Azimuthal anisotropy of charged particles from multiparticle correlations in pPb and PbPb collisions with CMS

Abstract

Motivated by two-and four-particle azimuthal correlation measurements that suggest possible collective flow for charged particles emitted in pPb collisions at √ s N N =5.02 TeV, we extend the correlation results for these collisions using the six-and eight-particle cumulant methods, and the Lee-Yang Zeros method. CMS has an extensive program studying azimuthal harmonic coefficients for both PbPb and pPb collisions using various methods. The current pPb results will be presented in this context. The data were collected by the CMS experiment at the LHC using both minimum bias and highmultiplicity collision triggers over a wide range in pseudorapidity. The results are compared to 2.76 TeV semi-peripheral PbPb collision data collected in 2011 covering a similar range of particle multiplicities. The second-order azimuthal anisotropy Fourier harmonic (v 2 ) is shown for the different methods. A comparison of the six-and greater particle correlations to the previously published twoand four-particle correlation results sheds light on the multiparticle nature of the azimuthal anisotropy. The results are also discussed in terms of recent calculations that explore the role of participant fluctuations on measurements of higher-order particle correlations in pPb collisions. AbstractMotivated by two-and four-particle azimuthal correlation measurements that suggest possible collective flow for charged particles emitted in pPb collisions at √ s NN = 5.02 TeV, we extend the correlation results for these collisions using the six-and eight-particle cumulant methods, and the Lee-Yang Zeros method. CMS has an extensive program studying azimuthal harmonic coefficients for both PbPb and pPb collisions using various methods. The current pPb results will be presented in this context. The data were collected by the CMS experiment at the LHC using both minimum bias and high-multiplicity collision triggers over a wide range in pseudorapidity. The results are compared to 2.76 TeV semi-peripheral PbPb collision data collected in 2011 covering a similar range of particle multiplicities. The second-order azimuthal anisotropy Fourier harmonic (v 2 ) is shown for the different methods. A comparison of the six-and greater particle correlations to the previously published two-and four-particle correlation results sheds light on the multiparticle nature of the azimuthal anisotropy. The results are also discussed in terms of recent calculations that explore the role of participant fluctuations on measurements of higher-order particle correlations in pPb collisions.

Plane correlations in small colliding systems

“…The idea relies on the knowledge that medium collectivity associates harmonic flow with event-by-event fluctuations in the initial density profile. The CMS Collaboration has updated its measurements of v 2 fluctuations in terms of cumulants v 2 {m} from multi-particle correlations [17,18]. The observed pattern of v 2 {m} agrees with cumulants of initial ellipticity which is purely driven by fluctuations [19,20], which coincides with the expectation from collective expansion.…”

Is hadronic flow produced in p–Pb collisions at the Large Hadron Collider?

“…For instance, a symmetric double ridge structure on both near-and away-side has been observed in high multiplicity p-Pb collisions by the ALICE Collaboration [38]. In addition, the CMS Collaboration has showed compatible results between multi-particle (including 4-, 6and 8-particles) and all-particle correlations with Lee-Yang Zero's (LYZ) [39], which corresponds to v 2 {4} ≈ v 2 {6} ≈ v 2 {8} ≈ v 2 {LYZ} [40] (These results have also been confirmed by the ATLAS [41] and ALICE Collaborations [42]). Recently, the measurements of azimuthal correlations have been extended to identified hadrons [43,44].…”

“…In this paper, the azimuthal correlations are calculated using 2-and 4-particle Q-Cumulant method [62,63], which were used in experiment at RHIC [64] and the LHC [18,33,40,65]. In this method, both 2and multi-particle azimuthal correlations are analytically expressed in terms of a Q-vector, which is defined as:…”

Long range azimuthal correlations of multiple gluons in gluon saturation limit

“…This shows that the correlation between hadron pairs becomes maximum when the hadrons are azimuthally collimated in the same direction or when they are back-to-back. Furthermore, these correlations in the near-and away-side are elongated in rapidity difference; the collimation and anti-collimation effects are robust even though the hadrons are separated in rapidity for multiple units [2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17].…”