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Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PRL. The superseded preliminary plots can be found here.

Figures	Summary	Additional Figures	References	CMS Publications

Figures
png pdf	Figure 1: A schematic illustration of a jet and its constituents in a pp collision, showing the idea of an initial scattered parton eventually exhibiting potential collective expansion in the direction transverse to the jet axis. A jet cone and emerging final state particles are drawn in a coordinate system, denoted the ``jet frame'', where the $z$ axis coincides with the jet direction. Cone size is not to scale.
png pdf	Figure 2: Examples of 2D two-particle angular correlation functions for particle 0.3 $< j_{\mathrm{T}} <$ 3 GeV from all jet multiplicities (left) and the highest (right) in-jet $N^{\text{j}}_{\text{ch}}$ , for anti- $k_{\text{T}}$ $R =$ 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6.
png pdf	Figure 3: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_T <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for two in-jet $N^{\text{j}}_{\text{ch}}$ classes in data (left), PYTHIA 8 (middle), and SHERPA (right). The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty
png pdf	Figure 3-a: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_T <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for two in-jet $N^{\text{j}}_{\text{ch}}$ classes in data (left), PYTHIA 8 (middle), and SHERPA (right). The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty
png pdf	Figure 3-b: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_T <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for two in-jet $N^{\text{j}}_{\text{ch}}$ classes in data (left), PYTHIA 8 (middle), and SHERPA (right). The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty
png pdf	Figure 3-c: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_T <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for two in-jet $N^{\text{j}}_{\text{ch}}$ classes in data (left), PYTHIA 8 (middle), and SHERPA (right). The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty
png pdf	Figure 4: The extracted two-particle Fourier coefficients $V^{\text{j}}_{n\Delta}$ (left) and single-particle elliptic anisotropies $v^{\text{j}}_{2}\{2\}$ (right), as a function of $N^{\text{j}}_{\text{ch}}$ , for anti- $k_{\text{T}}$ $R=$ 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6 jets in pp collisions at 13 TeV from data, PYTHIA 8, and SHERPA. Vertical bars on data points indicate statistical uncertainty in the extracted Fourier component. Vertical grey bands indicate the total systematic uncertainty.
png pdf	Figure 4-a: The extracted two-particle Fourier coefficients $V^{\text{j}}_{n\Delta}$ (left) and single-particle elliptic anisotropies $v^{\text{j}}_{2}\{2\}$ (right), as a function of $N^{\text{j}}_{\text{ch}}$ , for anti- $k_{\text{T}}$ $R=$ 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6 jets in pp collisions at 13 TeV from data, PYTHIA 8, and SHERPA. Vertical bars on data points indicate statistical uncertainty in the extracted Fourier component. Vertical grey bands indicate the total systematic uncertainty.
png pdf	Figure 4-b: The extracted two-particle Fourier coefficients $V^{\text{j}}_{n\Delta}$ (left) and single-particle elliptic anisotropies $v^{\text{j}}_{2}\{2\}$ (right), as a function of $N^{\text{j}}_{\text{ch}}$ , for anti- $k_{\text{T}}$ $R=$ 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6 jets in pp collisions at 13 TeV from data, PYTHIA 8, and SHERPA. Vertical bars on data points indicate statistical uncertainty in the extracted Fourier component. Vertical grey bands indicate the total systematic uncertainty.

Summary

In summary, the first search for long-range near-side correlations and QCD collective effects in jets produced in 13 TeV pp collisions is presented. The measurement is performed using a jet's charged particle constituents, after their kinematic variables have been calculated in a reference frame where the

$z$ -axis is defined as the jet direction. Two-particle correlations are studied as a function of the number of charged particle constituents in the jet,

$N^{\text{j}}_{\text{ch}}$ , which extends to values of over 100, and the transverse momentum with respect to the jet direction. The first three Fourier harmonics of long-range azimuthal correlations are extracted and compared with those of the PYTHIA8 and SHERPA Monte Carlo (MC) event generators. While data and the MC samples are in good agreement for particle correlations inside low- and mid-

$N^{\text{j}}_{\text{ch}}$ jets, the extracted long-range elliptic azimuthal anisotropy

$v^{\text{j}}_{2}\{2\}$ shows a distinct increase in data for

$N^{\text{j}}_{\text{ch}} >$ 80. Such a feature is not observed in any of MC event generators that model the parton fragmentation process. Therefore, results presented in this note may pave a new direction in uncovering novel effects related to nonperturbative QCD dynamics of parton fragmentation in the vacuum.

Additional Figures
png pdf	Additional Figure 1: Distribution of $N^{\text{j}}_{\text{ch}}$ in data. The events of primary interest, high- $N^{\text{j}}_{\text{ch}}$ jets with multiplicity approaching that of ``peripheral'' nucleus-nucleus at LHC [52], are exceptionally rare.
png pdf	Additional Figure 2: Distribution of $\eta^\ast$ for inclusive (red) and high- $N^{\text{j}}_{\text{ch}}$ (blue). The inclusive- $N^{\text{j}}_{\text{ch}}$ class is broader while the narrow high- $N^{\text{j}}_{\text{ch}}$ reaches $\textrm{d}N/\textrm{d}\eta^\ast$ values similar to that produced in ``peripheral'' nucleus-nucleus collisions at LHC [52].
png pdf	Additional Figure 3: Example of signal (left) and background (right) distributions for inclusive multiplicity jets. The signal is constructed from the average of correlations taken within a single jet, while the background is the average of particle pairs taken from separate jets and events.
png pdf	Additional Figure 4: Examples of 2D two-particle angular correlation functions from PYTHIA 8 [47] (top) and SHERPA [49] (bottom) MC generators for particle 0.3 $< j_{\textrm{T}} <$ 3 GeV from all jet multiplicities (left) and the highest (right) in-jet $N^{\text{j}}_{\text{ch}}$ class, for anti- $\textrm{k}_{\textrm{T}}$ R = 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6.
png	Additional Figure 4-a: Examples of 2D two-particle angular correlation functions from PYTHIA 8 [47] (top) and SHERPA [49] (bottom) MC generators for particle 0.3 $< j_{\textrm{T}} <$ 3 GeV from all jet multiplicities (left) and the highest (right) in-jet $N^{\text{j}}_{\text{ch}}$ class, for anti- $\textrm{k}_{\textrm{T}}$ R = 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6.
png	Additional Figure 4-b: Examples of 2D two-particle angular correlation functions from PYTHIA 8 [47] (top) and SHERPA [49] (bottom) MC generators for particle 0.3 $< j_{\textrm{T}} <$ 3 GeV from all jet multiplicities (left) and the highest (right) in-jet $N^{\text{j}}_{\text{ch}}$ class, for anti- $\textrm{k}_{\textrm{T}}$ R = 0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6.
png pdf	Additional Figure 5: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_{\textrm{T}} <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for the four highest in-jet $N^{\text{j}}_{\text{ch}}$ classes in data. The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty.
png pdf	Additional Figure 5-a: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_{\textrm{T}} <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for the four highest in-jet $N^{\text{j}}_{\text{ch}}$ classes in data. The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty.
png pdf	Additional Figure 5-b: Examples of 1D two-particle angular correlation functions projected on $\Delta\phi^\ast$ for particles in the range 0.3 $< j_{\textrm{T}} <$ 3 GeV and $\|\Delta\eta^\ast\| >$ 2, for the four highest in-jet $N^{\text{j}}_{\text{ch}}$ classes in data. The solid black line shows the fitted Fourier function and the colored dashed lines show the individual Fourier components. Vertical bars on data points indicate statistical uncertainty.
png pdf	Additional Figure 6: CMS event display of selected high- $N^{\text{j}}_{\text{ch}}$ jet event with a lab-frame $p_{\mathrm{T}}$ cut of 1.5GeV.
png pdf	Additional Figure 7: CMS event display of selected high- $N^{\text{j}}_{\text{ch}}$ jet event with no lab-frame $p_{\mathrm{T}}$ cut.
png pdf	Additional Figure 8: The extracted single-particle elliptic anisotropies $v_2\{2\}$ , as a function of $N^{\text{j}}_{\text{ch}}$ , for anti- $\textrm{k}_{\textrm{T}}$ R=0.8 jets with jet $p_{\mathrm{T}} >$ 550 GeV and $\|\eta\| <$ 1.6 jets in pp collisions at 13 TeV from data (black squares) and PYTHIA 8 [47] CP2 [48] (dashed red), PYTHIA 8 CP5 [48] (dashed blue), and Ropewalk with string shoving enabled [47] (dashed green) tunes and settings, respectively. Vertical bars on data points indicate statistical uncertainty in the extracted Fourier component. Vertical grey bands indicate the total systematic uncertainty.

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