Evidence for Flow from Hydrodynamic Simulations ofp−PbCollisions at 5.02 TeV fromv2Mass Splitting

Abstract

We show that a fluid dynamical scenario describes quantitatively the observed mass splitting of the elliptical flow coefficients v2 for pions, kaons, and protons. This provides a strong argument in favor of the existence of a fluid dynamical expansion in pPb collisions at 5TeV.One of the strongest signals of collective flow in heavy ion collisions is the fact that the transverse momentum dependence of the elliptical flow coefficient v 2 (measuring the azimuthal asymmetry) depends in a very characteristic way on the mass of the observed hadrons. This has been predicted [1] and impressively confirmed experimentally later [2,3]. Can we also "prove" the existence of flow in small systems like proton-lead collisions, where such a collective behavior has not been expected?Information about flow asymmetries can be obtained via studying two particle correlations as a function of the pseudorapidity difference ∆η and the azimuthal angle difference ∆φ. So-called ridge structures (very broad in ∆η) have been observed first in heavy ion collisions, later also in pp [4] and very recently in pPb collisions [5][6][7]. In case of heavy ions, these structures appear naturally in models employing a hydrodynamic expansion, in an event-by-event treatment.To clearly pin down the origin of such structures in small systems, one needs to consider identified particles. In the fluid dynamical scenario, where particles are produced in the local rest frame of fluid cells characterized by transverse velocities, large mass particles (compared to low mass ones) are pushed to higher transverse momenta, visible in p t distributions, but also in the dihadron correlations. Both effects are clearly seen in experimental data. In this paper, we focus on dihadron correlations and the v 2 coefficients.The following discussion is based on ALICE results on dihadron correlations [5, 8] and EPOS3 simulations. EPOS3 is a major update of the work described in [11] (EPOS2). Here, we introduced a theoretical scheme which accounts for hydrodynamically expanding bulk matter, jets, and the interaction between the two. The whole transverse momentum range is covered, from very low to very high p t . In [11], we show that this approach can accommodate spectra of jets with p t up to 200 GeV/c in pp scattering at 7 TeV, as well as particle yields and harmonic flows with p t between 0 and 20 GeV/c in PbPb collisions at 2.76 TeV. To our knowledge, this is the only model able to describe the famous lambda to kaon enhancement correctly over the whole p t range. New features of EPOS3 are: an event-by-event 3D+1 viscous hydrodynamical evolution, and a new treatment of high parton densities, via an individual parton saturation scale Q s for each elementary scattering. More details will be given in the appendix. All results in this paper are based on EPOS3.074.As in [11], the basis of our approach is multiple scattering (even for pp), where a single scattering is a hard elementary scattering plus initial state radiation, the whole object being referred to as parton ladder or...

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

“…This mass ordering feature is similar with what observed in Pb-Pb collisions [10] and can be roughly reproduced by hydrodynamic calculations [4,5]. It was token as a strong evidence for the collective expansion of the p-Pb systems created in √ s NN = 5.02 TeV collisions.…”

EPOS LHC: Test of collective hadronization with data measured at the CERN Large Hadron Collider

“…We will call it the nuclear AA flow, characterized by the maximal radial rapidity y AA rad . But in the case of a light system, such as p-p, using EPOS 2 or 3 with a realistic treatment of the hydrodynamical evolution with proper hadronization, such an effect was not observed [16,20,28]. In that case the large flow comes from the quick expansion of the very small volume of the core.…”

“…The main reason to have different versions is that for a Pb-Pb central event EPOS 2 or 3 needs about one hour while EPOS LHC will generate it in a few tens of seconds, and EPOS LHC is not under development any more (public stable version). As a consequence EPOS LHC has more parameters (and less predictive power) than EPOS 2 or 3 [18][19][20] and should not be used for a precise study of p t distributions or particle correlations in HI collisions, but is a good alternative model for p-p and p-A minimum bias analysis.…”

Initial state fluctuations and final state correlations in relativistic heavy-ion collisions

“…20 by the CMS collaboration correlations the two-particle correlations show surprisingly similar correlation structures in peripheral heavy ion and elementary p-Pb collisions. At the moment, there are two main explanations, either the correlations are generated directly in the initial state, e.g., by early gluon dynamics in a saturation picture [168,169], or the correlations are generated by collective expansion, e.g., hydrodynamics [170,171,172,173,174]. Since p-Pb and p-p collisions are used as a reference for many hard observables it is important to understand them in more detail.…”