Study of charged particle multiplicity, average transverse momentum and azimuthal anisotropy in Xe+Xe collisions at s NN − − − − √ = 5.44 TeV using AMPT model
aa r X i v : . [ nu c l - e x ] A ug Study of charged particle multiplicity, average transverse momentum and azimuthalanisotropy in Xe+Xe collisions at √ s NN = 5.44 TeV using AMPT model Sourav Kundu, ∗ Dukhishyam Mallick, † and Bedangadas Mohanty ‡ School of Physical Sciences, National Institute of Science Education and Research, HBNI, Jatni 752050, India (Dated: August 30, 2018)We have studied the average charged particle density ( < dN ch /d η> ), transverse momentum ( p T )spectra,
and azimuthal anisotropies of inclusive charged particles produced in Xe+Xe col-lisions at √ s NN = 5.44 TeV using A Multiphase Transport Model (AMPT), which includes thedeformation of Xe nucleus. Calculations have been performed with the string melting versionof AMPT model and compared with the recent measurements from the ALICE experiment. Themodel results over predict the measured < dN ch /d η> for central collisions, agree with the data formid-central collisions and under predict the measurements for peripheral collisions. The centralitydependence of
of charged particles measured in ALICE is not reproduced by the model re-sults. The calculated elliptic flow ( v ) from AMPT model overpredicts the ALICE measurements incentral collisions but are consistent with the data in mid central collisions. We find that the modelshows a mild centrality dependence of triangular flow and overestimates the ALICE measurements.Within the model framework, we have also studied various collision configurations of Xe nuclei suchas body-body, tip-tip, side-side and random. We find a strong dependence of the above observableon the collision configurations. I. INTRODUCTION
After a successful heavy-ion program at the LargeHadron Collider (LHC) and Relativistic Heavy-ionCollider (RHIC) facilities, observations such as large v and its number of constituent quark scaling [1], jetquenching [2], suppression in the production of high p T ( p T > c ) hadrons compare to p+p collisions haveconfirmed the presence of a deconfined state of quarksand gluons (QGP) [3, 4] in ultrarelativistic heavy-ioncollisions. In the ALICE experiment so far heavy-ioncollisions have been carried out by Pb nucleus [5].Recently on 9 th October 2017, ALICE collected data forXe+Xe collisions [6] over a time period of 8 hours. Xenucleus has a moderate prolate deformation [7] comparedto spherical Pb nucleus. A deformed Xe nucleus collisionallow us to probe a different initial condition comparedto the collision of spherical Pb nucleus.Deformed shape of Xe nucleus provide us with varioustype of collision configurations such as body-body, tip-tip and side-side depending upon the angle of collidingXe nucleus with respect to reaction plane. Centralheavy-ion collisions with a spherically symmetric nucleussuch as Au, Pb always give a circular overlapping regionin xy plane but in the case of deformed nucleus theoverlapping region need not be circular. Measurement ofazimuthal anisotropies and their fluctuations in varioustype of collision configurations and comparison with theresults from the collision of the spherical nucleus willprovide us the necessary input to constraining the initialcondition models. Hydrodynamical model simulationpredicts an increase of elliptic flow ( v ) by 10% for a ∗ [email protected] † [email protected] ‡ [email protected] deformed shape of Xe nucleus compared to the sphericalshaped Xe nucleus in the central collisions and theeffect of deformation on v vanishes beyond the collisioncentrality class of higher than 15% [8].Furthermore, heavy-ion collisions with deformed nu-cleus provide us with a better handle of backgroundmeasurement for chiral magnetic effect (CME) [9, 10].Magnetic field produced due to the presence of spectatornucleons is a source of CME observable whereas v actsas a background for CME observable. In the centralcollisions both v and magnetic field are less whereasin mid-central collisions both of them are large. So,the most preferable collisions to observe CME are thosewhere the magnetic field is finite and v is minimum andthis type of scenario can be achieved in the collision ofthe deformed nucleus by selecting body-tip type collisionconfiguration.In a recent hydrodynamic based model calculations [8] ofvarious observables in Xe+Xe collisions, the informationof the deformed Xe nucleus has been considered. In thiswork, we present result from a transport based modelstudy of Xe+Xe collisions after including the deforma-tion information. We also compare the model resultsto those measured in the ALICE experiment [6, 11]. Inaddition, we also present results for different Xe+Xe col-lision configurations for observables such as < dN ch /d η> , p T spectra,
and azimuthal anisotropies.This paper is organized as follows, in the next section wedescribe AMPT model and implementation of Xe+Xecollisions in AMPT [12, 13] model, along with thespecific configurations of Xe+Xe collisions. In sectionIII we compare the AMPT model results with ALICEmeasurements and present our results on < dN ch /d η> , p T spectra,
and azimuthal anisotropies for dif-ferent collision configurations. Finally in section IV wesummarize our observations. II. IMPLEMENTATION OF XE+XECOLLISIONS IN AMPT MODEL
We have used the AMPT model with string melting(SM) version. It is a hybrid transport model. This modelcontains four basic stages:1)Initial condition: Based on the initial condition fromHIJING [14].2)Partonic scattering: Zhang’s parton cascade model [15]has been used for scattering among the partons. Par-ton scattering is calculated by using two body scatteringcross section from pQCD with screening masses.3)Hadronization: In SM version of AMPT soft partonsare created from string and a quark coalescence model isused to combine parton into hadrons.4)Hadronic interaction: A Relativistic Transport(ART) [16] model is used for the evaluation of hadronicmatter, which includes the interaction between hadrons.For our study, we have used AMPT version 2.25t7 withpartonic cross section of 3mb. String melting parame-ters a and b are 0.3 and 0.15 respectively. We have im-plemented the deformation information of Xe nucleus inAMPT by using a deformed Woods-Saxon [17] profile, ρ = ρ exp ( | r − R | /d ) (1) R = R [1 + β Y ( θ ) + β Y ( θ )] (2) ρ is the normal nuclear density, R is the radius of Xenucleus, d is the diffuseness parameter and β , β are thedeformed parameters for Xe nucleus. We have used R = 5.4 fm, d = 0.59 fm, β = 0.162 and β = -0.003 [7]. Y ml ( θ ) is the spherical harmonics. This implementationis similar to those for U+U collisions as discussed in [18]In this work, we have studied 3 type of collision configu-ration of Xe nucleus which are chosen by the orientationof semi-major axis and semi-minor axis of the collidingdeformed Xe nucleus. We compare results from theseconfigurations with the Xe+Xe collision which havea random orientation of deformed Xe nucleus (usualexperimental situation). In addition, we also presentthe result of Xe+Xe collision where the Xe nucleus isconsidered to be spherically symmetric in order to seethe effect of deformation on the observables studied.This configuration is created by setting the deformationparameters β and β to zero. Three type of specialconfigurations which we have chosen for this analysis arenamed as body-body, tip-tip and side-side. Orientationunbiased configuration is named as random and collisionswith spherical Xe nucleus is named as spherical.Details about the angular orientation of various config-urations in terms of θ and φ are given in Table I. Insimulation of random configuration both the colliding Xenucleus are randomly rotated along the polar directionby using a uniform distribution in θ with a weight ofsin θ and in an azimuthal direction with a uniform φ distribution. We have analyzed 50000 minimum bias TABLE I. Details of angular configuration in Xe+Xe colli-sions. t and p in subscript denotes the target and projectilerespectively.Configuration θ p θ t φ p φ t Impact parameter directiongeneral 0- π π π π randomtip-tip 0 0 0-2 π π minor axisbody-body π /2 π /2 0 0 major axisside-side π /2 π /2 π /2 π /2 minor axis events for each configuration for the analysis carried outin the paper. III. RESULTS
In order to compare the model result with the ALICEexperiment, we select those events which have at least1 charged particle present in | η | <
1. This conditionis similar to INEL > η range 2.8 < η < < η < -1.7. This centrality selection, is chosen to mimicthe centrality selection used in the ALICE experiment.
1. Multiplicity, Pseudorapidity distribution, Transversemomentum spectra and Mean Transverse momentum
Figure 1 shows < dN ch /d η> per participating nucleonsas a function of centrality in | η | < < dN ch /d η> /( < N part > /
2) is similar inrandom and spherical case. Among the all collision con-figurations, the tip-tip configuration has largest chargedparticle multiplicity in central collisions whereas body-body configuration has largest charged particle multiplic-ity in mid central and peripheral collisions.The upper panel of Fig. 2 shows η distribution of pro-duced charged particles in 0-5% central Xe+Xe collisionsat √ s NN = 5.44 TeV for various collision configurationswhereas the lower panel shows the same but with a tunedcentrality in the model so that the < N part > of data andmodel are the same. AMPT model is able to describethe shape of η distribution but overpredicts the ALICEmeasurement [19].Charged particle p T spectra in the centrality class 0-5%, / ) pa r t > / ( N η / d c h < d N ALICEsphericalrandombody-bodyside-sidetip-tip = 5.44 TeV NN s Xe+Xe, AMPT-SM| < 0.5 η | FIG. 1. (Color online) < dN ch /d η> per participating nucleonsas a function of centrality for different Xe+Xe collision con-figurations from AMPT model. The model results are com-pared with the corresponding measurements from the ALICEexperiment [19]. − − − − η η / d c h d N ALICEsphericalrandombody-bodyside-sidetip-tip = 5.44 TeV NN s Xe+Xe, AMPT-SMCentralty 0-5%4 − − − − η η / d c h d N ALICEsphericalrandombody-bodyside-sidetip-tip = 5.44 TeV NN s Xe+Xe, AMPT-SMCentralty 0-5%
FIG. 2. (Color online) Upper panel: Pseudorapidity ( η )distribution of inclusive charged particles in 0-5 % centralXe+Xe collisions at √ s NN = 5.44 TeV for different configu-rations. Results from AMPT model are compared with themeasurements from the ALICE experiment [19]. Lower panel:Same as the upper panel but with centrality tuned in themodel such that data and model have same < N part > . √ s NN = 5.44TeV are shown in Fig. 3. Results from AMPT model arecompared with the measurements from the ALICE ex-periment [11]. For p T < c model overpredictsthe data in central collisions, it is consistent with thedata in mid central collisions and the model results un-derpredict the data for the peripheral collisions. For p T > c model underpredicts the data for all thecentrality classes studied. The p T spectra obtained fromboth spherical and random configurations are found tobe consistent with each other.Figure 4 shows
of charged particles as a func-tion of < N part > for various Xe+Xe collision configura-tions in AMPT model. We find that in AMPT modelwith string melting
does not show a strong cen-trality dependence and underpredicts the ALICE mea-surements except for the most peripheral collisions. Incentral and mid central Xe+Xe collisions the tip-tip con-figuration provides a higher value of
however inperipheral collisions body-body configuration leads to ahigher
value compare to other configuration.
2. Elliptic and Triangular flow
Elliptic flow ( v ) and triangular flow ( v ) are re-spectively the second order and third order Fouriercoefficients of particle distributions [20]. We havecalculated these flow coefficients by scalar productmethod. The n th order flow coefficient in scalar productmethod [21] is defined as, v n { , | ∆ η | > } = << u n,k Q ∗ n >> q (3)where flow vector Q n = P nj =1 e inφ j and u n,k = e inφ k . φ j is the azimuthal angle of j th particle and k is the particleof interest. Double bracket ( <<>> ) corresponds to anaverage over all particles in all events and single bracket( <> ) corresponds to an average over all events. ∗ is thecomplex conjugate. We calculate flow vectors Q n , Q An and Q Bn in 2.8 < η < < η < | η | < | ∆ η | > η gaps betweenthe regions where u n,k , Q n , Q An and Q Bn are calculated.Figure 5 shows p T integrated
10 110 - ) c ( G e V / T p d η d c h N d e v en t N = 5.44 TeV NN s Xe+Xe, AMPT-SM| < 0.8 η |ALICEsphericalrandombody-bodytip-tipside-side c (GeV/ T p M ode l / D a t a c (GeV/ T p c (GeV/ T p FIG. 3. (Color online) Upper panels show charged particle transverse momentum ( p T ) spectra at | η | < √ s NN = 5.44 TeValong with the measurement from the ALICE experiment [11]. Lower panels show the ratios of model to data. part of charged particle as a functionof < N part > at | η | < √ s NN = 5.44 TeV. observation is consistent with hydrodynamics predictionis given in [8]. side-side configuration leads to a larger In this work, we have implemented the deformation ofXe nucleus in AMPT model framework which gives us aunique possibility to study the different type of collisionconfiguration. These collision configurations are sensi-tive to initial conditions and can be used to constrainvarious initial state models. Deformation of Xe nucleusis implemented by using a deformed Woods-saxon pro-file of nucleon distribution. We have studied 3 specialtype of collision configurations of Xe nucleus which havebeen achieved by rotating the Xe nucleus in polar andazimuthal directions. We compare various result fromAMPT model with the corresponding ALICE measure-ments. To understand the effect of deformation on themeasured observable, we compare the AMPT model re-sults from the collisions of deformed Xe nucleus with the results from the collisions where Xe nucleus is conder tobe spherically symmetric. In addition to that we alsocompare the results between different Xe+Xe collisionconfigurations.We find that the observed < dN ch /d η> at | η | < √ s NN = 5.44 TeV, consistentwith the data in mid central collisions and underpredictsthe ALICE measurements in peripheral collisions. Thecalculated < p T > from AMPT model does not show anysignificant centrality dependence and underpredicts theALICE measurements for all centrality classes except forthe most peripheral collisions. random configuration ofAMPT model gives a slightly higher value of v compareto ALICE measurements in central collisions, however itis consistent with the data above 30% centrality. Mea-sured v in ALICE experiment is overpredicted by theresults from the random Xe+Xe configuration of AMPTmodel.We observe a dependence on the Xe+Xe collision config-urations for the measured observables. In central colli-sion the tip-tip configuration leads to a higher value of < dN ch /d η> compared to other configurations, whereasin peripheral collision the body-body configuration givesthe largest value of < dN ch /d η> . In central and mid cen-tral Xe+Xe collisions the tip-tip configuration provides ahigher value of however in peripheral collisions thebody-body configuration leads to a higher valuecompare to other configurations.We do not observe anysignificant change in charged particle multiplicity and between the spherical case and random case. El-liptic flow in the central collision is enhanced by ∼ v and body-body configuration gives a smallervalue of v relative to other configurations. The calcu-lated elliptic flow in side-side configuration is more than50% larger compared to other configuration in centraland mid-central collisions.If ALICE experiment could trigger these different colli-sion configurations then these different initial state con-dition can be possibly accessed. The future scope of ourstudy includes the study of CME observable in centralbody-tip type collisions where we expect a finite amountof magnetic field and less value of v . We would also liketo develop a way to select such special types of collisionconfigurations in the experiment. V. ACKNOWLEDGWMENTS