Multiparticle azimuthal correlations inp-Pb and Pb-Pb collisions at the CERN Large Hadron Collider

Abstract

Measurements of multiparticle azimuthal correlations (cumulants) for charged particles in p-Pb at √ s NN = 5.02 TeV and Pb-Pb at √ s NN = 2.76 TeV collisions are presented. They help address the question of whether there is evidence for global, flowlike, azimuthal correlations in the p-Pb system. Comparisons are made to measurements from the larger Pb-Pb system, where such evidence is established. In particular, the second harmonic two-particle cumulants are found to decrease with multiplicity, characteristic of a dominance of few-particle correlations in p-Pb collisions. However, when a | η| gap is placed to suppress such correlations, the two-particle cumulants begin to rise at high multiplicity, indicating the presence of global azimuthal correlations. The Pb-Pb values are higher than the p-Pb values at similar multiplicities. In both systems, the second harmonic four-particle cumulants exhibit a transition from positive to negative values when the multiplicity increases. The negative values allow for a measurement of v 2 {4} to be made, which is found to be higher in Pb-Pb collisions at similar multiplicities. The second harmonic six-particle cumulants are also found to be higher in Pb-Pb collisions.In Pb-Pb collisions, we generally find v 2 {4} v 2 {6} = 0 which is indicative of a Bessel-Gaussian function for the v 2 distribution. For very high-multiplicity Pb-Pb collisions, we observe that the four-and six-particle cumulants become consistent with 0. Finally, third harmonic two-particle cumulants in p-Pb and Pb-Pb are measured. These are found to be similar for overlapping multiplicities, when a | η| > 1.4 gap is placed.

Theory of pp/pA/small systems

“…In p+Pb collisions all higher cumulants are expected to be of similar magnitude, only the second moment is larger. Such a hierarchy has been observed experimentally [22,36,37,38], with v 2 {2} > v 2 {4} v 2 {6} v 2 {8} (Fig. 1, right panel).…”

Quantifying properties of hot and dense QCD matter through systematic model-to-data comparison

“…Higher-order cumulants are currently out of reach due to insufficient quantities of events. Postprocessed observables are compared to corresponding experimental results recently measured by the ALICE experiment at the LHC for Pb-Pb collisions at √ sNN = 2.76 TeV [42]. All observables are subjected to the same kinematic cuts as the ALICE detector, namely charged particles with |η| < 1 and 0.2 < pT < 3.0 GeV.…”

Charge-dependent flow and the search for the chiral magnetic wave in Pb-Pb collisions atsNN=2.76TeV

“…where ϕ is the azimuthal angle of the particle, v n is the Fourier coefficient, and ψ n is the symmetry plane, which in principle can be different for each harmonic number n. Taking into account the well-known modulation of particle emission due to elliptic flow parameterized with the Fourier coefficient v 2 (see, e.g., [25]), one can write the azimuthal distribution of charged particles due to the CMW [24] as…”