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  TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of 1.4<|Δη|<1.8 and a transverse momentum of 1<p_{T}<2  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  GeV and sqrt[s]=183-209  GeV, where initial-state effects such as preequilibrium dynamics and collision geometry are not expected to play a role. In the multiplicity range 8≲⟨N_{ch}⟩≲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.