Previous studies of two-particle angular correlations in proton-proton, proton-lead, and lead-lead collisions at the LHC have provided important insight on the physics of the particle production process. A common way to express the angular correlations is as a function of Δϕ – the difference between the azimuthal angles (the angles in the plane transverse to the beam) of the particles – and Δη – the difference between the pseudo-rapidities of the two particles. Pseudorapidity is a parameter related to the angle of the particle with respect to the beam.

Past measurements of this two-particle correlation function have shown an enhancement for Δϕ near 0 that extends over a wide range of Δη values. This correlation is commonly called the “ridge” due to its characteristic shape in the Δη-Δϕ plane (see Figure 1).


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Figure 1: Two-particle correlations in events with low multiplicity, 10120 (right panel), of produced particles. The right plot shows long-range correlations (ridge) along the Δη axis at Δϕ ~0. (Image: CERN)


In nucleus-nucleus collisions, the ridge is understood to arise from collective dynamics of the quark gluon plasma. However, its origin in proton-nucleus and proton-proton collisions is a matter of considerable debate. To ascertain the origin of this long-range correlation (or “ridge”), it is important to study the dependence its strength on collision energy, the number of particles produced in the collision (multiplicity), and the transverse momenta of the particles (pT).

On 24 July, ATLAS presented new preliminary measurements of two-particle correlations. These correlations were measured for proton-proton collisions of different charged particle multiplicity (Nchrec), which counts the number of reconstructed charged particles having transverse momenta (pT) greater than 0.4 GeV. The ridge is found to be present in events with Nchrec > 40, and its magnitude increases rapidly for higher multiplicities. Similar to past measurements, the correlation extends over the full Δη range that can be measured, which for ATLAS is -5 to 5. The strength of the ridge increases with pT until ~2.5 GeV and then decrease at higher pT, similar to the behaviour observed in proton-nucleus and nucleus-nucleus collisions.

Surprisingly, the strength of the ridge at a given pT and multiplicity is similar to that observed in previous 7 TeV proton-proton measurements made by the CMS collaboration. These results may provide insight on the origin of the ridge in proton-proton collisions.