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Register a new userHadronic effects on charmonium elliptic flows in heavy-ion collisions
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Transport and Langevin equations are employed to study hadronic medium effects on charmonium elliptic flows in heavy-ion collisions. In Pb-Pb collisions, the anisotropic energy density of the quark-gluon plasma (QGP) in the transverse plane is transformed into hadron momentum anisotropy after the phase transition. Charmonia with high transverse momentum $p_T$ are produced via the primordial hard process and undergo different degrees of dissociation along different paths in the QGP. They then scatter with light hadrons in the hadron phase. Both contributions to the charmonium elliptic flows are studied at moderate and high transverse momenta. The elliptic flows of the prompt $J/psi$ are found to be considerably enhanced at high transverse momentum when the charmonium diffusion coefficients in the hadronic medium are parametrized through the geometry scale approximation. This hadronic medium effect is negligible for quarkonia with larger mass such as bottomonia.
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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 and is substantially dependent on the size of the overlap area only, while the eccentricity $varepsilon_2$ is mainly related to the average collision geometry. Elliptic flow, therefore, is weakly dependent on the event-by-event fluctuations everywhere except of the very central collisions 0--2%, whereas triangular flow is mostly determined by the fluctuations. The scaling dependence of the magnitude of the flow harmonics on atomic number, $v_n propto A^{-1/3}$, is predicted for this centrality interval.
We study effects of eccentricity fluctuations on the elliptic flow coefficient v_2 at mid-rapidity in both Au+Au and Cu+Cu collisions at sqrt{s_NN}=200 GeV by using a hybrid model that combines ideal hydrodynamics for space-time evolution of the quark gluon plasma phase and a hadronic transport model for the hadronic matter. For initial conditions in hydrodynamic simulations, both the Glauber model and the color glass condensate model are employed to demonstrate the effect of initial eccentricity fluctuations originating from the nucleon position inside a colliding nucleus. The effect of eccentricity fluctuations is modest in semicentral Au+Au collisions, but significantly enhances v_2 in Cu+Cu collisions.
A significant excess of J/$psi$ yield at very low transverse momentum ($p_T < 0.3$ GeV/c) was observed by the ALICE and STAR collaborations in peripheral A+A collisions, which points to evidence of coherent photoproduction of J/$psi$ in violent hadronic interactions. Theoretically, the photoproduction of J$/psi$ in hadronic collisions raises questions about how spectator and non-spectator nucleons participate in the coherent reaction. For the first time, we argue that the strong interactions in the overlapping region of incoming nuclei may disturb the coherent production, leaving room for different coupling assumptions. Furthermore, first considerations of the destructive interference between photoproduction on ions moving in opposite directions in hadronic heavy-ion collisions are included. This letter presents calculations of J$/psi$ production from coherent photon-nucleus ($gamma + A rightarrow text{J}/psi + A$) interactions in hadronic A+A collisions at RHIC and LHC energies with both nucleus and spectator coupling hypotheses. The coherent J/$psi$ yield as a function of centrality and differential distributions as a function of transverse momentum, azimuthal angle and rapidity in different centrality bins are shown and found to be significantly different for different coupling scenarios, calling for future experimental measurements.
The direct photon spectra and flow ($v_2$, $v_3$) in heavy-ion collisions at SPS, RHIC and LHC energies are investigated within a relativistic transport approach incorporating both hadronic and partonic phases -- the Parton-Hadron-String Dynamics (PHSD). In the present work, four extensions are introduced compared to our previous calculations: (i) going beyond the soft-photon approximation (SPA) in the calculation of the bremsstrahlung processes $meson+mesonto meson+meson+gamma$, (ii) quantifying the suppression due to the Landau-Pomeranchuk-Migdal (LPM) coherence effect, (iii) adding the additional channels $V+Nto N+gamma$ and $Deltato N+gamma$ and (iv) providing predictions for Pb+Pb collisions at $sqrt{s_{NN}}$ = 2.76 TeV. The first issue extends the applicability of the bremsstrahlung calculations to higher photon energies in order to understand the relevant sources in the region $p_T=0.5-1.5$ GeV, while the LPM correction turns out to be important for $p_T<0.4$ GeV in the partonic phase. The results suggest that a large elliptic flow $v_2$ of the direct photons signals a significant contribution of photons produced in interactions of secondary mesons and baryons in the late (hadronic) stage of the heavy-ion collision. In order to further differentiate the origin of the direct photon azimuthal asymmetry (late hadron interactions vs electromagnetic fields in the initial stage), we provide predictions for the triangular flow $v_3(p_T)$ of direct photons. Additionally, we illustrate the magnitude of the photon production in the partonic and hadronic phases as functions of time and local energy density. Finally, the cocktail method for an estimation of the background photon elliptic flow, which is widely used in the experimental works, is supported by the calculations within the PHSD transport approach.
Higher-order anisotropic flows in heavy-ion collisions are affected by nonlinear mode coupling effects. It has been suggested that the associated nonlinear hydrodynamic response coefficients probe the transport properties and are largely insensitive to the spectrum of initial density fluctuations of the medium created in these collisions. To test this suggestion, we explore nonlinear mode coupling effects in event-by-event viscous fluid dynamics, using two different models for the fluctuating initial density profiles, and compare the nonlinear coupling coefficients between the initial eccentricity vectors before hydrodynamic expansion and the final flow vectors after the expansion. For several mode coupling coefficients we find significant sensitivity to the initial fluctuation spectrum. They all exhibit strong sensitivity to the specific shear viscosity at freeze-out, but only weak dependence on the shear viscosity during hydrodynamic evolution.
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