Elliptic and Triangular Flow inp-Pb and Peripheral Pb-Pb Collisions from Parton Scatterings

Abstract

Using a multiphase transport model (AMPT) we calculate the elliptic, v2, and triangular, v3, Fourier coefficients of the two-particle azimuthal correlation function in proton-nucleus (p+Pb) and peripheral nucleus-nucleus (Pb+Pb) collisions. Our results for v3 are in a good agreement with the CMS data collected at the Large Hadron Collider. The v2 coefficient is very well described in p+Pb collisions and is underestimated for higher transverse momenta in Pb+Pb interactions. The characteristic mass ordering of v2 in p+Pb is reproduced whereas for v3 this effect is not observed. We further predict the pseudorapidity dependence of v2 and v3 in p+Pb and observe that both are increasing when going from a proton side to a Pb-nucleus side. Predictions for the higher order Fourier coefficients, v4 and v5, in p+Pb are also presented. , with a modest elastic parton-parton crosssection σ = 1.5 − 3 mb, allows to understand qualitatively and quantitatively the long-range two-particle azimuthal correlation functions in proton-lead (p+Pb) and high-multiplicity proton-proton (p+p) collisions. Such correlations were recently observed by the CMS [3-5], ALICE [6,7] and ATLAS [8,9] Collaborations at the Large Hadron Collider (LHC), and by the PHENIX Collaboration in deuteron-gold collisions at the Relativistic Heavy Ion Collider (RHIC) [10].Interestingly all features of the two-particle azimuthal correlation function observed in p+Pb collisions are very similar to those observed in A+A interactions, where such correlations are commonly attributed to the hydrodynamic expansion of the produced fireball. This naturally suggests that collective physics is present in p+A collisions [1,[11][12][13][14][15][16][17]. Particularly strong evidence in favour of hydrodynamics (or any other approach where the initial coordinate space anisotropy is transformed into the final momentum anisotropy) in p+A and peripheral A+A collisions is an approximate equality of multiparticle elliptic flow cumulants, v 2 {4} ≈ v 2 {6} ≈ v 2 {8}, as predicted in Ref. [18], see also [19,20], and confirmed recently by the CMS Collaboration [21]. Other strong evidence is the characteristic mass ordering of the elliptic flow, v 2 , observed by the ALICE Collaboration in Ref. [7], and successfully reproduced by hydrodynamic calculations [22,23].The experimental data for the two-particle azimuthal correlation function can be also fitted within the color glass condensate framework [24], where the interesting part of the two-particle correlation function comes from the emission of two gluons in the so-called glasma diagram [25]. For a detailed discussion of this approach we refer the reader to Refs. [25][26][27].It is important to clarify whether the signal in p+A collisions comes form the initial or final (or both) state effects. To this end several interesting observations were recently published [28][29][30][31][32][33][34][35] which could help to distinguish between competing models of p+A interactions. A simple conformal scaling argument, presented in Ref.[31], indica...

Collective effects in light–heavy ion collisions

“…Reasons for this could be the neglected initial state correlations and/or the lack of a detailed description of the fluctuating subnucleonic structure of the proton. Our results for p+A collisions differ significantly from those in [28,29,30,31], suggesting that the details of the initial shape in small systems are of paramount importance. 3 We predict an increase of both v 3 and v 5 in 3 He+Au collisions compared to d+Au collisions, while the even harmonics are comparable in both systems.…”

Origin of the mass splitting of elliptic anisotropy in a multiphase transport model

“…The string melting version of AMPT [25,26] reasonably reproduces particle yields, p ⊥ spectra, and v 2 of low-p ⊥ pions and kaons in central and mid-central Au+Au collisions at 200A GeV and Pb+Pb collisions at 2760A GeV [27]. The small system data can be also satisfactorily described by AMPT [28]. The successful description by AMPT of experimental data, especially the heavy ion data, did not come as a surprise, because it has been thought that the transport models have approached hydrodynamic limit due to high energy densities and/or large parton-parton interaction cross sections.…”

Nuclear cluster structure effect on elliptic and triangular flows in heavy-ion collisions

“…AMPT has been extensively applied to simulate heavy-ion collisions in a wide colliding energy range from the SPS to the LHC, which contains the initial conditions from the HIJING model [46,47], partonic interactions modelled by a parton cascade model (ZPC) [48], hadronization in a simple quark coalescence model, and hadronic rescattering simulated by a relativistic transport (ART) model [49]. AMPT is successful to describe physics in relativistic heavy-ion collision for the RHIC [45] and the LHC energies [50], including pion-pair correlations [51], di-hadron azimuthal correlations [52] as well as collective flow etc [53,54].…”