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 <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"><a:msqrt><a:mi>s</a:mi></a:msqrt><a:mo>=</a:mo><a:mn>13</a:mn><a:mtext> </a:mtext><a:mtext> </a:mtext><a:mrow><a:mi>TeV</a:mi></a:mrow></a:math>, corresponding to an integrated luminosity of <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"><c:mn>138</c:mn><c:mtext> </c:mtext><c:mtext> </c:mtext><c:msup><c:mi>fb</c:mi><c:mrow><c:mo>−</c:mo><c:mn>1</c:mn></c:mrow></c:msup></c:math>. Jets are reconstructed with the anti-<e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"><e:msub><e:mi>k</e:mi><e:mi mathvariant="normal">T</e:mi></e:msub></e:math> algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity <h:math xmlns:h="http://www.w3.org/1998/Math/MathML" display="inline"><h:mrow><h:mrow><h:mo stretchy="false">|</h:mo><h:msup><h:mrow><h:mi>η</h:mi></h:mrow><h:mrow><h:mtext>jet</h:mtext></h:mrow></h:msup><h:mo stretchy="false">|</h:mo></h:mrow><h:mo>&lt;</h:mo><h:mn>1.6</h:mn></h:mrow></h:math>. Two-particle correlations among the charged particles within the jets are studied as functions of the particles’ azimuthal angle and pseudorapidity separations (<l:math xmlns:l="http://www.w3.org/1998/Math/MathML" display="inline"><l:mi mathvariant="normal">Δ</l:mi><l:msup><l:mi>ϕ</l:mi><l:mo>*</l:mo></l:msup></l:math> and <o:math xmlns:o="http://www.w3.org/1998/Math/MathML" display="inline"><o:mi mathvariant="normal">Δ</o:mi><o:msup><o:mi>η</o:mi><o:mo>*</o:mo></o:msup></o:math>) in a jet coordinate basis, where particles’ <r:math xmlns:r="http://www.w3.org/1998/Math/MathML" display="inline"><r:msup><r:mi>η</r:mi><r:mo>*</r:mo></r:msup></r:math>, <t:math xmlns:t="http://www.w3.org/1998/Math/MathML" display="inline"><t:msup><t:mi>ϕ</t:mi><t:mo>*</t:mo></t:msup></t: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 <v:math xmlns:v="http://www.w3.org/1998/Math/MathML" display="inline"><v:msubsup><v:mi>N</v:mi><v:mi>ch</v:mi><v:mi mathvariant="normal">j</v:mi></v:msubsup><v:mo>≈</v:mo><v:mn>100</v:mn></v:math>. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for <y:math xmlns:y="http://www.w3.org/1998/Math/MathML" display="inline"><y:mrow><y:mo stretchy="false">|</y:mo><y:mrow><y:mi mathvariant="normal">Δ</y:mi><y:msup><y:mi>η</y:mi><y:mo>*</y:mo></y:msup></y:mrow><y:mo stretchy="false">|</y:mo></y:mrow><y:mo>&gt;</y:mo><y:mn>2</y:mn></y:math>. For low-<db:math xmlns:db="http://www.w3.org/1998/Math/MathML" display="inline"><db:msubsup><db:mi>N</db:mi><db:mi>ch</db:mi><db:mi mathvariant="normal">j</db:mi></db:msubsup></db:math> jets, the long-range elliptic anisotropic harmonic, <gb:math xmlns:gb="http://www.w3.org/1998/Math/MathML" display="inline"><gb:msubsup><gb:mi>v</gb:mi><gb:mn>2</gb:mn><gb:mo>*</gb:mo></gb:msubsup></gb:math>, is observed to decrease with <ib:math xmlns:ib="http://www.w3.org/1998/Math/MathML" display="inline"><ib:msubsup><ib:mi>N</ib:mi><ib:mi>ch</ib:mi><ib:mi mathvariant="normal">j</ib:mi></ib:msubsup></ib:math>. This trend is well described by Monte Carlo event generators. However, a rising trend for <lb:math xmlns:lb="http://www.w3.org/1998/Math/MathML" display="inline"><lb:msubsup><lb:mi>v</lb:mi><lb:mn>2</lb:mn><lb:mo>*</lb:mo></lb:msubsup></lb:math> emerges at <nb:math xmlns:nb="http://www.w3.org/1998/Math/MathML" display="inline"><nb:msubsup><nb:mi>N</nb:mi><nb:mi>ch</nb:mi><nb:mi mathvariant="normal">j</nb:mi></nb:msubsup><nb:mo>≳</nb:mo><nb:mn>80</nb:mn></nb: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. © 2024 CERN, for the CMS Collaboration 2024 CERN