The new results confirm that the ridges in proton-proton, proton-nucleus, and nucleus-nucleus collisions have a similar origin. The results also show that the observed weak dependence on the numbers of charged particles and the centre-of-mass energy should provide strong constraints on the mechanism responsible for producing the ridge in proton-proton, and, maybe, proton-nucleus collisions.

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Figure 1: Two-particle correlation function in 13 TeV pp collisions with Nchrec > 120. The ridge is seen as the enhancement at Δϕ near zero that extends over the full range of Δη. (Image: CERN)

The term “ridge” is commonly used to refer to a feature observed in measurements of two-particle angular correlations in proton-proton, proton-nucleus, and nucleus-nucleus collision as a function of Δϕ. This is the difference between the azimuthal angles – the angles in the plane transverse to the beam – of the particles and Δη – the difference between the pseudorapidities of the two particles. Pseudorapidity is related to the angle the particle makes with respect to the beam. The ridge is an enhancement seen in the correlation function at small Δϕ that extends over the measured Δη range (see Figure 1).

The ridge in proton-nucleus and nucleus-nucleus collisions is known to result from a sinusoidal modulation of the single-particle azimuthal angle distributions that produce a corresponding modulation in the two-particle Δϕ distribution. In nucleus-nucleus collisions, this single-particle modulation is believed to result from collective expansion of the hot, dense medium created in the nuclear collisions. The possibility that similar collective expansion is responsible for the ridge in proton-nucleus collisions is currently under debate. Prior to the new ATLAS result, it was not known whether the ridge in proton-proton collisions arose from a similar single-particle modulation.

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Figure 2: The long-range two-particle correlation function projected onto the Δϕ axis, Y(Δϕ), for events with different numbers of charged particles, Nchrec The black points show the Y(Δϕ) distribution for Nchrec ≥ 90. The open points show the scaled correlation function measured in 0 < Nchrec < 20, the blue dashed line shows the sinusoidal contribution, and the red curve shows the sum of these two contributions which reproduces the data well. (Image: CERN)

Preliminary measurements by ATLAS in July 2015, showing a strong ridge signal in 13 TeV proton-proton collisions, provided the impetus to study the origins of the ridge in the same data. Separate proton-proton data collected in 2013 at a much lower collision energy (2.76 TeV) provided the opportunity to study the ridge over a large range of centre-of-mass energies.

An analysis of the long-range component of the two-particle correlations projected onto the Δϕ axis, Y(Δϕ), showed that this distribution in events with many particles could be well-described by the sum of the (scaled) Y(Δϕ) measured in events with few particles and a sinusoidal function like that observed in proton-nucleus and nucleus-nucleus collisions (see Figure 2).

The sinusoidal function is responsible for the ridge but also contributed to pairs with Δϕ = pi. This analysis suggests a common origin for the ridge in proton-proton, proton-nucleus and nucleus-nucleus collisions. This sinusoidal modulation is frequently characterised by the parameter, v2, which describes the fractional amplitude of the modulated component. The v2 parameters extracted from the 2.76 TeV and 13 TeV data are shown in Figure 3. They are observed to be approximately constant as a function of the number of charged particles and to be nearly the same at the two centre-of-mass energies.

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Figure 3: The extracted fractional amplitudes of the single-particle sinusoidal modulation as a function of Nchrec for 2.76 TeV (left) and 13 TeV (right). (Image: CERN)

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