Energy Dependence Of Elliptic Flow Research Articles

How much does the hadronic phase contribute to the observed anisotropic flow at the LHC?

Elliptic flow signals the presence of multiple interactions between the constituents of the created matter in heavy-ion collisions. This includes possible contributions from the different phases, including the hadronic phase. In these proceedings I will first show that the energy dependence of elliptic flow, based on recent ALICE and STAR beam energy scan measurements, can largely be understood in terms of a boosted thermal system. In addition, a detailed comparison between the identified particle elliptic flow measured by the ALICE collaboration and viscous hydrodynamical model calculations with and without a hadronic afterburner is performed to constrain the possible effects of individual hadron-hadron re-interaction cross-sections.

Centrality and energy dependence of elliptic flow of light nuclei and hadrons in STAR

We present the measurement of azimuthal anisotropy in the distributions of deuteron ( d ), anti-deuteron ( d ¯ ) , triton ( t ) and helium ( 3 He) nuclei produced at midrapidity ( | η | < 1.0 ) in heavy-ion collisions at s NN = 200 , 62.4 , 39 , 27 , 19.6 , 11.5 and 7.7 GeV . We also present systematic study of the difference between particle and anti-particle elliptic flow ( Δ v 2 ) for identified hadrons ( π ± , K ± , p , p ¯ , Λ , Λ ¯ and Ξ ± ) at mid-rapidity for Au+Au collisions at RHIC Beam Energy Scan (BES) energies. Elliptic flow coefficient v 2 of nuclei has been compared with those for p , p ¯ . We observe mass ordering in nuclei v 2 ( p T ) for low transverse momenta ( p T < 2.0 GeV / c ) and a clear centrality dependence. The triton and helium, having almost the same mass, show similar magnitude of v 2 within the available event statistics. Mass number ( A ) scaling of nuclei v 2 ( p T ) has been observed, which indicates the possibility of coalescence mechanism to be the underlying physics behind nuclei formation in heavy ion collisions. New results on Δ v 2 for all particles, studied in 10–40% centrality, show similar trend as was observed for minimum bias (0–80%) events. The proton and anti-proton Δ v 2 for three different collision centralities is presented as a function of s NN .

Evolution of elliptic and triangular flow as a function ofsNNin a hybrid model

We study the collision energy dependence of elliptic flow v_2 and triangular flow v_3 in Au+Au collisions within the energy range sqrt(s_{NN}) = 5-200 GeV, utilizing a transport + hydrodynamics hybrid model. The transport part is described by the Ultrarelativistic Quantum Molecular Dynamics (UrQMD) approach, combined with an intermediate (3+1)-dimensional ideal hydrodynamical evolution phase using a chiral model equation of state. We find the decrease of v_2 produced by hydrodynamics at lower collision energies partially compensated by the transport dynamics. This does not apply to v_3, which falls to 0 in midcentral collisions at sqrt(s_{NN}) = 5 GeV. We conclude that the triangular flow provides the clearer signal for the formation of low-viscous fluid in heavy ion collisions.

Energy dependence of elliptic flow of $\phi$-meson in STAR at RHIC

We present the measurement of elliptic flow ($v_{2}$) as a function of transverse momentum ($p_{T}$) for the $\phi$-meson in Au+Au collisions at $\sqrt{s_{NN}}$ = 7.7, 11.5 and 39 GeV. At low $p_{T}$ $\phi$-meson $v_{2}$ decreases with decrease in beam energy. The number of constituent quark ($n_{\rm{cq}}$) scaled $v_{2}$ for $\phi$-meson vs ($m_{T} - m_{0}$)/$n_{\rm{cq}}$ follows similar trend as other hadrons for $\sqrt{s_{NN}}$= 39 GeV whereas the $n_{\rm{cq}}$ scaled $v_{2}$ for $\phi$-mesons follows a different trend compared to the other hadrons at $\sqrt{s_{NN}}$= 11.5 GeV.

Energy dependence of elliptic flow from heavy-ion collision models

We have compared the experimental data on charged particle elliptic flow parameter (v2) in Au+Au collisions at midrapidity for \surd sNN = 9.2, 19.6, 62.4 and 200 GeV with results from various models in heavy-ion collisions like UrQMD, AMPT, and HIJING. We observe that the average <v2> from the transport model UrQMD agrees well with the measurements at \surd sNN = 9.2 GeV but increasingly falls short of the experimental <v2> values as the beam energy increases. The difference in <v2> being of the order of 60% at \surd sNN = 200 GeV. The <v2> results from HIJING is consistent with zero, while those from AMPT with default settings, a model based on HIJING with additional initial and final state rescattering effects included, gives a <v2> value of about 4% for all the beam energies studied. This is in contrast to increase in <v2> with beam energy for the experimental data. A different version of the AMPT model, which includes partonic effects and quark coalescence as a mechanism of hadronization, gives higher values of <v2> among the models studied and is in agreement with the measured <v2> values at \surd sNN = 200 GeV. These studies show that the experimental < v2 > has substantial contribution from partonic interactions at \surd sNN = 200 GeV whose magnitude reduces with decrease in beam energy. We also compare the available data on the transverse momentum and pseudorapidity dependence of v2 to those from the above models.

Can the energy dependence of elliptic flow reveal the QGP phase transition?

Ideal hydrodynamic simulations are performed to compute the evolution with collision energy of hadron spectra and elliptic flow between AGS and LHC energies. We argue that viscous effects should decrease with increasing energy, improving the applicability of ideal fluid dynamics at higher energies. We show that the increasing radial flow at higher energies pushes the elliptic flow to larger transverse momenta, leading to a peaking and subsequent decrease of the elliptic flow at fixed p T with increasing collision energy, independent of whether or not there is a phase transition in the equation of state.

Energy Dependence of Elliptic Flow over a Large Pseudorapidity Range inAu+AuCollisions at the BNL Relativistic Heavy Ion Collider

This Letter describes the measurement of the energy dependence of elliptic flow for charged particles in Au+Au collisions using the PHOBOS detector at the Relativistic Heavy Ion Collider. Data taken at collision energies of square root of s(NN)=19.6, 62.4, 130, and 200 GeV are shown over a wide range in pseudorapidity. These results, when plotted as a function of eta(')=|eta|-y(beam), scale with approximate linearity throughout eta('), implying no sharp changes in the dynamics of particle production as a function of pseudorapidity or increasing beam energy.

Systematics of elliptic flow in heavy-ion collisions

We analyze elliptic flow from SIS to RHIC energies systematically in a realistic dynamical cascade model. We compare our results with the recent data from STAR and PHOBOS collaborations on elliptic flow of charged particles at midrapidity in Au+ Au collisions at RHIC. In the analysis of elliptic flow at RHIC energy, we find a good fitting with data at 1.5 times a scaling factor to our model, which characterizes that the model is required to have extra pressure generated from the subsequent parton scattering after the initial minijet production. In energy dependence of elliptic flow, we notice re-hardening nature at RHIC energies. Both these two observations would probably imply the possible formation of quark-gluon plasma.