arXiv subjects

Ulrich W Heinz

Publications and source records attributed to Ulrich W Heinz.

11 recordsLinked to original sources

arXiv2013

HBT interferometry relative to the triangular flow plane in heavy-ion collisions

The PHENIX Collaboration has reported third-order harmonic oscillations of the source radius parameters when measuring the Hanbury Brown-Twiss correlation function for charged hadrons relative to the triangular flow angle. We explore possible origins of such third-order oscillations with a simple Gaussian source featuring both a triangular geometric deformation and triangular flow. Third-order oscillations of the HBT radii can arise from a purely geometric triangular deformation superimposed on an azimuthally symmetric radial flow, or from a radially symmetry spatial distribution which expands anisotropically with a triangular component in the flow velocity profile. In both cases the final particle momentum distribution features triangular flow. We show that the two alternatives can be distinguished experimentally through the phase of the azimuthal oscillations of the HBT radii relative to the triangular flow plane.

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arXiv2013

Collective flow and viscosity in relativistic heavy-ion collisions

Collective flow, its anisotropies and its event-to-event fluctuations in relativistic heavy-ion collisions, and the extraction of the specific shear viscosity of quark-gluon plasma (QGP) from collective flow data collected in heavy-ion collision experiments at RHIC and LHC are reviewed. Specific emphasis is placed on the similarities between the Big Bang of our universe and the Little Bangs created in heavy-ion collisions.

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arXiv2011

Event-by-event hydrodynamics for heavy-ion collisions

We compare v2/ε2 and v3/ε3 from single-shot and event-by-event (2+1)-dimensional hydrodynamic calculations and discuss the validity of using single-shot calculations as substitutes for event-by-event calculations. Further we present a proof- of-concept calculation demonstrating that v2 and v3 together can be used to strongly reduce initial condition ambiguities.

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arXiv2011

The QGP shear viscosity -- elusive goal or just around the corner?

With the new viscous hydrodynamic + hadron cascade hybrid code VISHNU, a rather precise (O(25%)) extraction of the QGP shear viscosity (eta/s)_QGP from heavy-ion elliptic flow data is possible if the initial eccentricity of the collision fireball is known with <5% accuracy. At this point, eccentricities from initial state models differ by up to 20%, leading to an O(100%) uncertainty for (eta/s)_QGP. It is shown that a simultaneous comparison of elliptic and triangular flow, v_2 and v_3, puts strong constraints on initial state models and can largely eliminate the present uncertainty in (eta/s)_QGP. The variation of the differential elliptic flow v_2(p_T) for identified hadrons between RHIC and LHC energies provides additional tests of the evolution model.

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arXiv2011

Hadron spectra and elliptic flow for 200 A GeV Au+Au collisions from viscous hydrodynamics coupled to a Boltzmann cascade

It is shown that the recently developed hybrid code VISHNU, which couples a relativistic viscous fluid dynamical description of the quark-gluon plasma (QGP) with a microscopic Boltzmann cascade for the late hadronic rescattering stage, yields an excellent description of charged and identified hadron spectra and elliptic flow measured in 200 A GeV Au+Au collisions at the Relativistic Heavy-Ion Collider (RHIC). Using initial conditions that incorporate event-by-event fluctuations in the initial shape and orientation of the collision fireball and values eta/s for the specific shear viscosity of the quark-gluon plasma that were recently extracted from the measured centrality dependence of the eccentricity-scaled, p_T-integrated charged hadron elliptic flow, we obtain universally good agreement between theory and experiment for the p_T-spectra and differential elliptic flow v_2(p_T) for both pions and protons at all collision centralities.

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arXiv2011

Hydrodynamic flow in heavy-ion collisions with large hadronic viscosity

Using the (2+1)-dimensional viscous hydrodynamic code VISH2+1 with a temperature dependent specific shear viscosity (eta/s)(T), we present a detailed study of the influence of a large hadronic shear viscosity and its corresponding relaxation time tau_pi on the transverse momentum spectra and elliptic flow of hadrons produced in 200 A GeV Au+Au collisions. Although theory, in principle, predicts a well-defined relation T*tau_pi = kappa(T)*(eta/s)(T), the precise form of kappa(T) for the matter created in relativistic heavy-ion collisions is not known. For the popular choice kappa=3 the hadron spectra are found to be insensitive to a significant rise of eta/s in the hadronic stage, whereas their differential elliptic flow v_2(p_T) is strongly suppressed by large hadronic viscosity. The large viscous effects on v_2 are strongly reduced if (as theoretically expected) kappa(T) is allowed to grow with decreasing temperature in the hadronic stage. This implies that, until reliable calculations of kappa(T) become available, an extraction of the hadronic shear viscosity from a comparison between VISH2+1 and a microscopic hadron cascade or experimental data requires a simultaneous fit of (eta/s)(T) and kappa(T).

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arXiv2009

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.

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arXiv2009

Interplay of shear and bulk viscosity in generating flow in heavy-ion collisions

We perform viscous hydrodynamic calculations in 2+1 dimensions to investigate the influence of bulk viscosity on the viscous suppression of elliptic flow in non-central heavy-ion collisions at RHIC energies. Bulk and shear viscous effects on the evolution of radial and elliptic flow are studied with different model assumptions for the transport coefficients. We find that the temperature dependence of the relaxation time for the bulk viscous pressure, especially its critical slowing down near the quark-hadron phase transition at T_c, partially offsets effects from the strong growth of the bulk viscosity itself near T_c, and that even small values of the specific shear viscosity eta/s of the fireball matter can be extracted without large uncertainties from poorly controlled bulk viscous effects.

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arXiv2008

Hydrodynamic radial and elliptic flow in heavy-ion collisions from AGS to LHC energies

Using ideal relativistic hydrodynamics in 2+1 dimensions, we study the collision energy dependence of radial and elliptic flow, of the emitted hadron spectra, and of the transverse momentum dependence of several hadronic particle ratios, covering the range from Alternating Gradient Synchrotron (AGS) to Large Hadron Collider (LHC) energies. These calculations establish an ideal fluid dynamic baseline that can be used to assess non-equilibrium features manifest in future LHC heavy-ion experiments. Contrary to earlier suggestions we find that a saturation and even decrease of the differential elliptic flow v_2(p_T) with increasing collision energy cannot be unambiguously associated with the QCD phase transition.

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arXiv2008

Causal relativistic hydrodynamics for viscous fluids

We report on recent results from VISH2+1, a code that solves the relativistic Israel-Stewart equations for causal viscous hydrodynamics for heavy-ion collisions with longitudinal boost invariance. We find that even ``minimal'' shear viscosity eta/s=hbar/(4pi) leads to a large reduction of elliptic flow compared to ideal fluid dynamics. We explore systematically the sensitivity of this reduction to the equation of state, system size, initial conditions, and the microscopic relaxation time in different formulations of the Israel-Stewart equations.

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arXiv2008

Causal viscous hydrodynamics in 2+1 dimensions for relativistic heavy-ion collisions

We explore the effects of shear viscosity on the hydrodynamic evolution and final hadron spectra of Cu+Cu collisions at ultrarelativistic collision energies, using the newly developed (2+1)-dimensional viscous hydrodynamic code VISH2+1. Based on the causal Israel-Stewart formalism, this code describes the transverse evolution of longitudinally boost-invariant systems without azimuthal symmetry around the beam direction. Shear viscosity is shown to decelerate the longitudinal and accelerate the transverse hydrodynamic expansion. For fixed initial conditions, this leads to a longer quark-gluon plasma (QGP) lifetime, larger radial flow in the final state, and flatter transverse momentum spectra for the emitted hadrons compared to ideal fluid dynamic simulations. We find that the elliptic flow coefficient v_2 is particularly sensitive to shear viscosity: even the lowest value allowed by the AdS/CFT conjecture, eta/s=1/4pi, suppresses v_2 enough to have significant consequences for the phenomenology of heavy-ion collisions at the Relativistic Heavy Ion Collider. A comparison between our numerical results and earlier analytic estimates of viscous effects within a blast-wave model parametrization of the expanding fireball at freeze-out reveals that the full dynamical theory leads to much tighter constraints for the specific shear viscosity eta/s, thereby supporting the notion that the quark-gluon plasma created at RHIC exhibits almost ``perfect fluidity''.

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