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Register a new userElliptic flow from a parton cascade
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Bin Zhang
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The dependence of elliptic flow at RHIC energies on the effective parton scattering cross section is calculated using the ZPC parton cascade model. We show that the v_2 measure of elliptic flow saturates early in the evolution before the hadronization transition to a rather large value ~0.05-0.15 as sigma_g varies from 2-10 mb and thus is a sensitive probe of the dynamics in the plasma phase.
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The directed flow of particles produced in ultrarelativistic heavy ion collisions at SPS and RHIC is so small that currently available methods of analysis are at the border of applicability. Standard two-particle and flow-vector methods are biased by large nonflow correlations. On the other hand, cumulants of four-particle correlations, which are free from this bias, are plagued by large statistical errors. Here, we present a new method based on three-particle correlations, which uses the property that elliptic flow is large at these energies. This method may also be useful at intermediate energies, near the balance energy where directed flow vanishes.
A current goal of relativistic heavy ion collisions experiments is the search for a Color Glass Condensate as the limiting state of QCD matter at very high density. In viscous hydrodynamics simulations, a standard Glauber initial condition leads to estimate $4pi eta/s sim 1$, while a Color Glass Condensate modeling leads to at least a factor of 2 larger $eta/s$. Within a kinetic theory approach based on a relativistic Boltzmann-like transport simulation, we point out that the out-of-equilibrium initial distribution proper of a Color Glass Condensate reduces the efficiency in building-up the elliptic flow. Our main result at RHIC energy is that the available data on $v_2$ are in agreement with a $4pi eta/s sim 1$ also for Color Glass Condensate initial conditions, opening the possibility to describe self-consistently also higher order flow, otherwise significantly underestimated, and to pursue further the search for signatures of the Color Glass Condensate.
Cascade solutions of the Boltzmann equation suffer from causality violation at large densities and/or scattering cross sections. Although the particle subdivision technique can reduce the causality violation, it alters event-by-event correlations and fluctuations and is also computationally expensive. Here we evaluate and then improve the accuracy of the ZPC parton cascade for elastic scatterings inside a box without using parton subdivision. We first test different collision schemes for the collision times and ordering time and find that the default collision scheme does not accurately describe the equilibrium momentum distribution at large opacities. We then find a specific collision scheme that can describe very accurately the equilibrium momentum distribution as well as the time evolution towards equilibrium, even at large opacities. We also calculate the shear viscosity and the $eta/s$ ratio of the parton systems and confirm that the new collision scheme is more accurate. In addition, we use a novel parton subdivision method to obtain the exact evolution of the system. This subdivision method is valid for such box calculations and is so much more efficient than the standard subdivision method that we use a subdivision factor of $10^6$ in this study.
The cumulant method is applied to study elliptic flow ($v_2$) in Au+Au collisions at $sqrt{s}=200$AGeV, with the UrQMD model. In this approach, the true event plane is known and both the non-flow effects and event-by-event spatial ($epsilon$) and $v_2$ fluctuations exist. Qualitatively, the hierarchy of $v_2$s from two, four and six-particle cumulants is consistent with the STAR data, however, the magnitude of $v_2$ in the UrQMD model is only 60% of the data. We find that the four and six-particle cumulants are good measures of the real elliptic flow over a wide range of centralities except for the most central and very peripheral events. There the cumulant method is affected by the $v_2$ fluctuations. In mid-central collisions, the four and six-particle cumulants are shown to give a good estimation of the true differential $v_2$, especially at large transverse momentum, where the two-particle cumulant method is heavily affected by the non-flow effects.
The parton and hadron cascade model PACIAE 2.1 (cf. Comput. Phys. Commun.184 (2013) 1476) has been upgraded to the new issue of PACIAE 2.2. By this new issue the lepton-nucleon and lepton-nucleus (inclusive) deep inelastic scatterings can also be investigated. As an example, the PACIAE 2.2 model is enabled to calculate the specific charged hadron multiplicity in the $e^-$+p and $e^-$+D semi-inclusive deep-inelastic scattering at 27.6 GeV electron beam energy. The calculated results are well comparing with the corresponding HERMES data. Additionally, the effect of model parameters alpha and beta in the Lund string fragmentation function on the multiplicity is studied.
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