Flow Harmonics Research Articles

Mixed higher-order flow harmonics and nonlinear response coefficients in PbPb collisions at 2.76 and 5.02 TeV with CMS

The mixed higher-order flow harmonics and nonlinear response coefficients of charged particles are presented as a function of pT and centrality in PbPb collisions at sNN=2.76 TeV and 5.02 TeV with the CMS detector. The results are obtained using the scalar product method, and cover a pT range from 0.3 GeV/c to 8.0 GeV/c, pseudorapidity |η|<2.4, and a centrality range of 0-60%. The mixed harmonic results at 5.02 TeV are compared to the matching higher-order flow harmonics from two-particle correlations, which measure vn values with respect to the n-th order event plane. It is observed that the nonlinear response coefficients of the odd harmonics are larger than the even harmonics ones. The results are compared with hydrodynamic predictions using different shear viscosity to entropy density ratios and different initial conditions.

Collective flow measurements with HADES in Au+Au collisions at 1.23A GeV

HADES has a large acceptance combined with a good mass-resolution and therefore allows the study of dielectron and hadron production in heavy-ion collisions with unprecedented precision. With the statistics of seven billion Au-Au collisions at 1.23A GeV recorded in 2012, the investigation of higher-order flow harmonics is possible. At the BEVALAC and SIS18 directed and elliptic flow has been measured for pions, charged kaons, protons, neutrons and fragments, but higher-order harmonics have not yet been studied. They provide additional important information on the properties of the dense hadronic medium produced in heavy-ion collisions. We present here a high-statistics, multidifferential measurement of v1 and v2 for protons in Au+Au collisions at 1.23A GeV.

Global view on coupled dynamics of heavy and light flavor observables from EPOSHQ

We present an analysis of the comparison between 2nd and 3rd flow harmonics of π, protons and D mesons produced in ultrarelativistic heavy ion collisions. We advocate that such an analysis could turn out to be a good strategy in order to quantify the off-equilibrium dynamics of heavy flavor at lower transverse momentum pT.

Event-by-event vn correlations of soft hadrons and heavy mesons in heavy ion collisions

Combining event-by-event hydrodynamics with heavy quark energy loss we compute correlations between the heavy and soft sectors for elliptic and triangular flow harmonics v2 and v3 of D0 mesons in PbPb collisions at 2.76 TeV and 5.02 TeV. Our results indicate that v3 is strongly influenced by the fragmentation temperature and that it builds up later than v2 during the evolution of the system.

Collective flow in 2.76 and 5.02 A TeV Pb\xa0+\xa0Pb collisions

In this paper, we study and predict flow observables in 2.76 and 5.02 A TeV Pb + Pb collisions, using the iEBE-VISHNU hybrid model with TRENTo and AMPT initial conditions and with different forms of the QGP transport coefficients. With properly chosen and tuned parameter sets, our model calculations can nicely describe various flow observables in 2.76 A TeV Pb + Pb collisions, as well as the measured flow harmonics of all charged hadrons in 5.02 A TeV Pb + Pb collisions. We also predict other flow observables, including v_n(p_T) of identified particles, event-by-event v_n distributions, event-plane correlations, (normalized) symmetric cumulants, non-linear response coefficients and p_T-dependent factorization ratios, in 5.02 A TeV Pb + Pb collisions. We find many of these observables to remain approximately the same values as the ones in 2.76 A TeV Pb + Pb collisions. Our theoretical studies and predictions could shed light to the experimental investigations in the near future.

Measurement of azimuthal flow of soft and high-pT charged particles in 5.02 TeV Pb+Pb collisions with the ATLAS detector

The experimental data collected by the ATLAS experiment during the 2015 heavy ion LHC run offers new opportunities to study properties of Quark-Gluon Plasma at unprecedented high temperatures and densities. Study of the azimuthal anisotropy of produced particles not only constrains our understanding of initial conditions of nuclear collisions and soft particle collective dynamics, but also sheds light on jet-quenching phenomena via measurement of flow harmonics at high transverse momenta. A new ATLAS measurement of elliptic flow and higher-order Fourier harmonics of charged particles in Pb+Pb collisions at sNN=5.02 TeV in a wide range of transverse momenta, pseudorapidity (|η|<2.5) and collision centrality is presented. These measurements are based on the Scalar Product and Two Particle Correlation methods. The results obtained are compared with experimental results at lower collision energies.

Resolving the RAA to vn puzzle

After 10 years of struggling to simultaneously describe the nuclear modification factor RAA and flow harmonics vn's at high pT, now theoretical models are able to reproduce experimental data well. The necessary theoretical developments such as event-by-event fluctuations, choice of initial conditions, and the scalar product method to calculate flow harmonics at high pT are reviewed. Additionally, a discussion of new proposed experimental observables known as Soft Hard Event Engineering (SHEE) that are sensitive to the path length dependence of the energy loss is included.

Heavy meson flow harmonics in event-by-event viscous relativistic hydrodynamics

Event-by-event viscous hydrodynamics is combined with heavy quark energy loss models to compute heavy flavor flow cumulants v2{2}, v3{2}, and v2{4} as well as the nuclear modification factors of D0 and B0 mesons in PbPb collisions at 2.76 TeV. Our results indicate that bottom quarks can flow as much as charm quarks in the pT range 8–30 GeV.

Correlations of flow harmonics in 2.76A TeV Pb-Pb collisions

Using the event-by-event viscous hydrodynamics VISH2+1 with MC-Glauber, MC-KLN, and AMPT initial conditions, we investigate the correlations of flow harmonics, including the symmetric cumulants $SC^{v}(m, n)$, the normalized symmetric cumulants $NSC(m, n)$, and the Pearson correlation coefficients $C(v_{m}^{2}, v_{n}^{2})$ in 2.76A TeV Pb--Pb collisions. We find $SC^{v}(m, n)$ is sensitive to both initial conditions and the specific shear viscosity $\eta/s$. A comparison with the recent ALICE data show that our hydrodynamic calculations can qualitatively describe the data of $SC^{v}(3, 2)$ and $SC^{v}(4, 2)$ for various initial conditions, which demonstrate that $v_2$, $v_4$ are correlated and $v_2$, $v_3$ are anti-correlated. Meanwhile, the predicted symmetric cumulants $SC^{v}(5, 2)$, $SC^{v}(5, 3)$, and $SC^{v}(4, 3)$ reveal that $v_2$ and $v_5$, $v_3$ and $v_5$ are correlated, $v_3$ and $v_4$ are anti-correlated in most centrality classes. We also find $NSC^{v}(3, 2)$ and $C(v_{3}^{2}, v_{2}^{2})$, which are insensitive to $\eta/s$, are mainly determined by corresponding $NSC^{\varepsilon}(3, 2)$ and $C(\varepsilon_{3}^{2}, \varepsilon_{2}^{2})$ correlators from the initial state. In contrast, other $NSC^{v}(m, n)$ and $C(v_{m}^{2}, v_{n}^{2})$ correlators are influenced by both initial conditions and $\eta/s$, which illustrates the non-linear mode couplings in higher flow harmonics with $n \geq 4$.

Cumulants and nonlinear response of high pT harmonic flow at sNN=5.02 TeV

Event-by-event fluctuations caused by quantum mechanical fluctuations in the wave function of colliding nuclei in ultrarelativistic heavy ion collisions were recently shown to be necessary for the simultaneous description of $R_{AA}$ as well as the elliptic and triangular flow harmonics at high $p_T$ in PbPb collisions at the Large Hadron Collider. In fact, the presence of a finite triangular flow as well as cumulants of the flow harmonic distribution that differ from the mean are only possible when these event-by-event fluctuations are considered. In this paper we combine event-by-event viscous hydrodynamics and jet quenching to make predictions for high $p_T$ $R_{AA}$, $v_2\{2\}$, $v_3\{2\}$, and $v_2\{4\}$ in PbPb collisions at $\sqrt{s_{NN}}=5.02$ TeV. With an order of magnitude larger statistics we find that high $p_T$ elliptic flow does not scale linearly with the soft elliptical flow, as originally thought, but has deviations from perfectly linear scaling. A new experimental observable, which involves the difference between the ratio of harmonic flow cumulants at high and low $p_T$, is proposed to investigate the fluctuations of high $p_T$ flow harmonics and measure this nonlinear response. By varying the path length dependence of the energy loss and the viscosity of the evolving medium we find that $R_{AA}(p_T)$ and $v_2\{2\}(p_T)$ strongly depend on the choice for the path length dependence of the energy loss, which can be constrained using the new LHC run 2 data.

Systematic study of symmetric cumulants at sNN=200 GeV in Au+Au collisions using a transport approach

Measurement of 4-particle symmetric cumulants has been considered to be a good tool to study the correlations between amplitudes of different orders of anisotropic flow harmonics in the heavy-ion collision. These new observables not yet been measured at RHIC. Using A Multi-Phase Transport model, a set of predictions for the centrality dependence of the normalized 4-particle symmetric cumulants in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV has been given. In addition, the effects of shear viscosity and hadronic rescattering on the magnitude of symmetric cumulants are discussed using AMPT model at $\sqrt{s_{NN}}$ = 200 GeV. It is shown that $sc(2,3)$ is found to be more sensitive to hadronic rescattering, whereas $sc(2,4)$ is more sensitive to the shear viscosity. Rapidity dependence of symmetric cumulants is also shown. A relation between symmetric cumulant and event plane correlation is investigated using AMPT model.

Towards a quantitative understanding of high pT flow harmonics

In this proceedings I briefly review the recent progress achieved on the calculation of vn at high pT via the coupling of a jet energy loss model with full event-by-event viscous hydrodynamics. It is shown that this framework can simultaneously describe experimental data for RAA, v2, and v3 at high pT. High pT v2 is found to be approximately linearly correlated with the soft v2 on an event-by-event basis, which opens up a new way to correlate soft and hard observables in heavy ion collisions.

Influence of jets and decays of resonances on the triangular flow in ultrarelativistic heavy-ion collisions

Triangular flow $v_3$ of identified and inclusive particles in Pb+Pb collisions at $\sqrt{s_{NN}} = 2.76$ TeV is studied as a function of centrality and transverse momentum within the \textsc{hydjet++} model. The model enables one to investigate the influence of both hard processes and final-state interactions on the harmonics of particle anisotropic flow. Decays of resonances are found to increase the magnitude of the $v_3(p_{\rm T})$ distributions at $p_{\rm T} \geq 2$ GeV/$c$ and shift their maxima to higher transverse momenta. The $p_{\rm T}$-integrated triangular flow, however, becomes slightly weakened for all centralities studied. The resonance decays also modify the spectra towards the number-of-constituent-quark scaling fulfillment for the triangular flow, whereas jets are the main source of the scaling violation at the energies available at the CERN Large Hadron Collider (LHC). Comparison with the corresponding spectra of elliptic flow reveals that resonance decays and jets act in a similar manner on both $v_3(p_{\rm T})$ and $v_2(p_{\rm T})$ behavior. Obtained results are also confronted with the experimental data on differential triangular flow of identified hadrons, ratio $v_3^{1/3}(p_{\rm T}) / v_2^{1/2}(p_{\rm T})$ and $p_{\rm T}$-integrated triangular flow of charged hadrons.

Azimuthal Correlations and Mixed Higher Order Flow Harmonics from CMS at the LHC

Azimuthal Correlations and Mixed Higher Order Flow Harmonics from CMS at the LHC

Study of the long-range azimuthal correlations in pp and p+Pb collisions with the ATLAS detector at the LHC

ATLAS measurements of azimuthal correlations between particle pairs at large pseudorapidity separation in pp and p+Pb collisions are presented. The data were collected using a combination of the minimum-bias and high track-multiplicity triggers. The correlations in the “ridge” region are analysed using a new template fit method for extraction of flow harmonics. A comparison of this method with the previously used ZYAM method and a detailed study of the dependence of their results on the charged particle multiplicity and the transverse momentum of the particles forming a pair are shown.

Investigating the correlations of flow harmonics in 2.76A TeV Pb–Pb collisions

This proceeding briefly summarizes our recent investigations on the correlations of flow harmonics in 2.76A TeV Pb–Pb collisions with viscous hydrodynamics VISH2+1. We calculated both the symmetric cumulants SCv(m, n) and the normalized symmetric cumulants NSCv(m, n), and found v2 and v4, v2 and v5, v3 and v5 are correlated, v2 and v3, v3 and v4 are anti-correlated. We also found NSCv(3, 2) are insensitive to the QGP viscosity, which are mainly determined by the initial conditions.

Correlated Event-by-Event Fluctuations of Flow Harmonics in Pb-Pb Collisions at sqrt[s_{NN}]=2.76 TeV.

We report the measurements of correlations between event-by-event fluctuations of amplitudes of anisotropic flow harmonics in nucleus-nucleus collisions, obtained for the first time using a new analysis method based on multiparticle cumulants in mixed harmonics. This novel method is robust against systematic biases originating from nonflow effects and by construction any dependence on symmetry planes is eliminated. We demonstrate that correlations of flow harmonics exhibit a better sensitivity to medium properties than the individual flow harmonics. The new measurements are performed in Pb-Pb collisions at the center-of-mass energy per nucleon pair of sqrt[s_{NN}]=2.76 TeV by the ALICE experiment at the Large Hadron Collider. The centrality dependence of correlation between event-by-event fluctuations of the elliptic v_{2} and quadrangular v_{4} flow harmonics, as well as of anticorrelation between v_{2} and triangular v_{3} flow harmonics are presented. The results cover two different regimes of the initial state configurations: geometry dominated (in midcentral collisions) and fluctuation dominated (in the most central collisions). Comparisons are made to predictions from MonteCarlo Glauber, viscous hydrodynamics, ampt, and hijing models. Together with the existing measurements of the individual flow harmonics the presented results provide further constraints on the initial conditions and the transport properties of the system produced in heavy-ion collisions.

Initial partonic eccentricity fluctuations in a multiphase transport model

Initial partonic eccentricities in Au+Au collisions at center-of-mass energy $\sqrt{s_{NN}}$ = 200 GeV are investigated using a multi-phase transport model with string melting scenario. The initial eccentricities in different order of harmonics are studied using participant and cumulant definitions. Eccentricity in terms of second-, fourth- and sixth order cumulants as a function of number of participant nucleons are compared systematically with the traditional participant definition. The ratio of the cumulant eccentricities $\varepsilon\left\{4\right\}/\varepsilon\left\{2\right\}$ and $\varepsilon\left\{6\right\}/\varepsilon\left\{4\right\}$ are studied in comparison with the ratio of the corresponding flow harmonics. The conversion coefficients ($v_n/\varepsilon_n$) are explored up to fourth order harmonic based on cumulant method. Furthermore, studies on transverse momentum ($p_T$) and pseudo-rapidity ($\eta$) dependencies of eccentricities and their fluctuations are presented. As in ideal hydrodynamics initial eccentricities are expected to be closely related to the final flow harmonics in relativistic heavy-ion collisions, studies of the fluctuating initial condition in the AMPT model will shed light on the tomography properties of the initial source geometry.

Measurement of the correlation between flow harmonics of different order in lead–lead collisions at [formula omitted] with ATLAS

Measurement of the correlation between elliptic or triangular flow coefficients vm (m = 2, 3) and other flow harmonics vn (n = 2 to 5) in sNN=2.76 TeV Pb+Pb collisions using the data collected by the ATLAS experiment at the LHC is presented. The correlations are measured for the different centrality intervals. Each centrality interval is sub divided into event shape classes defined using the ellipticity or triangularity measured in the forward detectors, and for each of the event shape classes the vm−vn correlations are measured in the midrapidity region. The v2 and v3 values show anticorrelation with each other while positive correlations are observed between v2−v4,v2−v5 and v3−v5. The vm−vn correlations (for n = 4, 5) are not reproduced by the ϵm−ϵn correlations predicted by initial geometry models, but are found to be well described by a two component model with a linear response contribution and a nonlinear response contribution which is proportional to v22 in the case of v4 and to v2v3 in the case of v5. The linear and nonlinear contributions are extracted from the measured correlations and compared with results from previous event-plane correlation measurements.

Hydrodynamic fluctuations and dissipation in an integrated dynamical model

We develop a new integrated dynamical model to investigate the effects of the hydrodynamic fluctuations on observables in high-energy nuclear collisions. We implement hydrodynamic fluctuations in a fully 3-D dynamical model consisting of the hydrodynamic initialization models of the Monte-Carlo Kharzeev-Levin-Nardi model, causal dissipative hydrodynamics and the subsequent hadronic cascades. By analyzing the hadron distributions obtained by massive event-by-event simulations with both of hydrodynamic fluctuations and initial-state fluctuations, we discuss the effects of hydrodynamic fluctuations on the flow harmonics, vn and their fluctuations.