Observation of enhanced long-range elliptic anisotropies inside high-multiplicity jets in pp collisions at $\sqrt{s}$ = 13 TeV

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 $\mid \eta \mid$ $<$ 1.6. Two-particle correlations among the charged constituents within the jets are studied as functions of the particles’ azimuthal angle and pseudorapidity separations ($\mathrm{\Delta }{\phi }^{\ast }$ and $\mathrm{\Delta }{\eta }^{\ast }$) 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 $\mid \mathrm{\Delta }{\eta }^{\ast }\mid$ $>$ 2. For low-$N_{\text{ch}}^{\mathrm{j}}$, the long-range elliptic anisotropic harmonic, $v_2^{\ast }$, 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 $v_2^{\ast }$ 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.