $ 0.8 jets with $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6 in pp collisions at 13 TeV. Data points include statistical uncertainties (vertical bars), while systematic uncertainties are represented by shaded boxes. The shaded envelope around the PYTHIA8 model curves indicates statistical uncertainty.

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Figure 4:
Average $ j_{\mathrm{T}} $\ of charged particles inside jets as a function of $ N_\text{ch}^\mathrm{j} $\ corrresponding to $ \eta^{\ast} < $ 2 (black) and $ \eta^{\ast} < $ 5 (red), for anti-$ k_{\mathrm{T}} \mathrm{R}= $ 0.8 jets with $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6 in pp collisions at 13 TeV from data and PYTHIA8. Vertical bars on data points indicate statistical uncertainties, while shaded boxes represent systematic uncertainties.

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Figure 5:
The first- and-third order two-particle Fourier coefficients, $ V^{\ast}_{\text{1}\Delta} $ (red) and $ V^{\ast}_{\text{3}\Delta} $ (blue), as a function of $ j_{\mathrm{T}}^{\text{trg}} $ for different $ N_\text{ch}^\mathrm{j} $ intervals, with associated particles in the range 0.3 $ < j_{\mathrm{T}}^{\text{assoc.}} < $ 3 GeV. Results are shown for anti-$ k_{\mathrm{T}} \mathrm{R}= $ 0.8 jets with $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6 in pp collisions at $ \sqrt{s} = $ 13 TeV, comparing data with PYTHIA8 predictions.

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Figure 6:
The second-order two-particle Fourier coefficient, $ V^{\ast}_{\text{2}\Delta} $, as a function of $ j_{\mathrm{T}}^{\text{trg}} $ for different $ N_\text{ch}^\mathrm{j} $ ranges, with associated particles in the range 0.3 $ < j_{\mathrm{T}}^{\text{assoc.}} < $ 3 GeV. Results are shown for anti-$ k_{\mathrm{T}} \mathrm{R}= $ 0.8 jets with $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6 in pp collisions at $ \sqrt{s} = $ 13 TeV, comparing data with PYTHIA8 predictions.

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Figure 7:
Average $ z_g\theta_g^\beta $ as a function of $ N_\text{ch}^\mathrm{j} $, for anti-$ k_{\mathrm{T}} \mathrm{R}= $ 0.8 jets with $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6 in pp collisions at 13 TeV from data and PYTHIA8. Shaded boxes represent systematic uncertainties, while statistical uncertainties are smaller than the marker size.

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Figure 8:
The 2D two-particle angular correlation functions in the jet frame for charged particles with 0.3 $ < j_{\mathrm{T}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets, with further classification based on the jet substructure variable: $ z_g\theta_g^\beta < $ 0.25 (top) or $ z_g\theta_g^\beta > $ 0.25 (bottom). The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 8-a:
The 2D two-particle angular correlation functions in the jet frame for charged particles with 0.3 $ < j_{\mathrm{T}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets, with further classification based on the jet substructure variable: $ z_g\theta_g^\beta < $ 0.25 (top) or $ z_g\theta_g^\beta > $ 0.25 (bottom). The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 8-b:
The 2D two-particle angular correlation functions in the jet frame for charged particles with 0.3 $ < j_{\mathrm{T}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets, with further classification based on the jet substructure variable: $ z_g\theta_g^\beta < $ 0.25 (top) or $ z_g\theta_g^\beta > $ 0.25 (bottom). The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

png pdf
Figure 8-c:
The 2D two-particle angular correlation functions in the jet frame for charged particles with 0.3 $ < j_{\mathrm{T}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets, with further classification based on the jet substructure variable: $ z_g\theta_g^\beta < $ 0.25 (top) or $ z_g\theta_g^\beta > $ 0.25 (bottom). The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 8-d:
The 2D two-particle angular correlation functions in the jet frame for charged particles with 0.3 $ < j_{\mathrm{T}} < $ 3.0 GeV. Results are shown for low-$ N_\text{ch}^\mathrm{j} $ (left) and high-$ N_\text{ch}^\mathrm{j} $ (right) jets, with further classification based on the jet substructure variable: $ z_g\theta_g^\beta < $ 0.25 (top) or $ z_g\theta_g^\beta > $ 0.25 (bottom). The jets are reconstructed using the anti-$ k_{\mathrm{T}} $ algorithm with $ \mathrm{R}= $ 0.8, requiring $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV and $ |\eta^{\rm jet}| < $ 1.6.

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Figure 9:
The elliptic anisotropy $ v^{\ast}_2 $, obtained from two-particle correlations, as a function of $ N_\text{ch}^\mathrm{j} $ for two jet substructure classes, $ z_g\theta_g^\beta < $ 0.25 (red) and $ z_g\theta_g^\beta > $ 0.25 (blue). Results are presented for two transverse momentum ranges, 0.3 $ < j_{\mathrm{T}} < $ 3 GeV (left) and 0.5 $ < j_{\mathrm{T}} < $ 3 GeV (right), using anti-$ k_{\mathrm{T}} $ jets with $ \mathrm{R}= $ 0.8, $ p_{\mathrm{T}}^{\rm jet} > $ 550 GeV, and $ |\eta^{\rm jet}| < $ 1.6 in pp collisions at $ \sqrt{s} = $ 13 TeV. The data points include statistical uncertainties (vertical bars), while systematic uncertainties are indicated by shaded boxes. The shaded envelopes around the PYTHIA8 curves represent statistical uncertainties.
Summary
In this paper, we present a systematic investigation of the long-range azimuthal anisotropy observed in high-multiplicity jets produced in proton-proton collisions using data collected by the CMS experiment at $ \sqrt{s}= $ 13 TeV. The second-order Fourier coefficient ($ v^{\ast}_2 $) is studied in a jet-centric reference frame, revealing a clear dependence on both the jet multiplicity and transverse momentum of constituent particles. The most pronounced anisotropy signal is found for particles with higher transverse momentum. Additionally, the $ v^{\ast}_2 $ dependence on the pseudorapidity separation ($ \Delta\eta^{\ast} $) exhibits contrasting behavior for different jet multiplicities: for low-multiplicity jets, $ v^{\ast}_2 $ shows a monotonic decrease with increasing $ |\Delta\eta^{\ast}| $, whereas for high-multiplicity jets, it appears to saturate and remains approximately constant for $ |\Delta\eta^{\ast}| > $ 2. The predictions from PYTHIA8 Monte-Carlo simulations are generally in agreement with the measured trends in the low-multiplicity regime. For high-multiplicity jets, a significant deviation from PYTHIA8 predictions is observed, hinting at the presence of a novel underlying mechanism. Further investigation into the jet substructure reveals that $ v^{\ast}_2 $ is more pronounced in jets exhibiting a two-prong structure, indicating a potential connection between the jet shape and the emergence of collective effects. The two-prong topology is consistent with hard splitting of the initial-state parton, leading to a wider energy distribution within the jet. Thus, the substructure-dependent $ v^{\ast}_2 $ enhancement may suggest that the long-range correlations could originate from intrinsic partonic dynamics together with final-state interactions. While PYTHIA8 serves as a valuable baseline model, its treatment of higher order perturbative quantum chromodynamics processes, particularly those involving multijet production, may introduce uncertainties in its comparison with the observed high-multiplicity jet phenomena. This will be further explored in future works to establish possible new insights into the interplay of jet fragmentation, partonic geometry, and potential medium-like effects.
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