A detailed study of elliptical and triangular eccentricities in the initial state of relativistic heavy ion collisions is presented. A model of randomly distributed sources of energy density in the transverse plane based on the effective theory of Color Glass Condensate is used. This model describes well the ALICE and ATLAS data for Pb+Pb collisions at center-of-mass energy 5.02 TeV per nucleon pair in a wide range of centralities, if the second $v_2$ and third $v_3$ harmonics of the anisotropic flow are simply implied to be proportional to the eccentricities $\varepsilon_2$ and $\varepsilon_3$ as a reasonable approximation. The eccentricity $\varepsilon_2$ is closely related with the collision geometry and its centrality dependence is mainly determined by the edge diffuseness of the region of the uniform distribution of the saturation pulse of the oncoming nucleus. The eccentricity $\varepsilon_3$ is completely determined by the chaotic fluctuations of the source position in the region of overlapping nuclei and is substantially dependent on the overlap area only.
The triangular flow in ultrarelativistic $^3$He-Au collisions at RHIC energies is enhanced due to the triangular arrangement of the nucleon configurations in $^3$He. We study the fireball eccentricities in the Glauber Monte Carlo model and find that since the configurations of the projectile $^3$He are elongated triangles, the created fireball has a large ellipticity and a smaller triangularity. The dependence of the triangularity on centrality is weak, so it cannot be extracted from the centrality dependence of the triangular flow $v_3$, as it is dominated by the centrality dependence of the …
In this paper we propose to thoroughly investigate asymmetric nuclear collisions both in the fixed target mode at the laboratory energy below 5 GeV per nucleon and in the collider mode with a center of mass energy below 11 GeV per nucleon. Using the UrQMD transport model, we demonstrate a strong enhancement of directed and elliptic flow coefficients for the midcentral asymmetric nuclear collisions compared to symmetric collisions. We argue that such an enhancement is due to the disappearance of the nuclear shadowing effect on the side of the smaller projectile nucleus. An analysis of the energ…
We investigate the elliptic and the triangular flow of heavy mesons in ultrarelativistic heavy-ion collisions at RHIC and the LHC. The dynamics of heavy quarks is coupled to the locally thermalized and fluid dynamically evolving quark-gluon plasma. The elliptic flow of $D$ mesons and the centrality dependence measured at the LHC is well reproduced for purely collisional and bremsstrahlung interactions. Due to the event-by-event fluctuating initial conditions from the EPOS2 model, the $D$ meson triangular flow is predicted to be nonzero at $\sqrt{s}=200$ GeV and $\sqrt{s}=2.76$ TeV. We study th…
We carry out a combined analysis of elliptic and triangular flow data using viscous relativistic hydrodynamics. We show that these data allow to put tight constraints on models of the early dynamics of a nucleus-nucleus collision. Specifically, the rms values of the initial ellipticity $\varepsilon_2$ and the initial triangularity $\varepsilon_3$ are constrained to lie within a narrow band for each centrality. We use these constraints as a filter for existing Monte-Carlo models of initial state, and provide a simple test that can be performed on any candidate model to determine its compatibili…
The multihadron production in nucleus-nucleus collisions and its interrelation with that in (anti)proton-proton interactions are studied by exploring the charged particle mean multiplicity collision-energy and centrality dependencies in the measurements to date. The study is performed in the framework of the recently proposed effective-energy approach which, under the proper scaling of the collision energy, combines the constituent quark picture with Landau relativistic hydrodynamics counting for the centrality-defined effective energy of participants and relating different types of collisions…
We studied the relation between the initial geometry anisotropy and the anisotropic flow in a multiphase transport model (AMPT) for both Au+Au and Cu+Cu collisions at $\sqrt{s_{NN}}$ = 200 GeV. It is found that unlike the elliptic flow $v_2$, little centrality dependence of the triangular flow $v_3$ is observed. After removing the initial geometry effect, $v_3/ε_3$ increases with the transverse particle density, which is similar to $v_2/ε_2$. The transverse momentum ($p_T$) dependence of $v_3$ from identified particles is qualitatively similar to the $p_T$ dependence of $v_2$.
The first calculation of triangular flow $v_3$ in Au+Au collisions at $\sqrt{s_{\rm NN}}=200A$ GeV from an event-by-event (3+1)-d transport+hydrodynamics hybrid approach is presented. As a response to the initial triangularity $ε_3$ of the collision zone, $v_3$ is computed in a similar way to the standard event-plane analysis for elliptic flow $v_2$. It is found that the triangular flow exhibits weak centrality dependence and is roughly equal to elliptic flow in most central collisions. We also explore the transverse momentum and rapidity dependence of $v_2$ and $v_3$ for charged particles as …
We introduce the concepts of participant triangularity and triangular flow in heavy-ion collisions, analogous to the definitions of participant eccentricity and elliptic flow. The participant triangularity characterizes the triangular anisotropy of the initial nuclear overlap geometry and arises from event-by-event fluctuations in the participant-nucleon collision points. In studies using a multi-phase transport model (AMPT), a triangular flow signal is observed that is proportional to the participant triangularity and corresponds to a large third Fourier coefficient in two-particle azimuthal …
We present results for the elliptic and triangular flow coefficients in Au+Au collisions at root-s=200 AGeV using event-by-event (3+1)D viscous hydrodynamic simulations. We study the effect of initial state fluctuations and finite viscosities on the flow coefficients v_2 and v_3 as functions of transverse momentum and pseudo-rapidity. Fluctuations are essential to reproduce the measured centrality dependence of elliptic flow. We argue that simultaneous measurements of v_2 and v_3 can determine eta/s more precisely.
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 sl…
A simple geometrical model with event-by-event fluctuations is suggested to study elliptical and triangular eccentricities in the initial state of relativistic heavy-ion collisions. This model describes rather well the ALICE and ATLAS data for Pb+Pb collisions at center-of-mass energy $\sqrt{s_{NN}} = 5.02$~TeV per nucleon pair, assuming that the second, $v_2$, and third, $v_3$, harmonics of the anisotropic flow are simply linearly proportional to the eccentricities $\varepsilon_2$ and $\varepsilon_3$, respectively. We show that the eccentricity $\varepsilon_3$ has a pure fluctuation origin an…
We apply the hydrodynamic model for the dynamics of matter created in p-Pb collisions at 4.4TeV and d-Pb collisions at 3.11TeV. The fluctuating initial conditions are calculated in the Glauber Monte-Carlo model for several centrality classes. The expansion is performed event by event in 3+1-dimensional viscous hydrodynamics. Noticeable elliptic and triangular flows appear in the distributions of produced particles.
Anomalous centrality evolution of two-particle angular correlations observed in Au-Au collisions at $\sqrt{s_{NN}} = 62$ and 200 GeV and the onset of ridge structures are considered in the model of interacting quark-gluon strings. We assume that at the given energy of nucleus-nucleus collisions the critical energy density may be reached at the specific centrality. In a string percolation model this might be treated equivalently to a formation of a large cluster of strings characterized by the critical string density, with a size comparable to the whole area of interaction of two nuclei. This h…
The ATLAS Collaboration has measured for the first time the fourth cumulant of quadrangular flow, $v_4\{4\}^4$. Unlike the fourth cumulants of elliptic and triangular flows, it presents a change of sign above 30\% centrality. We show that this change of sign is predicted by event-by-event hydrodynamics. We argue that it results from the combined effects of a nonlinear hydrodynamic response, which couples quadrangular flow to elliptic flow, and elliptic flow fluctuations.
We perform a systematic study on the decorrelation of anisotropic flows along the pseudorapidity in relativistic heavy-ion collisions at the LHC and RHIC energies. The dynamical evolution of the QGP fireball is simulated via the CLVisc (ideal) (3+1)-dimensional hydrodynamics model, with the fully fluctuating initial condition from A-Multi-Phase-Transport (AMPT) model. Detailed analysis is performed on the longitudinal decorrelations of elliptic, triangular and quadrangular flows in terms of flow vectors, flow magnitudes and flow orientations (event planes). It is found that pure flow magnitude…
We study the development of heavy-flavor flow harmonics in high-energy nuclear collisions. The elliptic and triangular flow of heavy-flavor hadrons, arising from the finite impact parameter of the two nuclei and from event-by-event fluctuations of the initial geometry, is analyzed in detail, considering the contribution from particles decoupling from the fireball at various times. We also study the dependence of the flow harmonics on the event-shape fluctuations, considering events belonging to the same centrality class but characterized by very different eccentricities (or vice-versa).
In this paper, the centrality dependent $\Zn$ and $\Wpm$ production and the isospin effect in $\Wpm$ production are investigated with a parton and hadron cascade model PACIAE in Pb--Pb collisions at $\sNN=5.02$~TeV. ALICE data of $\Zn$ production in Pb--Pb collisions at $\sNN=5.02$~TeV are found to be reproduced fairly well. The prediction on $\Wpm$ production in the same collision system is given as well. An interesting isospin effect is observed in exploring the charge asymmetry between $\Wp$ and $\Wm$ as a function of the asymmetry between number of valence $u$- and $d$-quarks varied from s…
The first results from heavy ion collisions at the Large Hadron Collider for charged particle spectra and elliptic flow are compared to an event-by-event hybrid approach with an ideal hydrodynamic expansion. This approach has been shown to successfully describe bulk observables at RHIC. Without changing any parameters of the calculation the same approach is applied to Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. This is an important test if the established understanding of the dynamics of relativistic heavy ion collisions is also applicable at even higher energies. Specifically, we employ…
We present a simple description of the energy density profile created in a nucleus-nucleus collision, motivated by high-energy QCD. The energy density is modeled as the sum of contributions coming from elementary collisions between localized charges and a smooth nucleus. Each of these interactions creates a sharply-peaked source of energy density falling off at large distances like $1/r^2$, corresponding to the two-dimensional Coulomb field of a point charge. Our model reproduces the one-point and two-point functions of the energy density field calculated in the framework of the color glass co…
The LHC data on azimuthal anisotropy harmonics from PbPb collisions at center-of-mass energy 2.76 TeV per nucleon pair are analyzed and interpreted in the framework of the HYDJET++ model. The cross-talk of elliptic $v_2$ and triangular $v_3$ flow in the model generates both even and odd harmonics of higher order. Comparison with the experimental data shows that this mechanism is able to reproduce the $p_{\rm T}$ and centrality dependencies of quadrangular flow $v_4$, and also the basic trends for pentagonal $v_5$ and hexagonal $v_6$ flows.
We study, within the Monte-Carlo Glauber model, centrality dependence of the midrapidity charged multiplicity density $dN_{ch}/dη$ and of the anisotropy coefficients $\varepsilon_{2,3}$ in Pb+Pb collisions at $\sqrt{s}=5.02$ TeV and in Xe+Xe collisions at $\sqrt{s}=5.44$ TeV. Calculations are performed for versions with and without nucleon meson cloud. The fraction of the binary collisions, $α$, has been fitted to the data on $dN_{ch}/dη$ in Pb+Pb collisions. We obtain $α\approx 0.09(0.13)$ with (without) meson cloud. The effect of meson cloud on the $dN_{ch}/dη$ is relatively small. For Xe+Xe…
On the Role of Prompt Photons in the Anisotropic Emission of Direct Photons -- Direct Photons from Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV with IP-Glasma Initial Condition
Institute of Particle Physics, Central China Normal University（华中师范大学粒子物理研究所）
Higher harmonics of anisotropic flow ($v_n$ with $n\ge 4$) in heavy-ion collisions can be measured either with respect to their own plane, or with respect to a plane constructed using lower-order harmonics. We explain how such measurements are related to event-plane correlations. We show that CMS data on $v_4$ and $v_6$ are compatible with ATLAS data on event-plane correlations. If one assumes that higher harmonics are the superposition of non-linear and linear responses, then the linear and non-linear parts can be isolated under fairly general assumptions. By combining analyses of higher harm…
Fluctuations in the initial transverse energy-density distribution lead to anisotropic flows as observed in central high-energy heavy-ion collisions. Studies of longitudinal fluctuations of the anisotropic flows can shed further light on the initial conditions and dynamical evolution of the hot quark-gluon matter in these collisions. Correlations between anisotropic flows with varying pseudorapidity gaps in Pb+Pb collisions at the CERN Large Hadron Collider are investigated using both an event-by-event (3+1)-D ideal hydrodynamical model with fluctuating initial conditions and the a multiphase …
The observation of large azimuthal anisotropy or $v_2$ for hadrons above $p_T>5$ GeV/$c$ in Au+Au collisions at $\sqrt{s_{\rm nn}}=200$ GeV has been a longstanding challenge for jet quenching models based on perturbative QCD (pQCD). Using a simple jet absorption model, we seek to clarify the situation by exploring in detail how the calculated $v_2$ varies with choices of the collision geometry as well as choices of the path length dependence and thermalization time $τ_0$ in the energy loss formula. Besides the change of eccentricity due to distortion from gluon saturation or event-by-event flu…
We present the results of an improved Monte Carlo Glauber (MCG) model of relevance for collisions involving nuclei at center-of-mass energies of BNL RHIC ($\sqrt{s_{\rm NN}}=0.2$ TeV), CERN LHC ($\sqrt{s_{\rm NN}}=2.76$-$8.8$ TeV), and proposed future hadron colliders ($\sqrt{s_{\rm NN}}\approx 10$-$63$ TeV). The inelastic pp cross sections as a function of $\sqrt{s_{\rm NN}}$ are obtained from a precise data-driven parametrization that exploits the many available measurements at LHC collision energies. We describe the nuclear transverse profile with two separated 2-parameter Fermi distributio…
Since their discovery, fluctuations in the initial state of heavy-ion collisions have been understood as originating mostly from the random positions of nucleons within the colliding nuclei. We consider an alternative approach where all the focus is on fluctuations generated by QCD interactions, that we evaluate at leading logarithmic accuracy in the color glass condensate effective theory. We validate our approach using BNL Relativistic Heavy Ion Collider (RHIC) and CERN Large Hadron Collider (LHC) data on anisotropic flow. In particular, we show that, compared to standard Glauber-inspired ca…
In the hydrodynamic model description of heavy ion collisions, the elliptic flow $v_2$ and triangular flow $v_3$ are sensitive to the quadrupole deformation $β_2$ and octupole deformation $β_3$ of the colliding nuclei. The relations between $v_n$ and $β_n$ have recently been clarified and were found to follow a simple parametric form. The STAR Collaboration have just published precision $v_n$ data from isobaric $^{96}$Ru+$^{96}$Ru and $^{96}$Zr+$^{96}$Zr collisions, where they observe large differences in central collisions $v_{2,\mathrm{Ru}}>v_{2,\mathrm{Zr}}$ and $v_{3,\mathrm{Ru}}<v_{3,\mat…
$α$-clustered structures in light nuclei could be studied through "snapshots" taken by relativistic heavy-ion collisions. A multiphase transport (AMPT) model is employed to simulate the initial structure of collision nuclei and the proceeding collisions at center of mass energy $\sqrt{s_{NN}}$ = 6.37 TeV. This initial structure can finally be reflected in the subsequent observations, such as elliptic flow ($v_{2}$), triangular flow ($v_{3}$) and quadrangular flow ($v_4$). Three sets of the collision systems are chosen to illustrate system scan is a good way to identify the exotic $α$-clustered…
Probing initial geometrical anisotropy and final azimuthal anisotropy in heavy-ion collisions at Large Hadron Collider energies through event-shape engineering
Exploring the chiral magnetic effect in Au+Au collisions at $\sqrt{s_{NN}}=7.7-200$ GeV through Chiral Anomaly Transport
University of Chinese Academy of Sciences（中国科学院大学）；Institue of High Energy Physics, Chinese Academy of Sciences（中国科学院高能物理研究所）；China University of Mining and Technology（中国矿业大学）；Fudan University（复旦大学）
Electromagnetic probes such as direct photons and dileptons are crucial to study the properties of a Quark-Gluon Plasma (QGP) created in heavy ion collisions. Based on the (3+1)-dimensional event-by-event viscous hydrodynamic model EPOS 3.102, we calculated the anisotropic emission of thermal photons and dileptons in Au+Au collisions at the Relativistic Heavy Ion Collider (RHIC) energy $\sqrt{s_{NN}}=200$~GeV. Thermal emissions from both QGP phase and hadronic gas phase are considered, with AMY rate for photons and Lattice QCD based rates for dileptons in QGP phase. For emission from hadron ga…
Jet substructure observables are crucial for exploring the effects of the hot, dense medium and differentiating between quark and gluon jets. In this paper, we investigate the modification of jet shape by calculating the differential girth at LHC energies, ranging from pp to Pb--Pb collisions. The differential girth distribution exhibits a wave-like pattern, which offers a better explanation for the radial spreading of jet energy and large-angle radiation effects. We fit the differential girth distribution with a sine wave function to obtain the amplitude, angular frequency, initial phase, and…
