Multiplicity and transverse-momentum dependence of two- and

 four-particle correlations in pPb and PbPb collisions

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Abstract

Measurements of two- and four-particle angular correlations for charged particles emitted in pPb collisions are presented over a wide range in pseudorapidity and full azimuth. The data, corresponding to an integrated luminosity of approximately 31 inverse nanobarns, were collected during the 2013 LHC pPb run at a nucleon-nucleon center-of-mass energy of 5.02 TeV by the CMS experiment. The results are compared to 2.76 TeV semi-peripheral PbPb collision data, collected during the 2011 PbPb run, covering a similar range of particle multiplicities. The observed correlations are characterized by the near-side (abs(Delta(phi)~0) associated pair yields and the azimuthal anisotropy Fourier harmonics (v[n]). The second-order (v[2]) and third-order (v[3]) anisotropy harmonics are extracted using the two-particle azimuthal correlation technique. A four-particle correlation method is also applied to obtain the value of v[2] and further explore the multi-particle nature of the correlations. Both associated pair yields and anisotropy harmonics are studied as a function of particle multiplicity and transverse momentum. The associated pair yields, the four-particle v[2], and the v[3] become apparent at about the same multiplicity. A remarkable similarity in the v[3] signal as a function of multiplicity is observed between the pPb and PbPb systems. Predictions based on the color glass condensate and hydrodynamic models are compared to the experimental results.

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Anisotropy as a signature of transverse collective flow

Jean-Yves Ollitrault (1992)

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Elliptic and triangular flow in event-by-event (3+1)D viscous hydrodynamics

Bjoern Schenke, Sangyong Jeon, Charles Gale (2010)

We present results for the elliptic and triangular flow coefficients in Au+Au collisions at root-s=200 AGeV using event-by-event (3+1)D viscous hydrodynamic simulations. We study the effect of initial state fluctuations and finite viscosities on the flow coefficients v_2 and v_3 as functions of transverse momentum and pseudo-rapidity. Fluctuations are essential to reproduce the measured centrality dependence of elliptic flow. We argue that simultaneous measurements of v_2 and v_3 can determine eta/s more precisely.

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Triangularity and Dipole Asymmetry in Heavy Ion Collisions

Li Yan, Derek Teaney (2010)

We introduce a cumulant expansion to parameterize possible initial conditions in relativistic heavy ion collisions. We show that the cumulant expansion converges and that it can systematically reproduce the results of Glauber type initial conditions. At third order in the gradient expansion, the cumulants characterize the triangularity \( \) and the dipole asymmetry \( \) of the initial entropy distribution. We show that for mid-peripheral collisions the orientation angle of the dipole asymmetry \(\psi_{1,3}\) has a \(20%\) preference out of plane. This leads to a small net \(v_1\) out of plane. In peripheral and mid-central collisions the orientation angles \(\psi_{1,3}\) and \(\psi_{3,3}\) are strongly correlated. We study the ideal hydrodynamic response to these cumulants and determine the associated \(v_1/\epsilon_1\) and \(v_3/\epsilon_3\) for a massless ideal gas equation of state. \(v_1\) and \(v_3\) develop towards the edge of the nucleus, and consequently the final spectra are more sensitive to the viscous dynamics of freezeout. The hydrodynamic calculations for \(v_3\) are compared to Alver and Roland fit of two particle correlation functions. Finally, we propose to measure the \(v_1\) associated with the dipole asymmetry and the correlations between \(\psi_{1,3}\) and \(\psi_{3,3}\) by measuring a two particle correlation with respect to the participant plane, \( \). The hydrodynamic prediction for this correlation function is several times larger than a correlation currently measured by the STAR collaboration, \( \).