Recent experimental results revealed large elliptic flow of the charmed hadrons at LHC energies, unexpectedly similar to that of the charged pions. The mentioned measurements are often interpreted using transport models, which incorporate dissociation and recombination mechanisms for charm quarks. In this report, a modified version of the Monte Carlo model with string repulsion is used to calculate azimuthal correlations with charmed hadrons. The string repulsion mechanism may provide significant angular correlations and can be considered as an alternative to thermalization picture

Altsybeev, I G

Feofilov, G A

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I G Altsybeev

G A Feofilov

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Angular Correlations with Charmed Hadrons in the Monte-Carlo Model with String Repulsion

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 KnE Energy & PhysicsICPPA 2017The 3rd International Conference on Particle Physics and AstrophysicsVolume 2018Conference PaperAngular correlations with charmed hadrons inthe Monte-Carlo model with string repulsionI G Altsybeev and G A FeofilovSaint-Petersburg State University, 7/9 Universitetskaya nab., St. Petersburg, 199034 RussiaAbstractRecent experimental results revealed large elliptic flow of the charmed hadrons atLHC energies, unexpectedly similar to that of the charged pions. The mentionedmeasurements are often interpreted using transport models, which incorporatedissociation and recombination mechanisms for charm quarks. In this report, amodified version of the Monte Carlo model with string repulsion is used to calculateazimuthal correlations with charmed hadrons. The string repulsion mechanism mayprovide significant angular correlations and can be considered as an alternative tothermalization picture.1. IntroductionThis is usually argued that heavy quarks (charm and beauty) are predominantly pro-duced at the very early stages of a heavy-ion collision via the hard scattering processesand, therefore, they experience all subsequent stages of the system evolution (seesome recent reviews in [1, 2]).A rather strong modification of the transverse momentum (𝑝𝑇 ) distributions ofheavy-flavour hadrons in heavy-ion collisions in comparison to pp collisions, wasobserved in the experiments at RHIC [3, 4] and LHC (see [5] for ALICE data). It maybe considered as one of the indications of the interactions of heavy quarks withthe medium. A large suppression of heavy-flavour hadron yields for 𝑝𝑇 > 4-5 GeV/𝑐in central collisions is usually attributed to parton energy loss in the medium [6–8]. Another evidence comes from the recent measurements of the second Fouriercoefficient 𝑣2 (called an elliptic flow) of D mesons in Pb–Pb collisions at LHC energies(√𝑠𝑁𝑁=2.76 and 5.02 TeV) [9]. It was found that the magnitude of the 𝑣2 turned outto be similar to that of charged pions. These measurements are usually comparedwith theoretical model calculations which implement charm-quark transport in ahydrodynamically expanding medium.How to cite this article: I G Altsybeev and G A Feofilov, (2018), “Angular correlations with charmed hadrons in the Monte-Carlo model with stringrepulsion” in The 3rd International Conference on Particle Physics and Astrophysics, KnE Energy & Physics, pages 280–285. DOI 10.18502/ken.v3i1.1755 Page 280Corresponding Author:I G AltsybeevIgor.Altsybeev@cern.chReceived: 25 December 2017Accepted: 2 February 2018Published: 9 April 2018Publishing services provided byKnowledge EI G Altsybeev and G AFeofilov. This article isdistributed under the terms ofthe Creative CommonsAttribution License, whichpermits unrestricted use andredistribution provided that theoriginal author and source arecredited.Selection and Peer-reviewunder the responsibility of theICPPA Conference Committee. KnE Energy & Physics ICPPA 20172. The MC model with string repulsionIn the current work, we try to qualitatively describe the behaviour of D mesons inthe framework of the Monte Carlo (MC) model with string repulsion. We will try toseparate so-called “soft” and “hard” sources of particle production, and will see howthe interplay between these two contributions is reflected on some observables.The “soft” part of particle production comes from the hadronization of the quark-gluon strings. The strings are formed at the early stages of hadron-hadron collisions,they may overlap in case of sufficiently high densities and interact by repelling orattracting each other, depending on the direction of the color fluxes [10]. For instance,in the hypothesis of repulsive interaction, strings may acquire, before the hadroniza-tion, an additional transverse boost. This produces additional transverse momenta tothe particles formed in string decays over a wide range of rapidity, thus leading tomodification of observables. Details of the Monte Carlo implementation of this modelare described in [11], and results on azimuthal asymmetry of two-particle correlationsobtained with this model are shown in [12].In order to parametrize the spectra of particles coming from breaking of a string inits rest frame, the identified particle spectra of pions, kaons and protons measuredby ALICE [13] in proton-proton collisions at √𝑠 = 2.76 TeV were used. Namely, foreach particle species, the exponential fit for the soft part of the spectrum (usuallyat 𝑝𝑇 < 1 GeV/𝑐) was performed, and then utilized in the MC model to assign thetransverse momentum for the particles emitted from the strings. The remaining partof each spectrum, obtained after subtraction of the exponential part, was used as a“hard” component for corresponding particle species. This component comes from thehard scatterings of partons, and for high 𝑝𝑇 the spectra have a power-law shape, asexpected from the perturbative QCD calculations.The important feature of the model presented here is that the hard part is notaffected by the transverse boosts of the strings, acquired due to the repulsive interac-tion. As a consequence, in the non-peripheral (i.e. central) nucleus-nucleus collisions,where strings accumulate significant transverse boosts, the soft component of thespectrum is being “blue-shifted” onto the hard one, leading to the modification of theoverall shapes of the spectra in nucleus-nucleus collisions, simulated in this MC model.There is a very small probability for the quark-gluon string to break into charm andanti-charm quark pair via the Schwinger mechanism (∼ 10−11, [14]). Nevertheless,we assume in the present MC model, that 𝑐-𝑐 quark pairs can be produced in thestring hadronization process with a sufficient probability, providing the exponentialpart of the spectra of charmed hadrons, which is supplemented by the hard part. SuchDOI 10.18502/ken.v3i1.1755 Page 281 KnE Energy & Physics ICPPA 2017growth of the open charm production can be explained by themodification of the stringtension due to string overlapping and interaction in a form of fusion. String breakinginto 𝑐-𝑐 quark pairs was studied in [15].As a proxy for the charmed hadrons, D mesons were taken, and their spectrummeasured in pp collisions by ALICE [16] was used to parametrize the MC model, in thesame manner as spectra of 𝜋, K, p.For the simplified version of the MC model used in this work, it is supposed also thatit is possible to neglect resonance decays, jets and final state effects like rescatteringof hadrons.3. Nuclear modification factor for A–A collisionsIn order to compare to the experiment, it is interesting to capture the 𝑝𝑇 –dependenceof the nuclearmodification factor (𝑅𝐴𝐴) in the framework of the string repulsionmodel.The𝑅𝐴𝐴 factors, calculated within the MCmodel for Pb–Pb collisions in centrality rangeof 20–40% for pions, kaons and protons, are shown in Figure 1 as lines of differentcolors. Markers represent values measured by ALICE [17]. Correspondence betweenthe model and data is only qualitative. However, it can be seen that the so-calledmass ordering is correctly reproduced by the MC model, and also a decreasing trendis observed after some maximum value for each particle species, similarly to data.This change of behavior reflects the interplay between the soft and hard components,implemented in the MC model.Some differences for pions at small 𝑝𝑇 could come from decays of resonances,which are switched off in the model. Peaks for 𝜋, K and p in the model calculationsare shifted to the right with respect to data points, indicating that radial expansion isslightly stronger in the simulation than in data. Also, the known increasing trend of𝑅𝐴𝐴, observed in data for 𝑝𝑇 > 8 GeV/𝑐 up to very high 𝑝𝑇 values, is not capturedin the given simulations, since we do not introduce any modification of the “hard”component in this variant of the model.4. Differential elliptic flowThe mass ordering of particle species was observed also in the differential elliptic flow,𝑣2(𝑝𝑇 ), for instance in Pb-Pb collisions [18]. Recently, the 𝑣2(𝑝𝑇 ) has been measuredalso for D meson in semi-central Pb–Pb events [9], and the values were found to besimilar to that of lighter particles.DOI 10.18502/ken.v3i1.1755 Page 282 KnE Energy & Physics ICPPA 2017cpππscentrality 20-40%Figure 1: 𝑅𝐴𝐴 for Pb–Pb collisions in centrality range of 20–40%. Model calculations (lines) are comparedwith ALICE data (markers).centrality 20-40%Figure 2: Differential elliptic flow for 𝜋, K, p and D mesons in centrality range of 20–40% in the MC model.Markers are for ALICE data (for D mesons, centrality range in experiment was 30–50%).In the present MC model, the mass ordering of 𝑣2 obtained for different particlespecies is qualitatively similar to the experiment (Figure 2). Data points for D mesonsare also fairly described in the model. For pions, after some maximum value, one maysee a decreasing trend of 𝑣2 at high 𝑝𝑇 , as it was observed for 𝑅𝐴𝐴 in Figure 1. This isdue to increasing fraction of the hard contribution at higher 𝑝𝑇 , which is not affectedby the “collective” repulsive motion of the strings. In order to see the same change ofDOI 10.18502/ken.v3i1.1755 Page 283 KnE Energy & Physics ICPPA 2017behavior for other particle species, more subtle tuning of the interplay between softand hard components of the particle production is needed.5. ConclusionsIt is shown that some certain signatures of collectivity, like 𝑅𝐴𝐴 or elliptic flow,observed experimentally in A–A collisions, can be obtained in the model with therepulsing strings. Namely, the model qualitatively reproduces the mass ordering ofthe nuclear modification factor and the azimuthal flow both for the light flavours andfor heavier hadrons, for instance, D mesons.The main challenge for the model is to reproduce correctly the interplay betweensoft and hard contributions in the intermediate 𝑝𝑇 range (from ≈ 1 to 8 GeV/𝑐, depend-ing on the particle species).AcknowledgementsThis work is supported by the Russian Science Foundation, grant 16-12-10176.References[1] Andronic A et al. Eur. Phys. J. C 76 (2016) 107[2] Prino F and Rapp R, J. Phys.G 43 (2016) 093002[3] PHENIX Collaboration Phys. Rev. C 84 (2011) 044905[4] STAR Collaboration Phys. Rev. Lett. 113 (2014) 142301[5] ALICE Collaboration JHEP 03 (2016) 081[6] ALICE Collaboration Phys. Rev. C 94 (2016) 054908[7] ALICE Collaboration Phys. Lett. B 754 (2016) 81[8] ALICE Collaboration Phys. Lett. B 770 (2017) 459[9] ALICE Collaboration CERN-EP-2017-153 (2017) arXiv:1707.01005[10] Abramovsky V A and Kanchely V O, JETP letters 31, (1980) 566[11] Altsybeev I AIP Conf.Proc. 1701 (2016) 100002[12] Altsybeev I and Feofilov G EPJ Web of Conferences 125 (2016) 04011[13] ALICE Collaboration Phys. Lett. B 736 (2014) 196[14] Buckley A et al. Physics Reports 504 (2011) 145[15] Kovalenko V et al. SQM2017 Proceedings (2017) arXiv:1711.00842DOI 10.18502/ken.v3i1.1755 Page 284 KnE Energy & Physics ICPPA 2017[16] ALICE Collaboration, JHEP 03 (2016) 081[17] ALICE Collaboration Phys. Rev. C 93 (2016) 034913[18] ALICE Collaboration, JHEP 06 (2015) 190DOI 10.18502/ken.v3i1.1755 Page 285

Angular Correlations with Charmed Hadrons in the Monte-Carlo Model with String Repulsion

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