Azimuthal Anisotropy Of Charged Particles Research Articles

Study of transverse spherocity and azimuthal anisotropy in Pb–Pb collisions at TeV using a multi-phase transport model

Transverse spherocity is an event shape observable having a unique capability to separate the events based on their geometrical shapes. Recent results from experiments at the LHC suggest that transverse spherocity is an important event classifier in small collision systems. In this work, we use transverse spherocity for the first time in heavy-ion collisions and perform an extensive study on azimuthal anisotropy of charged particles produced in Pb–Pb collisions at TeV using a multi-phase transport model (AMPT). The azimuthal anisotropy is estimated using the two-particle correlation method, which suppresses the non-flow effects significantly with an appropriate pseudorapidity gap of particle pairs. The results from AMPT are compared with estimations from PYTHIA8 (Angantyr) model and it is found that with the chosen pseudorapidity gap, the residual non-flow effects become negligible. We found that the high spherocity events have nearly zero elliptic flow while low spherocity events contribute significantly to elliptic flow of spherocity-integrated events.

Transverse momentum and process dependent azimuthal anisotropies in sqrt{s_{mathrm {NN}}}=8.16\xa0TeV p+Pb collisions with the ATLAS detector

The azimuthal anisotropy of charged particles produced in sqrt{s_{mathrm {NN}}}=8.16 TeV p+Pb collisions is measured with the ATLAS detector at the LHC. The data correspond to an integrated luminosity of 165 mathrm {nb}^{-1} that was collected in 2016. Azimuthal anisotropy coefficients, elliptic v_2 and triangular v_3, extracted using two-particle correlations with a non-flow template fit procedure, are presented as a function of particle transverse momentum (p_mathrm {T}) between 0.5 and 50 GeV. The v_2 results are also reported as a function of centrality in three different particle p_mathrm {T} intervals. The results are reported from minimum-bias events and jet-triggered events, where two jet p_mathrm {T} thresholds are used. The anisotropies for particles with p_mathrm {T} less than about 2 GeV are consistent with hydrodynamic flow expectations, while the significant non-zero anisotropies for p_mathrm {T} in the range 9–50 GeV are not explained within current theoretical frameworks. In the p_mathrm {T} range 2–9 GeV, the anisotropies are larger in minimum-bias than in jet-triggered events. Possible origins of these effects, such as the changing admixture of particles from hard scattering and the underlying event, are discussed.

Multiparticle azimuthal correlations for extracting event-by-event elliptic and triangular flow in Au + Au collisions at sNN=200 GeV

We present measurements of elliptic and triangular azimuthal anisotropy of charged particles detected at forward rapidity 1<|η|<3 in Au + Au collisions at sNN=200 GeV, as a function of centrality. The multiparticle cumulant technique is used to obtain the elliptic flow coefficients v2{2},v2{4},v2{6}, and v2{8}, and triangular flow coefficients v3{2} and v3{4}. Using the small-variance limit, we estimate the mean and variance of the event-by-event v2 distribution from v2{2} and v2{4}. In a complementary analysis, we also use a folding procedure to study the distributions of v2 and v3 directly, extracting both the mean and variance. Implications for initial geometrical fluctuations and their translation into the final-state momentum distributions are discussed.9 MoreReceived 26 April 2018DOI:https://doi.org/10.1103/PhysRevC.99.024903©2019 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCollective flowParticle correlations & fluctuationsRelativistic heavy-ion collisionsPhysical SystemsHadronsQuarksTechniquesHadron collidersNuclear Physics

Measurement of the azimuthal anisotropy of charged particles in 5.02 TeV Pb+Pb and 5.44 TeV Xe+Xe collisions with ATLAS

The data collected by the ATLAS experiment during the 2015 Pb+Pb and 2017 Xe+Xe LHC runs offer new opportunities to study charged particle azimuthal anisotropy. The high-statistics Pb+Pb sample allows for a detailed study of the azimuthal anisotropy of produced particles. This should improve the understanding of initial conditions of nuclear collisions, hydrodynamical behavior of quark-gluon plasma and parton energy loss. New ATLAS measurements of differential and global Fourier harmonics of charged particles (νn) in 5.02 TeV Pb+Pb and 5.44 TeV Xe+Xe collisions in a wide range of transverse momenta, pseudorapidity (|η| < 2.5) and collision centrality are presented. The higher order harmonics, sensitive to fluctuations in the initial state, are measured up to n = 7 using the two-particle correlation, cumulant and scalar product methods. The dynamic properties of QGP are studied using a recently-proposed modified Pearson's correlation coefficient, ρ(νn2,pT), between the event-wise mean transverse momentum and the magnitude of the flow vector in 5.02 TeV Pb+Pb and p+Pb collisions. Several important observations are made. The elliptic and triangular flow harmonics show an interesting universal pT-scaling. A linear correlation between the ν2 and ν3 coefficients at low and high pT ranges is observed and quantified. The correlation coefficient for ν2 is found to be negative in peripheral and positive in central Pb+Pb collisions. The value for ν3 is found to be much smaller than for ν2 and have similar centrality behavior as the ν2.

Measurements of Fourier Harmonics of Azimuthal Anisotropy in Pb+Pb and Xe+Xe Collisions

The high-statistics data sets collected by the LHC experiments during the heavy-ion runs allow for a detailed study of the azimuthal anisotropy of charged particles. This report presents the selected results of $v_{n}$ measured in Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV and Xe+Xe collisions at $\sqrt{s_\mathrm{NN}} = 5.44$ TeV, with emphasis on the higher order Fourier coefficients, $v_{n}$, sensitive to initial state fluctuations of the medium created in ultra-relativistic heavy-ion collisions.

Measurement of the azimuthal anisotropy of charged particles produced in sqrt{s_{_text {NN}}} = 5.02\xa0TeV Pb+Pb collisions with the ATLAS detector

Measurements of the azimuthal anisotropy in lead–lead collisions at sqrt{s_{_text {NN}}} = 5.02 TeV are presented using a data sample corresponding to 0.49 {mathrm {nb}}^{-1} integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for “ultra-central” collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, v_{2}–v_{7}, which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics v_{n} over wide ranges of the transverse momentum, 0.5 <p_{mathrm{T}}< 60 GeV, the pseudorapidity, |eta |< 2.5, and the collision centrality 0–80%. Results from different methods are compared and discussed in the context of previous and recent measurements in Pb+Pb collisions at sqrt{s_{_text {NN}}} = 2.76 mathrm{TeV} and 5.02 mathrm{TeV}. In particular, the shape of the p_{mathrm{T}}dependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the v_{n} and p_{mathrm{T}}values by constant factors over the centrality interval 0–60% and the p_{mathrm{T}}range 0.5 < p_{mathrm{T}}< 5 GeV.

Azimuthal anisotropy of charged particles from multiparticle correlations in pPb and PbPb collisions with CMS

Abstract Motivated by two- and four-particle azimuthal correlation measurements that suggest possible collective flow for charged particles emitted in pPb collisions at s NN = 5.02 TeV , we extend the correlation results for these collisions using the six- and eight-particle cumulant methods, and the Lee–Yang Zeros method. CMS has an extensive program studying azimuthal harmonic coefficients for both PbPb and pPb collisions using various methods. The current pPb results will be presented in this context. The data were collected by the CMS experiment at the LHC using both minimum bias and high-multiplicity collision triggers over a wide range in pseudorapidity. The results are compared to 2.76 TeV semi-peripheral PbPb collision data collected in 2011 covering a similar range of particle multiplicities. The second-order azimuthal anisotropy Fourier harmonic ( v 2 ) is shown for the different methods. A comparison of the six- and greater particle correlations to the previously published two- and four-particle correlation results sheds light on the multiparticle nature of the azimuthal anisotropy. The results are also discussed in terms of recent calculations that explore the role of participant fluctuations on measurements of higher-order particle correlations in pPb collisions.

Flow harmonics in Pb + Pb collisions at SNN=2.76 TeV with the ATLAS detector

The measurement of the azimuthal anisotropy of charged particles produced in lead–lead collisions at SNN=2.76TeV is reported. The Fourier coefficients vn of the azimuthal angle distribution are obtained using multi-particle cumulants. The results on the transverse momentum and centrality dependence of the elliptic flow (v2) obtained from two-, four-, six- and eight-particle cumulants are presented. Higher-order harmonics, v3 and v4, are also derived using two- and four-particle cumulants and shown as a function of centrality and transverse momentum. A reduction of contributions from correlations not related to the collective flow are discussed. Event-by-event fluctuations of the flow harmonics are also presented as a function of transverse momentum and the collision centrality.

Measurement of higher-order harmonic azimuthal anisotropy in PbPb collisions atsNN=2.76TeV

Measurements are presented by the CMS Collaboration at the Large Hadron Collider (LHC) of the higher-order harmonic coefficients that describe the azimuthal anisotropy of charged particles emitted in sqrt(s[NN]) = 2.76 TeV PbPb collisions. Expressed in terms of the Fourier components of the azimuthal distribution, the n = 3-6 harmonic coefficients are presented for charged particles as a function of their transverse momentum (0.3 < pt < 8.0 GeV), collision centrality (0-70%), and pseudorapidity (abs(eta) < 2.0). The data are analyzed using the event plane, multiparticle cumulant, and Lee-Yang zeros methods, which provide different sensitivities to initial-state fluctuations. Taken together with earlier LHC measurements of elliptic flow (n = 2), the results on higher-order harmonic coefficients develop a more complete picture of the collective motion in high-energy heavy-ion collisions and shed light on the properties of the produced medium.

Measurements of Elliptic Flow with the ATLAS Detector

The measurement of the azimuthal anisotropy of charged particles obtained with the multi-particle correlations method is presented and compared to the results obtained with the event plane method for Pb + Pb collisions at 2.76 TeV from the ATLAS experiment. Results on flow harmonics, determined from the cumulants of particle correlations, over wide transverse momenta ( 0.5 < p T < 20 GeV ), pseudorapidity ( | η | < 2.5 ) and centrality ranges are shown. The applied cumulant approach provides a unique handle on non-flow effects and allows for the evaluation of the genuine flow fluctuations. Derived estimates of the non-flow correlations and the magnitude of flow fluctuations are discussed. Centrality and pseudorapidity dependence of the elliptic flow measured with the event-plane method and integrated down to very low p T is also shown and compared to the results obtained by the other LHC experiments.

Azimuthal Anisotropy of Charged Particles at High Transverse Momenta in Pb-Pb Collisions atsNN=2.76 TeV

The azimuthal anisotropy of charged particles in Pb-Pb collisions at sqrt[s(NN)]=2.76 TeV is measured with the CMS detector at the LHC over an extended transverse momentum (p(T)) range up to approximately 60 GeV/c. The data cover both the low-p(T) region associated with hydrodynamic flow phenomena and the high-p(T) region where the anisotropies may reflect the path-length dependence of parton energy loss in the created medium. The anisotropy parameter (v2) of the particles is extracted by correlating charged tracks with respect to the event-plane reconstructed by using the energy deposited in forward-angle calorimeters. For the six bins of collision centrality studied, spanning the range of 0-60% most-central events, the observed v2 values are found to first increase with p(T), reaching a maximum around p(T)=3 GeV/c, and then to gradually decrease to almost zero, with the decline persisting up to at least pp(T)=40 GeV/c over the full centrality range measured.

Can we observe the elliptic flow in pp collisions at LHC?

The azimuthal anisotropy of charged particles in heavy ion collisions is an important probe of quark-gluon plasma evolution at early stages of reaction. Now, with the start of Large Hadron Collider (LHC), the collective effects in proton-proton collisions are also of great interest. In this work we analyze the ability of different methods to extract elliptic flow in pp collisions at LHC. The analysis is based on Monte Carlo simulations of proton-proton collisions with PYTHIA, PHOJET and EPOS event generators at energies = 900 GeV and 7TeV.

Elliptic flow studies using the CMS detector

The azimuthal anisotropy of charged particles in heavy ion collisions is an important probe of quark-gluon plasma evolution at early stages. The nuclear reaction plane can be determined independently by different detector subsystems and using different analysis methods. This paper reports the capability of the CMS detector at the LHC to reconstruct the reaction plane of the collision and to me asure elliptic flow with calorimetry and a tracking system. The analysis is based on a full CMS detector simulation of Pb + Pb events with the HYDJET event generator.