Importance of correlations and fluctuations on the initial source eccentricity in high-energy nucleus-nucleus collisions

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The elliptic, triangular, quadrangular and pentagonal anisotropic flow coefficients for π ± , K ± and p+p in Pb-Pb collisions at √ s NN = 2.76 TeV were measured with the ALICE detector at the Large Hadron Collider. The results were obtained with the Scalar Product method, correlating the identified hadrons with reference particles from a different pseudorapidity region. Effects not related to the common event symmetry planes (non-flow) were estimated using correlations in pp collisions and were subtracted from the measurement. The obtained flow coefficients exhibit a clear mass ordering for transverse momentum (p T ) values below ≈ 3 GeV/c. In the intermediate p T region (3 < p T < 6 GeV/c), particles group at an approximate level according to the number of constituent quarks, suggesting that coalescence might be the relevant particle production mechanism in this region. The results for p T < 3 GeV/c are described fairly well by a hydrodynamical model (iEBE-VISHNU) that uses initial conditions generated by A Multi-Phase Transport model (AMPT) and describes the expansion of the fireball using a value of 0.08 for the ratio of shear viscosity to entropy density (η/s), coupled to a hadronic cascade model (UrQMD). Finally, expectations from AMPT alone fail to quantitatively describe the measurements for all harmonics throughout the measured transverse momentum region. However, the comparison to the AMPT model highlights the importance of the late hadronic rescattering stage to the development of the observed mass ordering at low values of p T and of coalescence as a particle production mechanism for the particle type grouping at intermediate values of p T for all harmonics.

Section: Jhep09(2016)164mentioning

confidence: 99%

Section: Jhep09(2016)164mentioning

confidence: 99%

Higher harmonic flow coefficients of identified hadrons in Pb-Pb collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV

Adam¹,

Adamová²,

Aggarwal³

et al. 2016

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“…Each quantity mentioned above, v n , σ vn , v 2 n , σ vn / v n , v RP n and δ vn , has been the subject of extensive studies both experimentally [19,22,25] and in theoretical models [23,24,28]. Experimental measurement of the EbyE v n distributions can elucidate the relations between these quantities, as well as clarify the connections between various experimental methods.…”

Section: Jhep11(2013)183mentioning

confidence: 99%

Measurement of the distributions of event-by-event flow harmonics in lead-lead collisions at $ \sqrt{{{s_{NN }}}} $ = 2.76 TeV with the ATLAS detector at the LHC

Aad¹,

Abajyan²,

Abbott³

et al. 2013

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216

197

The distributions of event-by-event harmonic flow coefficients v n for n = 2-4 are measured in √ s N N = 2.76 TeV Pb+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using charged particles with transverse momentum p T > 0.5 GeV and in the pseudorapidity range |η| < 2.5 in a dataset of approximately 7 µb −1 recorded in 2010. The shapes of the v n distributions suggest that the associated flow vectors are described by a two-dimensional Gaussian function in central collisions for v 2 and over most of the measured centrality range for v 3 and v 4 . Significant deviations from this function are observed for v 2 in mid-central and peripheral collisions, and a small deviation is observed for v 3 in mid-central collisions. In order to be sensitive to these deviations, it is shown that the commonly used multi-particle cumulants, involving four particles or more, need to be measured with a precision better than a few percent. The v n distributions are also measured independently for charged particles with 0.5 < p T < 1 GeV and p T > 1 GeV. When these distributions are rescaled to the same mean values, the adjusted shapes are found to be nearly the same for these two p T ranges. The v n distributions are compared with the eccentricity distributions from two models for the initial collision geometry: a Glauber model and a model that includes corrections to the initial geometry due to gluon saturation effects. Both models fail to describe the experimental data consistently over most of the measured centrality range. The ATLAS collaboration 41 IntroductionHeavy ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC) create hot, dense matter that is thought to be composed of strongly interacting quarks and gluons. A useful tool to study the properties of this matter is the azimuthal anisotropy of particle emission in the transverse plane [1,2]. This anisotropy has been interpreted as a result of pressure-driven anisotropic expansion (referred to as "flow") of the created matter, and is described by a Fourier expansion of the particle distribution in azimuthal angle φ, around the beam direction:where v n and Φ n represent the magnitude and phase of the n th -order anisotropy of a given event in the momentum space. These quantities can also be conveniently represented by the per-particle "flow vector" [2]: ⇀ v n = (v n cos nΦ n , v n sin nΦ n ). The angles Φ n are commonly referred to as the event plane (EP) angles.In typical non-central [2] heavy ion collisions, the large and dominating v 2 coefficient is associated mainly with the "elliptic" shape of the nuclear overlap. However, v 2 in -1 - JHEP11(2013)183central (head-on) collisions and the other v n coefficients in general are related to various shape components of the initial state arising from fluctuations of the nucleon positions in the overlap region [3]. The amplitudes of these shape components, characterized by eccentricities ǫ n , can be estimated via a simple Glauber model from the...

“…Different Glauber MC samples were produced varying the Glauber parameters within the uncertainties from refs. [15] and [16]. The variation in the final results is quoted as the uncertainty in N part .…”

mentioning

confidence: 99%

Dependence on pseudorapidity and on centrality of charged hadron production in PbPb collisions at $ \sqrt {{{s_{\text{NN}}}}} = 2.76 $ TeV

Chatrchyan¹,

Nikolić²,

Erbacher³

et al. 2011

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117

A measurement is presented of the charged hadron multiplicity in hadronic PbPb collisions, as a function of pseudorapidity and centrality, at a collision energy of 2.76 TeV per nucleon pair. The data sample is collected using the CMS detector and a minimum-bias trigger, with the CMS solenoid off. The number of charged hadrons is measured both by counting the number of reconstructed particle hits and by forming hit doublets of pairs of layers in the pixel detector. The two methods give consistent results. The charged hadron multiplicity density, dN ch /dη| η=0 , for head-on collisions is found to be 1612 ± 55, where the uncertainty is dominated by systematic effects. Comparisons of these results to previous measurements and to various models are also presented. Keywords: Hadron-Hadron ScatteringOpen Access, Copyright CERN, for the benefit of the CMS collaboration doi:10.1007/JHEP08(2011)141 The CMS collaboration 21 JHEP08(2011)141 IntroductionQuantum chromodynamics (QCD), the theory of strong interactions, predicts a phase transition at high temperature between hadronic and deconfined matter [1]. Strongly interacting matter under extreme conditions can be studied experimentally using ultrarelativistic collisions of heavy nuclei. The field entered a new era in November 2010 when the Large Hadron Collider (LHC) produced the first PbPb collisions at a centre-of-mass energy per nucleon pair of 2.76 TeV. This represents an increase of more than one order of magnitude over the highest-energy nuclear collisions previously achieved in the laboratory. The multiplicity of charged particles produced in the central-rapidity region is a key observable characterising the properties of the quark-gluon matter created in these collisions [2]. Nuclei are extended objects, and their collisions occur at various impact parameters, referred to as "centralities". The studies of the dependence of the charged particle density on the type of colliding nuclei, on the centre-of-mass energy, and on the collision geometry are important for understanding the relative contributions of hard scattering and soft processes to particle production and provide insight into the partonic structure of the nuclei.-1 - JHEP08(2011)141In this paper we report measurements of the multiplicity density dN ch /dη of primary charged hadrons. The analysis is based on the 2.76 TeV-per-nucleon PbPb collision data recorded by the Compact Muon Solenoid (CMS) detector in December 2010, in runs without magnetic field. The pseudorapidity is defined as η = − ln[tan(θ/2)] with θ the polar angle with respect to the counterclockwise beam direction (the z axis). The number of primary charged hadrons N ch is defined as all charged hadrons produced in an event including decay products of particles with proper lifetimes less than 1 cm.A detailed description of the CMS experiment can be found in ref. [3]. The pixel tracker used for the analysis covers the region |η| < 2.5 and a full 2π in azimuth, with 66M detector channels out of which 97.5% were functional during data...