Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in <math display="inline"><mi>p</mi><mi>p</mi></math> Collisions at <math display="inline"><mrow><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>

Benecke, Anna;Bethani, Agni;Bruno, Giacomo;Caputo, Claudio;CMS;et.al.
(2024) Physical Review Letters — Vol. 133 (2024)

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Abstract
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 <math display="inline"><msqrt><mi>s</mi></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mrow><mi>TeV</mi></mrow></math>, corresponding to an integrated luminosity of <math display="inline"><mn>138</mn><mtext> </mtext><mtext> </mtext><msup><mi>fb</mi><mrow><mo>-</mo><mn>1</mn></mrow></msup></math>. Jets are reconstructed with the anti-<math display="inline"><msub><mi>k</mi><mi mathvariant="normal">T</mi></msub></math> algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity <math display="inline"><mrow><mrow><mo stretchy="false">|</mo><msup><mrow><mi>η</mi></mrow><mrow><mtext>jet</mtext></mrow></msup><mo stretchy="false">|</mo></mrow><mo>&lt;</mo><mn>1.6</mn></mrow></math>. Two-particle correlations among the charged particles within the jets are studied as functions of the particles’ azimuthal angle and pseudorapidity separations (<math display="inline"><mi mathvariant="normal">Δ</mi><msup><mi>ϕ</mi><mo>*</mo></msup></math> and <math display="inline"><mi mathvariant="normal">Δ</mi><msup><mi>η</mi><mo>*</mo></msup></math>) in a jet coordinate basis, where particles’ <math display="inline"><msup><mi>η</mi><mo>*</mo></msup></math>, <math display="inline"><msup><mi>ϕ</mi><mo>*</mo></msup></math> are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to <math display="inline"><msubsup><mi>N</mi><mi>ch</mi><mi mathvariant="normal">j</mi></msubsup><mo>≈</mo><mn>100</mn></math>. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for <math display="inline"><mrow><mo stretchy="false">|</mo><mrow><mi mathvariant="normal">Δ</mi><msup><mi>η</mi><mo>*</mo></msup></mrow><mo stretchy="false">|</mo></mrow><mo>&gt;</mo><mn>2</mn></math>. For low-<math display="inline"><msubsup><mi>N</mi><mi>ch</mi><mi mathvariant="normal">j</mi></msubsup></math> jets, the long-range elliptic anisotropic harmonic, <math display="inline"><msubsup><mi>v</mi><mn>2</mn><mo>*</mo></msubsup></math>, is observed to decrease with <math display="inline"><msubsup><mi>N</mi><mi>ch</mi><mi mathvariant="normal">j</mi></msubsup></math>. This trend is well described by Monte Carlo event generators. However, a rising trend for <math display="inline"><msubsup><mi>v</mi><mn>2</mn><mo>*</mo></msubsup></math> emerges at <math display="inline"><msubsup><mi>N</mi><mi>ch</mi><mi mathvariant="normal">j</mi></msubsup><mo>≳</mo><mn>80</mn></math>, 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.
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Benecke, A., Bethani, A., Bruno, G., Caputo, C., Delaere, C., Donertas, I. S., Giammanco, A., Jaffel, K., Jain, S., Jain, S., Lemaitre, V., Lidrych, J., Mastrapasqua, P., Mondal, K., Tran, T. T., Wertz, S., CMS, & et al. (2024). Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in <math display=“inline”><mi>p</mi><mi>p</mi></math> Collisions at <math display=“inline”><mrow><msqrt><mrow><mi>s</mi></mrow></msqrt><mo>=</mo><mn>13</mn><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math>. Physical Review Letters, 133. https://doi.org/10.1103/PhysRevLett.133.142301 (Original work published 2024)