The Compressed Baryonic Matter experiment (CBM) performance for proton anisotropic flow measurements is studied with Monte-Carlo simulations using collisions of gold ions at lab momentum of 12A GeV/c employing DCM-QGSM-SMM heavy-ion event generator. Realistic procedures are used for centrality estimation with the number of registered tracks and particle identification with information from Time-Of-Flight detector. Variation of directed flow estimates depending on various combinations of PSD modules is used to evaluate possible systematic biases due to collision symmetry plane estimation

Balzer, Matthias

Becker, Jürgen

Blank, Thomas

Bähr, Steffen

Caselle, Michele

Golosov, O.

Kashirin, E.

Klochkov, V.

Selyuzhenkov, I.

Sidorenko, Vladimir

Trifonova, Ekatarina

Unger, Kai Lukas

Weber, Marc

English

CBM Collaboration

KITopen

Journal of Physics: Conference SeriesPAPER • OPEN ACCESSPerformance for proton anisotropic flowmeasurement of the CBM experiment at FAIRTo cite this article: O Golosov et al 2020 J. Phys.: Conf. Ser. 1690 012104 View the article online for updates and enhancements.You may also likeCharged-particle pseudorapidity densityand anisotropic flow over a widepseudorapidity range using ALICE at theLHCK Gulbrandsen and the ALICECollaboration-The "Little Bang" at RHIC and at the LHCRaimond Snellings-Anisotropic flow analyses with multiparticleazimuthal correlationsA Bilandzic-This content was downloaded from IP address 129.13.72.195 on 15/11/2021 at 07:27Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distributionof this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Published under licence by IOP Publishing LtdICPPA 2020Journal of Physics: Conference Series 1690 (2020) 012104IOP Publishingdoi:10.1088/1742-6596/1690/1/0121041Performance for proton anisotropic flowmeasurement of the CBM experiment at FAIRO Golosov1, V Klochkov2,3, E Kashirin1 and I Selyuzhenkov1,2 for theCBM Collaboration1 National Research Nuclear University MEPhI (Moscow Engineering Physics Institute)Moscow, Russia2 GSI Helmholtzzentrum für Schwerionenforschung, Darmstadt, Germany3 Eberhard Karls University of Tübingen, Tübingen, GermanyE-mail: oleg.golosov@gmail.comAbstract. The Compressed Baryonic Matter experiment (CBM) performance for protonanisotropic flow measurements is studied with Monte-Carlo simulations using collisions of goldions at lab momentum of 12A GeV/c employing DCM-QGSM-SMM heavy-ion event generator.Realistic procedures are used for centrality estimation with the number of registered tracksand particle identification with information from Time-Of-Flight detector. Variation of directedflow estimates depending on various combinations of PSD modules is used to evaluate possiblesystematic biases due to collision symmetry plane estimation.1. IntroductionThe Compressed Baryonic Matter experiment (CBM) at FAIR is designed to study the phasediagram of strongly-interacting matter in the area of moderate temperatures and high net baryondensities with heavy ion collisions at beam momenta of 3.3–12A GeV/c per nucleon (√sNN=2.9–4.9 GeV). One of the most important observables to probe the properties of matter created inthese collisions is anisotropic transverse flow. In heavy ion collisions spatial asymmetry of initialenergy density in the overlap region is converted due to interaction between produced particlesto the asymmetry in final momentum distribution of collision products. This momentumasymmetry allows to constrain the transport parameters of the matter created in heavy-ioncollisions. It is quantified by the coefficients vn in a Fourier decomposition of the azimuthaldistribution of produced particles direction [1]:ρ(ϕ−ΨRP) =12π(1 + 2∞∑n=1vn cos[n(ϕ−ΨRP )]). (1)The Fourier decomposing is performed relative to the reaction plane spanned by the impactparameter and the beam. In the experiment reaction plane angle ΨRP can be estimated usingthe azimuthal distribution of produced particles or deflection of spectator fragments. Magnitudeof vn depends on the colliding system size and energy, centrality and other event and particleproperties.ICPPA 2020Journal of Physics: Conference Series 1690 (2020) 012104IOP Publishingdoi:10.1088/1742-6596/1690/1/01210422. CBM layout and simulation setupThe Compressed Baryonic Matter (CBM) is a future fixed target experiment at the currentlyconstructed FAIR facility. The relevant detector subsystems (figure 1 (a)) used in this analysisinclude [2] Micro-Vertex Detector (MVD), Silicon Tracking System (STS), and a forward hadroncalorimeter called the Projectile Spectator Detector (PSD).The MVD and STS were used to obtain information about particle momenta and definecentrality classes with track multiplicity, TOF for particle identification, PSD for reaction planeestimation. Twelve layers of silicon sensors of MVD and STS detectors cover the acceptance inpolar angle of 2.5◦ < Θ < 25◦. Hadron calorimeter PSD (figure 1 (b)) is situated at a distanceof 8 m from the target which is optimized for FAIR energy range and is shifted horizontallyin the transverse plane by about 10 cm to account for the beam deflection by the magneticfield with bending power of 1 Tm. The PSD consists of 44 modules and covers polar angles0.21◦ < Θ < 5.7◦ (4.3◦) in x (y) directions. A 20 cm hole in its center will allow to avoidradiation damage at high beam intensities expected at CBM.STS+MVDTOFPSD(a) (b)Figure 1. (a): layout of the CBM experiment. (b): transverse to the beam layout of thePSD modules. Colors show module groups used in the flow performance study: PSD1 (central),PSD2 (middle) and PSD3 (outer).Analysis of the directed flow was performed using a sample of 5M Au+Au collisions at beammomentum of 12A GeV/c produced with DCM-QGSM-SMM event generator (see [3, 4] andreferences therein). Detector response was simulated using GEANT4 transport code embeddedin the CbmRoot framework along with the reconstruction software.3. Analysis descriptionEvents with high quality of primary vertex reconstruction (χ2/NDF < 3) were selected. Onlytracks with good quality fit to primary vertex (χ2 < 18) and distance to the matched TOF hitless then 1.5 cm were used in the analysis. Centrality determination was performed using themultiplicity of tracks registered in MVD and STS as described in [5]. Particle identification wasbased on track charge, momentum and time of flight information and utilizes Bayesian approachdescribed in [6]. Figure 2 (a) shows the distribution of squared mass versus momentum forreconstructed tracks identified as protons and positive pions with purity higher than 90%.ICPPA 2020Journal of Physics: Conference Series 1690 (2020) 012104IOP Publishingdoi:10.1088/1742-6596/1690/1/0121043(a) (b)Figure 2. (a): distribution of squared mass versus momentum for selected protons and positivepions (purity > 90%). (b): Directed flow v1 of identified protons extracted relative to reactionplane or symmetry plane from PSD1 with the use of true and reconstructed resolution. Linesshow the values obtained from the generator input.Figure 3 shows the distribution of transverse momentum versus rapidity for generator protons(a) and selected proton tracks (b). To minimize the effect of finite efficiency on reconstructedflow values the analysis was performed in kinematic regions with higher tracking efficiency:pT ∈ [0.5, 1.2] for rapidity-differential and |y| ∈ [0.3, 0.4] for pT -differential calculations.(a) (b)Figure 3. Distribution of transverse momentum versus rapidity for generator protons (a) andidentified proton tracks (b).4. Flow extractionDirected flow v1 is reconstructed from the correlation of two-dimensional flow vectors q1 andQ1. The q1 vector is calculated event-by-event from the azimuthal angles φi of tracks identifiedas protons:q1 =1MM∑i=1u1,i, u1,i = (cosφi, sinφi), (2)ICPPA 2020Journal of Physics: Conference Series 1690 (2020) 012104IOP Publishingdoi:10.1088/1742-6596/1690/1/0121044where M is the number of identified proton tracks in the event, i track index. The spectatorsymmetry plane is estimated with the Q1 direction determined from the azimuthal asymmetryof the energy deposition in different PSD subevents (see figure 1 (b)):QPSDA1 =1EPSDANA∑i=1Eini, EPSDA =NA∑i=1Ei, (3)where the unit vector ni points in the direction of the center of i-th PSD module, Ei is the energydeposited in it, A denotes the PSD subevent used for event plane estimation, NA - number ofmodules in this subevent.Due to rectangular shape of the transverse layout of the detectors and presence of magneticfield the azimuthal acceptance of the CBM tracking system and the PSD are non-uniform inthe transverse plane, which introduces substantial bias in flow measurements. Non-uniformitymay vary with event and track properties. These effects were corrected for using a data-driven procedure [7] as implemented in the QnTools framework [8] which is an extension ofthe QnCorrections [9] framework originally developed for ALICE experiment at the LHC.Recentering, twist and rescaling corrections were applied as a function of time and collisioncentrality for Q1 and q1-vectors and for q-vectors additionally as a function of track transversemomentum and rapidity.Independent estimates of directed flow v1 are obtained with the scalar product method usingx and y components of flow vectors and different symmetry plane sources:v1,i{A} =2〈qiQA1,i〉RA1,i{B,C,D}, (4)where i = x, y are q1 and Q1 components, A, B, and C are different PSD subevents, andD is the Q1-vector calculated with the STS tracks. Symmetry plane resolution correctionfactors RA1,j{B,C,D} were calculated with the 4-subevent method, which reduces the bias fromadditional correlations between PSD subevents.5. Flow performanceFigure 2 (b) shows proton directed flow as a function of rapidity for the 10–25% centrality class.Dependencies obtained using information from Monte-Carlo simulation (lines) are comparedwith the calculations based on reconstructed tracks relative to:• true reaction plane (green triangles)• symmetry plane reconstructed using four central modules of forward hadron calorimeter(PSD1) with:– true resolution based on generator information about reaction plane angle (blue circles)– fully data driven result for resolution calculation (red squares).Solid lines (filled markers) show results obtained with x components of q1 and Q1 vectors,dashed lines (empty markers) represent the y component. Good agreement is observed betweengenerator input and all the three variants of reconstructed values in a wide rapidity range.Figure 4 presents the effect of choice of symmetry plane source. True dependencies arecompared with calculations relative to symmetry planes reconstructed using different groups ofPSD modules (see figure 1 (b)). One can see that with the shift to more peripheral modules forsymmetry plane estimation (from PSD1 to PSD3) additional correlations arise at higher rapidityreducing the agreement of reconstructed flow values with the generator input.ICPPA 2020Journal of Physics: Conference Series 1690 (2020) 012104IOP Publishingdoi:10.1088/1742-6596/1690/1/0121045(a) (b) (c)Figure 4. Directed flow of protons as a function of rapidity extracted relative to symmetryplane estimates from PSD1 (a), PSD2 (b) and PSD3 (c) subevents. Centrality 10-25%. Linesshow the values obtained from the generator input.(a) (b)Figure 5. Directed flow of protons as a function of transverse momentum in the backward (a)and forward (b) rapidity range (10–25% centrality class). Lines show the values obtained usingtrue information in the Monte-Carlo simulations.6. Summary and outlookPerformance for directed flow measurement for protons relative to the projectile spectatorsymmetry plane in Au+Au collisions at 12A GeV/c are presented as a function of transversemomentum and rapidity. Realistic procedures for centrality determination and particleidentification are used. Corrections are applied to account for effects of the detector azimuthalnon-uniformity. Good agreement has been shown between reconstructed and input values ofdirected flow in case symmetry plane is estimated from the most central group of PSD modules.The use of more peripheral groups of PSD modules for symmetry plane estimation leads toadditional correlations in the higher rapidity range. Future plans include application of efficiencycorrection and study of performance for measurement of higher order anisotropic flow with moreparticle species (protons, kaons, lambda).AcknowledgmentsThe work is supported by the Ministry of Science and Higher Education of the RussianFederation, Project “Fundamental properties of elementary particles and cosmology” No 0723-2020-0041, the Russian Foundation for Basic Research (RFBR) funding within the researchproject no. 18-02-40086, the European Union‘s Horizon 2020 research and innovation programICPPA 2020Journal of Physics: Conference Series 1690 (2020) 012104IOP Publishingdoi:10.1088/1742-6596/1690/1/0121046under grant agreement No. 871072, the National Research Nuclear University MEPhI inthe framework of the Russian Academic Excellence Project (contract no. 02.a03.21.0005,27.08.2013).References[1] Voloshin S A, Poskanzer A M and Snellings R 2010 Relativistic Heavy Ion Physics ed Stock R (Berlin:Springer-Verlag Berlin Heidelberg) chap Collective phenomena in non-central nuclear collisions, pp 293–333(Preprint 0809.2949)[2] Ablyazimov T et al. (CBM Collaboration) 2017 Eur. Phys. J. A 53 60 (Preprint 1607.01487)[3] Baznat M, Botvina A, Musulmanbekov G, Toneev V and Zhezher V 2020 Phys. Part. Nucl. Lett. 17 303–24(Preprint 1912.09277)[4] Botvina A et al. 1995 Nucl. Phys. A 584 737–56[5] Klochkov V and Selyuzhenkov I (CBM Collaboration) 2017 J. Phys. Conf. Ser. 798 012059[6] Anticic T et al. 2017 CBM Progress Report 2016 ed I Selyuzhenkov and A Toia (Darmstadt: GSI) URLhttp://repository.gsi.de/record/201318[7] Selyuzhenkov I and Voloshin S 2008 Phys. Rev. C 77 034904 (Preprint 0707.4672)[8] URL https://github.com/HeavyIonAnalysis/QnTools[9] URL https://github.com/FlowCorrections/FlowVectorCorrections

Performance for proton anisotropic flow measurement of the CBM experiment at FAIR

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Performance for proton anisotropic flow measurement of the CBM experiment at FAIR

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