Observation of long-range, near-side angular correlations in pPb collisions at the LHC

Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon– nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, η, and full azimuth, φ, as a function of charged particle multiplicity and particle transverse momentum, pT. In high-multiplicity events, a long-range (2 < |η| < 4), near-side (φ ≈ 0) structure emerges in the two-particle η–φ correlation functions. This is the first observation of such correlations in proton–nucleus collisions, resembling the ridge-like correlations seen in highmultiplicity pp collisions at √s = 7 TeV and in AA collisions over a broad range of center-of-mass energies. The correlation strength exhibits a pronounced maximum in the range of pT = 1–1.5 GeV/c and an approximately linear increase with charged particle multiplicity for high-multiplicity events. These observations are qualitatively similar to those in pp collisions when selecting the same observed particle multiplicity, while the overall strength of the correlations is significantly larger in pPb collisions. © 2012 CERN. Published by Elsevier B.V. 1. Introduction This Letter presents measurements of two-particle angular correlations in proton-lead (pPb) collisions at a nucleon–nucleon center-of-mass energy √sN N = 5.02 TeV, performed with the Compact Muon Solenoid (CMS) detector at the Large Hadron Collider (LHC). Two-particle correlations in high-energy collisions provide valuable information for characterizing Quantum Chromodynamics and have been studied previously for a broad range of collision energies in proton–proton (pp), proton–nucleus (pA), and nucleus– nucleus (AA) collisions. Such measurements can elucidate the underlying mechanism of particle production and possible collective effects resulting from the high particle densities accessible in these collisions. Studies of two-particle angular correlations are typically performed using two-dimensional η–φ correlation functions, where φ is the difference in azimuthal angle φ between the two particles and η is the difference in pseudorapidity η = −ln(tan(θ/2)). The polar angle θ is defined relative to the counterclockwise beam. Of particular interest in studies of collective effects is the longrange (large |η|) structure of two-particle correlation functions, which is less susceptible to known sources of correlations such E-mail address: cms-publication-committee-chair@cern.ch. as resonance decays and fragmentation of energetic jets. Measurements in high-energy AA collisions have shown significant modifications of the long-range structure compared with minimum bias pp collisions [1]. Novel correlation structures extending over large η at |φ| ≈ 0 and |φ| ≈ 2π/3 were observed in azimuthal correlations for intermediate particle transverse momenta, pT ≈ 1–5 GeV/c [2–10]. In AA collisions, long-range correlations are interpreted as a consequence of the hydrodynamic flow of the produced strongly interacting medium [11] and are usually characterized by the Fourier components of the azimuthal particle distributions [12]. Of particular importance are the second and third Fourier components, called elliptic and triangular flow, as they most directly reflect the medium response to the initial collision geometry and its fluctuations [13], and allow the study of fundamental transport properties of the medium using hydrodynamic models [14–16]. In current pp and pA Monte Carlo (MC) event generators, the dominant sources of such long-range correlations are momentum conservation and away-side (φ ≈ π) jet correlations. Measurements in pp collisions at 7 TeV have revealed the emergence of long-range, near-side (φ ≈ 0) correlations in a selection of collisions with very high final-state particle multiplicity [17]. A large variety of theoretical models have been proposed to explain the origin of these so-called ridge-like correlations (see Ref. [18] for a recent review). The proposed mechanisms range from color connections in hard scattering processes and collective effects in the initial interaction of the protons to hydrodynamic effects in the 0370-2693/ © 2012 CERN. Published by Elsevier B.V.