Observation of Enhanced Long-Range Elliptic Anisotropies Inside High-Multiplicity Jets in $pp$ Collisions at $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$

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 $\sqrt{s}=13\text{}\text{}\mathrm{TeV}$, corresponding to an integrated luminosity of $138\text{}\text{}{\mathrm{fb}}^{-1}$. Jets are reconstructed with the anti-${k}_{T}$ algorithm with a distance parameter of 0.8 and are required to have transverse momentum greater than 550 GeV and pseudorapidity $|{\eta }^{\text{jet}}|<1.6$. Two-particle correlations among the charged particles within the jets are studied as functions of the particles' azimuthal angle and pseudorapidity separations ($\Delta {\varphi }^{*}$ and $\Delta {\eta }^{*}$) in a jet coordinate basis, where particles' ${\eta }^{*}$, ${\varphi }^{*}$ are defined relative to the direction of the jet. The correlation functions are studied in classes of in-jet charged-particle multiplicity up to ${N}_{\mathrm{ch}}^{j}\approx 100$. Fourier harmonics are extracted from long-range azimuthal correlation functions to characterize azimuthal anisotropy for $|\Delta {\eta }^{*}|>2$. For low-${N}_{\mathrm{ch}}^{j}$ jets, the long-range elliptic anisotropic harmonic, ${v}_{2}^{*}$, is observed to decrease with ${N}_{\mathrm{ch}}^{j}$. This trend is well described by Monte Carlo event generators. However, a rising trend for ${v}_{2}^{*}$ emerges at ${N}_{\mathrm{ch}}^{j}\gtrsim 80$, 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 jet evolution in the vacuum.