Emergence of Long-Range Angular Correlations in Low-Multiplicity Proton-Proton Collisions

This Letter presents the measurement of near-side associated per-trigger yields, denoted ridge yields, from the analysis of angular correlations of charged hadrons in proton-proton collisions at $\sqrt{s}=13\mathrm{TeV}$. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of $1.4<\left|\Delta \eta \right|<1.8$ and a transverse momentum of $1<p_T<2\mathrm{GeV}/c$, as a function of the charged-particle multiplicity measured at midrapidity. This Letter extends the measurements of the ridge yield to the low multiplicity region, where in hadronic collisions it is typically conjectured that a strongly interacting medium is unlikely to be formed. The precision of the new low multiplicity results allows for the first direct quantitative comparison with the results obtained in $e^{+}{e}^{-}$ collisions at $\sqrt{s}=91\mathrm{GeV}$ and $\sqrt{s}=183–209\mathrm{GeV}$, where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range $8\lesssim ⟨N_{\mathrm{ch}}⟩\lesssim 24$ where the $e^{+}{e}^{-}$ results have good precision, the measured ridge yields in $pp$ collisions are substantially larger than the limits set in $e^{+}{e}^{-}$ annihilations. Consequently, the findings presented in this Letter suggest that the processes involved in $e^{+}{e}^{-}$ annihilations do not contribute significantly to the emergence of long-range correlations in $pp$ collisions.