Collective flow as a consequence of hydrodynamical evolution in heavy ion
collisions is intensively studied by theorists and experimentalists to
understand the behavior of hot quark matter. Due to their large mass, heavy
ions suffer collective effects even at low (SPS) or intermediate energies
(RHIC). In case of light systems such as (anti)proton-proton interactions,
collective effects was not expected. Within a global model such as EPOS, where
light and heavy systems are treated using the same physics, it appears that
Tevatron data are better described if a flow is introduced. Then the
extrapolation to LHC can easily be done and we can compare to first data from
ATLAS experiment.Comment: 4 pages, 6 figures, Proceeding of the 45th Rencontres de Moriond QC

Karpenko, Iurii

Pierog, Tanguy

Porteboeuf, Sarah

Werner, Klaus

English

arXiv

Collective flow as a consequence of hydrodynamical evolution in heavy ion collisions is intensively studied by theorists and experimentalists to understand the behavior of hot quark matter. Due to their large mass, heavy ions suffer collective effects even at low (SPS) or intermediate energies (RHIC). In case of light systems such as (anti)proton-proton interactions, collective effects was not expected. Within a global model such as EPOS, where light and heavy systems are treated using the same physics, it appears that Tevatron data are better described if a flow is introduced. Then the extrapolation to LHC can easily be done and we can compare to first data from ATLAS experiment.Collective flow as a consequence of hydrodynamical evolution in heavy ion collisions is intensively studied by theorists and experimentalists to understand the behavior of hot quark matter. Due to their large mass, heavy ions suffer collective effects even at low (SPS) or intermediate energies (RHIC). In case of light systems such as (anti)proton-proton interactions, collective effects was not expected. Within a global model such as EPOS, where light and heavy systems are treated using the same physics, it appears that Tevatron data are better described if a flow is introduced. Then the extrapolation to LHC can easily be done and we can compare to first data from ATLAS experiment.Collective flow as a consequence of hydrodynamical evolution in heavy ion collisions is intensively studied by theorists and experimentalists to understand the behavior of hot quark matter. Due to their large mass, heavy ions suffer collective effects even at low (SPS) or intermediate energies (RHIC). In case of light systems such as (anti)proton-proton interactions, collective effects was not expected. Within a global model such as EPOS, where light and heavy systems are treated using the same physics, it appears that Tevatron data are better described if a flow is introduced. Then the extrapolation to LHC can easily be done and we can compare to first data from ATLAS experiment

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arXiv:1005.4526v1  [hep-ph]  25 May 2010 Collective flow in (anti)proton-proton collision at Tevatron and LHCTanguyPierog(1), S. Porteboeuf(2), Iu.Karpenko(3,4), K.Werner(4)(1) Karlsruhe Institut of Technology, Institut fu¨r Kernphysik, Postfach 3640, 76021 Karlsruhe Germany(2) LLR, Ecole Polytechnique, Palaiseau, France(3) Bogolyubov Institute for Theoretical Physics, Kiev 143, 03680, Ukraine(4) SUBATECH, University of Nantes – IN2P3/CNRS– EMN, Nantes, FranceCollective flow as a consequence of hydrodynamical evolution in heavy ion collisions is in-tensively studied by theorists and experimentalists to understand the behavior of hot quarkmatter. Due to their large mass, heavy ions suffer collective effects even at low (SPS) or in-termediate energies (RHIC). In case of light systems such as (anti)proton-proton interactions,collective effects was not expected. Within a global model such as EPOS, where light andheavy systems are treated using the same physics, it appears that Tevatron data are betterdescribed if a flow is introduced. Then the extrapolation to LHC can easily be done and wecan compare to first data from ATLAS experiment.1 IntroductionThere seems to be little doubt that heavy ion collisions at RHIC energies produce matter whichexpands as an almost ideal fluid1,2. This observation is mainly based on the studies of azimuthalanisotropies, which can be explained on the basis of ideal hydrodynamics3. A big success of thisapproach was the correct description of the so-called mass splitting, which refers to quite differenttransverse momentum dependencies of the asymmetries for the different hadrons, depending ontheir masses.As it was pointing out already in 2007 in 4, a model which describe properly these effectsfor heavy ion interactions will predict the same mechanism already for (anti)proton-protoninteractions when the “centrality” is large enough (high multiplicity in the central region). Andit can be compared to Tevatron data5 where such effects are clearly visible in the dependence ofthe average transverse momentum with central multiplicity (including mass splitting) as shownfig. 1.After a short introduction on the EPOS model section 2, we will compare its results to thelatest ATLAS data at 900 GeV and show section 3 that the same effect is already visible eventat this relatively “low” energy. In the section 4, we will present how a correct calculation ofcollective effects can be done in the framework of the EPOS model.2 EPOS ModelOne may consider the simple parton model to be the basis of high energy hadron-hadron inter-action models, which can be seen as an exchange of a “parton ladder” between the two hadrons.In additions to the parton ladder, hadronized using strings (flux-tube), there is anothersource of particle production: the two off-shell remnants.0.40.60.811.21.41.61.820 5 10 15 20 25 30 Nch mean pt   |h|< 1  pt>0.40.40.60.811.21.41.61.820 5 10 15 20 25 30 Nch mean pt   |h|< 1  pt>0.4Figure 1: Average transverse momentum 〈pt〉 as a function of central multiplicity of charged particles for differentparticle types (from top to bottom : lambdas, kaons short and charged particles). Points are data from CDFexperiment 5. Line are simulations with EPOS without hydro (left hand-side) and with hydro (right hand-side).EPOS6 is a consistent quantum mechanical multiple scattering approach based on partonsand strings, where cross sections and the particle production are calculated consistently, takinginto account energy conservation in both cases. Nuclear effects related to Cronin transversemomentum broadening, parton saturation, and screening have been introduced into EPOS.Furthermore, high density effects leading to collective behavior in heavy ion collisions are alsotaken into account. In next section preliminary results are shown using an effective treatmentusing a parameterized flow (not using LHC data). The full hydrodynamic treatment describedin section 4 applied to (anti)proton-proton will be shown in a paper in preparation.3 Comparison to LHC datax [fm]-8 -6 -4 -2 0 2 4 6 8y [fm]-8-6-4-2024680102030405060energy density [GeV/fm3]   (eta_s= 0.0 , tau= 0.6fm)   C 1x [fm]-2 -1.5 -1 -0.5 0 0.5 1 1.5 2y [fm]-2-1.5-1-0.500.511.5201020304050energy density [GeV/fm3]   (eta_s= 0.0 , tau= 0.6fm/c)   C 1Figure 2: Energy density in central Au-Au (L) and pp (R) scattering at 200GeV resp. 7T´eV.Let us consider the energy density at an early time in a Au-Au scattering at RHIC, asobtained from an EPOS simulation 6. In the left hand-side of fig. 2, we plot the energy densityat the space-time rapidity ηs = 0, as a function of the transverse coordinates x and y . Weobserve a very bumpy structure concerning the x − y−dependence. There are in particularpeaks in the x− y−plane which are sub-flux-tubes which exhibit a long range structure in thelongitudinal variable ηs.In fig. 2 , we clearly identify several sub-flux-tubes, with a typical width of the order of afermi. This is exactly the width we obtain if we compute the initial energy density in protonscattering at the LHC (fig. 2 right hand-side). This means, if a hydrodynamic treatment isjustified for Au-Au collisions at RHIC, it is equally justified for pp scattering at the LHC,provided the energy densities are high enough. This latter condition can easily be satisfied,since in proton-proton scattering on has the possibility to trigger on high multiplicity events,with ten or twenty times the multiplicity compared to an average event.If collective effects are possible in light system, how can it be observed ? Besides correlationsbetween particles, one of the striking consequence of a collective hadronization is the creationof a radial velocity. In case of heavy ion collisions, the asymmetry created by the impactparameter allows the measurement of the v2 parameter of the flow to quantify this effect. Incase of anti(proton)-proton scattering, it is experimentally difficult to define a “collision plane”and actually it was never done or even foreseen to measure v2 in that case. But the radialflow increase the transverse momentum of the particles, so that this effect should change themean transverse momentum 〈pt〉. A measure of the energy density is given by the multiplicityof charged particles Nch event-by-event. As a consequence the variation of the 〈pt〉 as a functionof Nch has to be sensitive to the collective effects in a very light system.The effect shown in fig. 1 for Tevatron at 1.8TeV is actually confirmed at 900GeV by theATLAS experiment 8. In fig. 3 we can see the difference between simulation with (full line) orwithout (dashed line) hydrodynamic evolution of the high density region. This effect has onlylittle influence on the total multiplicity has shown in fig. 4.4 Hydrodynamics in EPOSIn future version of EPOS, we are going to employ a new tool for treating the hydrodynamicevolution, based on the following features (see 7 for details and tests with AuAu data):• initial conditions obtained from a flux tube approach (EPOS), compatible with the stringmodel used since many years for elementary collisions (electron-positron, proton proton),and the color glass condensate picture;• consideration of the possibility to have a (moderate) initial collective transverse flow;• event-by-event procedure, taking into the account the highly irregular space structure ofsingle events, being experimentally visible via so-called ridge structures in two-particlecorrelations;• core-corona separation, considering the fact that only a part of the matter thermalizes;• use of an efficient code for solving the hydrodynamic equations in 3+1 dimensions, includ-ing the conservation of baryon number, strangeness, and electric charge;• employment of a realistic equation-of-state, compatible with lattice gauge results – with across-over transition from the hadronic to the plasma phase;• use of a complete hadron resonance table, making our calculations compatible with theresults from statistical models;• hadronic cascade procedure after hadronization from the thermal system at an early stage.5 SummaryEPOS is an interaction model constructed on a solid theoretical basis. It has been tested verycarefully against all existing hadronic data. Designed to be compared to heavy ion collisions, thecollective effects are being implemented in a very sophisticated way. Based on a realistic event-by-event energy density with large fluctuations, a 3D hydrodynamical calculation is performed0.60.811.210 20 30 40 50 Nch mean pt p + p → chrg at 900 GeV | h|  < 2.5      pt > 500 MeVFigure 3: 〈pt〉 as a function of multiplicity of chargedparticles for proton-proton collisions at 900 GeV. AT-LAS data points8 are compared to EPOS simulationswith (full line) or without (dashed line) hydro evolu-tion.11.11.21.31.41.5-2 -1 0 1 2 peusdorapidity h dn/dh p + p → chrg at 900 GeV pt > 500 MeVFigure 4: Pseudorapidity distribution of charged par-ticles with pt > 500MeV for proton-proton collisionsat 900GeV. ATLAS data points 8 are compared toEPOS simulations with (full line) or without (dashedline) hydro evolution.until chemical freeze-out followed by a hadron cascade until thermal freeze-out. Applying thesame scheme for (anti)proton-proton scattering at high energy, the most inelastic collisionswill actually satisfy the conditions to create a small thermalized system which will hadronizedstatistically and with a non-negligible radial velocity. We showed that this effect is visible inthe dependence of the 〈pt〉 with the particle multiplicity of each event at Tevatron or LHCenergies, especially if we look at the dependence with the mass of the particles. Further detailedmeasurements at LHC (correlations) will allow us to actually test the space-time evolution ofthe energy density of hadronic interactions.AcknowledgmentsThis research has been carried out within the scope of the ERG (GDRE) “Heavy ions at ultra-relativistic energies”, a European Research Group comprising IN2P3/CNRS, Ecole des Mines deNantes, Universite´ de Nantes, Warsaw University of Technology, JINR Dubna, ITEP Moscow,and Bogolyubov Institute for Theoretical Physics NAS of Ukraine. Iu. K. acknowledges partialsupport by the MESU of Ukraine, and Fundamental Research State Fund of Ukraine, agreementNo F33/461-2009. Iu.K. and K.W. acknowledge partial support by the Ukrainian-French grant“DNIPRO”, an agreement with MESU of Ukraine No M/4-2009. T.P. and K.W. acknowledgepartial support by a PICS (CNRS) with KIT (Karlsruhe).References1. STAR Collaboration, J. Adams, et al, Nucl. Phys. A 757, 102 (2005)2. PHOBOS Collaboration, B.B. Back et al., Nucl. Phys. A 757, 28-101 (2005)3. Quark-Gluon Plasma 3, eds. R. C. Hwa and X. N. Wang (World Scientific, Singapore,2004)4. N. Armesto et al., J. Phys. G 35, 054001 (2008).5. CDF Collaboration, D. E. Acosta et al., Phys. Rev. D 72, 052001 (2005).6. K. Werner et al., Phys. Rev. C 74, 044902 (2006).7. K. Werner et al.,arXiv:1004.0805, submitted to Phys. Rev. C.8. ATLAS Collaboration, G. Aad et al., Phys. Lett. B 688, 21 (2010).

Collective flow in (anti)proton-proton collision at Tevatron and LHC

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