Measurements of two-particle angular correlations in proton–proton (pp) collisions at the LHC have shown a feature commonly called the “ridge”: an enhancement in production of particle pairs at small azimuthal angle separation, Δφ, that extends over a large pseudorapidity separation, Δη. This feature has been demonstrated and the two-particle correlation function has been measured in pp collisions at √s = 13 TeV. The ridge has been observed and studied in more detail in proton–nucleus (p+A) and nucleus–nucleus (A+A) collisions where a long-range correlation is observed on the away-side (Δφ ~ π) as well. Both the near- and away-side ridges in p+A and A+A collisions have been shown to result from a modulation of the single-particle azimuthal angle distributions, whose convolution produces the observed features in the two-particle Δφ distribution. However, prior to this measurement, it was not yet known whether the ridge in pp collisions arises from single-particle modulations or even if it is present on the away-side or not.
Recently, ATLAS has performed an analysis of the long-range component of the two-particle correlations in pp collisions at 2.76 TeV and 13 TeV in different intervals of charged-particle multiplicity, Nrecch. The yield, Y(Δφ), of particles associated with a “trigger” particle, for |Δη| > 2, is shown in the figure. A peak at Δφ ~ 0 is associated with the ridge while the peak at Δφ ~ π contains known contributions from dijets and, possibly, contributions from an away-side ridge. To disentangle the ridge and jet contributions, a new template fitting procedure, demonstrated in the figure, was used. The Y(Δφ) distributions were fitted by a sum (red curve) of two components: Y(Δφ) measured in low-multiplicity (0 < Nrecch < 20) collisions (open points), which accounts for the “jet” contribution, and a sinusoidally modulated component (blue dashed lines) which represents the long-range correlation like that observed in p+A and A+A collisions. These template fits successfully describe the two-particle correlations in all Nrecch intervals at both energies. Furthermore, the sinusoidal component was found to be present at all Nrecch intervals, indicating that the long-range correlation is a feature that is present at all multiplicities and not only in rare high-multiplicity events. The fractional amplitudes of the sinusoidal components are observed to be nearly constant with multiplicity and to be approximately the same at the two centre-of-mass energies.
These results suggest that the ridges in pp, p+A and A+A collisions arise from similar mechanisms. The observed weak dependence of the fractional amplitudes on Nrecch and centre-of-mass energy should provide a strong constraint on the physical mechanism responsible for producing the ridge.