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| CMS-HIN-21-013 ; CERN-EP-2023-281 | ||
| Observation of enhanced long-range elliptic anisotropies inside high-multiplicity jets in pp collisions at $ \sqrt{s}= $ 13 TeV | ||
| CMS Collaboration | ||
| 28 December 2023 | ||
| Submitted to Phys. Rev. Lett. | ||
| Abstract: A search for partonic collective effects inside jets produced in proton-proton collisions is performed via correlation measurements of charged constituents using the CMS detector at the CERN 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} $. Jets are reconstructed with the anti-$ k_{\mathrm{T}} $ algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $ |\eta| < $ 1.6. Two-particle correlations among the charged constituents within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations ($ \Delta\phi^{\ast} $ and $ \Delta\eta^{\ast} $) in a jet coordinate basis, where constituents' $ \eta^{\ast} $, $ \phi^{\ast} $ are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to $ N_\text{ch}^\mathrm{j}\approx $ 100. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $ |\Delta\eta^{\ast}| > $ 2. For low-multiplicity jets, the long-range elliptic anisotropic harmonic, $ v^{\ast}_2 $, is observed to decrease with $ N_\text{ch}^\mathrm{j} $. This trend is well described by Monte Carlo event generators. However, a rising trend for $ v^{\ast}_2 $ emerges at $ N_\text{ch}^\mathrm{j}\gtrsim $ 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 parton fragmentation processes in the vacuum. | ||
| Links: e-print arXiv:2312.17103 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; Physics Briefing ; CADI line (restricted) ; | ||
| Figures | |
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Figure 1:
An illustration showing the idea of an initially scattered parton evolving to a shower that eventually exhibits collective expansion transverse to the jet axis. A jet cone (not to scale) and emerging final state particles are drawn in a coordinate system, denoted as the ``jet basis,'' where the $ z $ axis coincides with the jet direction. The redefined pseudorapidity $ \eta^{\ast} $ is shown with key values in dotted blue lines, and the azimuthal angle $ \phi^{\ast} $ is shown with key values in solid red. |
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Figure 2:
Results for 2D two-particle angular correlation functions for particle 0.3 $ < j_{\mathrm{T}} < $ 3.0 GeV from all jet multiplicities (left) and the highest (right) in-jet $ N_\text{ch}^\mathrm{j} $, for anti-$ k_{\mathrm{T}} \mathrm{R}= $ 0.8 jets with jet $ p_{\mathrm{T}} > $ 550 GeV and $ |\eta| < $ 1.6. |
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Figure 3:
Examples of two-particle angular correlations projected onto 1D $ \Delta\phi^{\ast} $ for $ |\Delta\eta^{\ast}| > $ 2. The panels show (a) data and (b) PYTHIA 8 and SHERPA for inclusive $ N_\text{ch}^\mathrm{j} $ (upper) and high-$ N_\text{ch}^\mathrm{j} $ (lower) jet selections. Data and both MC models are compared in (c) with a continuous evolution of extracted two-particle Fourier coefficients $ V^{\ast}_{\mathrm{n}\Delta} $ as a function of $ N_\text{ch}^\mathrm{j} $. Vertical bars on data points indicate statistical uncertainty while shaded boxes represent systematic uncertainties. Projections are symmetrized about $ \Delta\phi^{\ast}= $ 0 and $ \pi $. |
png pdf |
Figure 3-a:
Examples of two-particle angular correlations projected onto 1D $ \Delta\phi^{\ast} $ for $ |\Delta\eta^{\ast}| > $ 2. The panels show (a) data and (b) PYTHIA 8 and SHERPA for inclusive $ N_\text{ch}^\mathrm{j} $ (upper) and high-$ N_\text{ch}^\mathrm{j} $ (lower) jet selections. Data and both MC models are compared in (c) with a continuous evolution of extracted two-particle Fourier coefficients $ V^{\ast}_{\mathrm{n}\Delta} $ as a function of $ N_\text{ch}^\mathrm{j} $. Vertical bars on data points indicate statistical uncertainty while shaded boxes represent systematic uncertainties. Projections are symmetrized about $ \Delta\phi^{\ast}= $ 0 and $ \pi $. |
png pdf |
Figure 3-b:
Examples of two-particle angular correlations projected onto 1D $ \Delta\phi^{\ast} $ for $ |\Delta\eta^{\ast}| > $ 2. The panels show (a) data and (b) PYTHIA 8 and SHERPA for inclusive $ N_\text{ch}^\mathrm{j} $ (upper) and high-$ N_\text{ch}^\mathrm{j} $ (lower) jet selections. Data and both MC models are compared in (c) with a continuous evolution of extracted two-particle Fourier coefficients $ V^{\ast}_{\mathrm{n}\Delta} $ as a function of $ N_\text{ch}^\mathrm{j} $. Vertical bars on data points indicate statistical uncertainty while shaded boxes represent systematic uncertainties. Projections are symmetrized about $ \Delta\phi^{\ast}= $ 0 and $ \pi $. |
png pdf |
Figure 4:
The single-particle elliptic anisotropies $ v^{\ast}_2 $, as a function of $ N_\text{ch}^\mathrm{j} $, for anti-$ k_{\mathrm{T}} \mathrm{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, while shaded boxes represent systematic uncertainties. The shaded envelope around the MC curves show statistical uncertainty. |
| Summary |
| In summary, the first search for long-range near-side correlations and quantum chromodynamics (QCD) collective effects in jets produced in $ \sqrt{s}= $ 13 TeV proton-proton collisions is presented. The measurement is performed using charged constituents from individual jets, after their kinematic variables have been calculated in a coordinate basis having the $ z $-axis coinciding with the jet direction. Two-particle correlations are studied as a function of the number of charged constituents in the jet, $ N_\text{ch}^\mathrm{j} $. The first three Fourier harmonics of long-range azimuthal correlations are extracted and compared with those calculated using the PYTHIA 8 and SHERPA Monte Carlo (MC) event generators which model the parton fragmentation process. While the data and MC predictions are in good agreement for particle correlations inside low- and mid-$ N_\text{ch}^\mathrm{j} $ jets, the extracted long-range elliptic azimuthal anisotropy $ v^{\ast}_2 $ shows a distinct increase in data for $ N_\text{ch}^\mathrm{j}\gtrsim $ 80, hinting at a possible onset of collective behavior, which is not reproduced by the MC simulations. Therefore, results presented in this Letter may pave a new direction in uncovering novel effects related to nonperturbative QCD dynamics of parton fragmentation in vacuum. |
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