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 root s=13 TeV. Long-range ridge yields are extracted for pairs of charged particles with a pseudorapidity difference of 1.4 < Delta eta < 1.8 and a transverse momentum of 1 < pr < 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 ethorne- collisions at root s=91 GeV and root 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(eh <=)24 where the ethorne- results have good precision, the measured ridge yields in pp collisions are substantially larger than the limits set in ethorneannihilations. Consequently, the findings presented in this Letter suggest that the processes involved in ethorne- annihilations do not contribute significantly to the emergence of long-range correlations in pp collisions.