arXiv subjects

Pasi Huovinen

Publications and source records attributed to Pasi Huovinen.

At least 37 records · Page 2Linked to original sources

arXiv2018

Temperature dependence of $η/s$: uncertainties from the equation of state

We perform a global model-to-data comparison on Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV and Pb+Pb collisions at $2.76$ TeV and $5.02$ TeV, using a 2+1D hydrodynamics model with the EKRT initial state and a shear viscosity over entropy density ratio $(η/s)(T)$ with a linear $T$ dependence. To quantify the amount of uncertainty due to the choice of the equation of state (EoS), we compare analysis results based on four different EoSs: the well known $s95p$ parametrisation, an updated parametrisation based on the same list of particles in hadron resonance gas, but using recent lattice results for the partonic part of the EoS, and two new parametrisations based on the Particle Data Group 2016 particle list and the recent lattice results. We find that the choice of the EoS does affect the favoured minimum value of $η/s$, although within the confidence limits of the analysis. On the other hand, our analysis hardly constrains the temperature dependence of $η/s$, no matter the EoS.

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arXiv2017

Dynamical freeze-out criterion in a hydrodynamical description of Au + Au collisions at $\sqrt{s_\mathrm{NN}}=200$ GeV and Pb + Pb collisions at $\sqrt{s_\mathrm{NN}}=2760$ GeV

In hydrodynamical modeling of ultrarelativistic heavy-ion collisions, the freeze-out is typically assumed to take place at a surface of constant temperature or energy density. A more physical approach is to assume that freeze-out takes place at a surface of constant Knudsen number. We evaluate the Knudsen number as a ratio of the expansion rate of the system to the pion scattering rate, and apply the constant Knudsen number freeze-out criterion to ideal hydrodynamical description of heavy-ion collisions at RHIC ($\sqrt{s_\mathrm{NN}}=200$ GeV) and the LHC ($\sqrt{s_\mathrm{NN}}=2760$ GeV) energies. We see that once the numerical values of freeze-out temperature and freeze-out Knudsen number are chosen to produce similar $p_T$ distributions, the elliptic and triangular anisotropies are similar too, in both event-by-event and averaged initial state calculations.

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arXiv2017

Effects of $ρ$-meson width on pion distributions in heavy-ion collisions

The influence of the finite width of $ρ$ meson on the pion momentum distribution is studied quantitatively in the framework of the S-matrix approach combined with a blast-wave model to describe particle emissions from an expanding fireball. We find that the proper treatment of resonances which accounts for their production dynamics encoded in data for partial wave scattering amplitudes can substantially modify spectra of daughter particles originating in their two body decays. In particular, it results in an enhancement of the low-$p_T$ pions from the decays of $ρ$ mesons which improves the quantitative description of the pion spectra in heavy ion collisions obtained by the ALICE collaboration at the LHC energy.

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arXiv2016

Violation of mass ordering for multi-strange hadrons at RHIC and LHC

We study effects of the hadronic rescattering on final observables especially for multi-strange hadrons such as $ϕ$, $Ξ$ and $Ω$ in high-energy heavy-ion collisions within an integrated dynamical approach. In this approach, (3+1)-dimensional ideal hydrodynamics is combined with a microscopic transport model, JAM. We simulate the collisions with or without hadronic rescatterings and compare observables between these two options so that we quantify the effects of the hadronic rescattering. We find that the mean transverse momentum and the elliptic flow parameter of multi-strange hadrons are less affected by hadronic rescattering and, as a result, the mass ordering of the $p_T$-differential elliptic flow parameter $v_2(p_T)$ is violated: At the RHIC and the LHC energies the $v_2(p_T)$ for $ϕ$-mesons is larger than that for protons in the low-$p_T$ regions.

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arXiv2015

Effects of hadronic rescattering on multistrange hadrons in high-energy nuclear collisions

We study the effects of hadronic rescattering on hadron distributions in high-energy nuclear collisions by using an integrated dynamical approach. This approach is based on a hybrid model combining (3+1)-dimensional ideal hydrodynamics for the quark gluon plasma (QGP), and a transport model for the hadron resonance gas. Since the hadron distributions are the result of the entire expansion history of the system, understanding the QGP properties requires investigating how rescattering during the hadronic stage affects the final distributions of hadrons. We include multistrange hadrons in our study, and quantify the effects of hadronic rescattering on their mean transverse momenta and elliptic flow. We find that multistrange hadrons scatter less during the hadronic stage than nonstrange particles, and thus their distributions reflect the properties of the system in an earlier stage than the distributions of nonstrange particles.

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arXiv2014

Influence of temperature dependent shear viscosity on elliptic flow at back- and forward rapidities in ultrarelativistic heavy-ion collisions

We explore the influence of a temperature-dependent shear viscosity over entropy density ratio $η/s$ on the azimuthal anisotropies v_2 and v_4 of hadrons at various rapidities. We find that in Au+Au collisions at full RHIC energy, $\sqrt{s_{NN}}=200$ GeV, the flow anisotropies are dominated by hadronic viscosity at all rapidities, whereas in Pb+Pb collisions at the LHC energy, $\sqrt{s_{NN}}=2760$ GeV, the flow coefficients are affected by the viscosity both in the plasma and hadronic phases at midrapidity, but the further away from midrapidity, the more dominant the hadronic viscosity is. We find that the centrality and rapidity dependence of the elliptic and quadrangular flows can help to distinguish different parametrizations of $(η/s)(T)$. We also find that at midrapidity the flow harmonics are almost independent of the decoupling criterion, but show some sensitivity to the criterion at back- and forward rapidities.

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arXiv2014

Equation of state at finite net-baryon density using Taylor coefficients up to sixth order

We employ the lattice QCD data on Taylor expansion coefficients up to sixth order to construct an equation of state at finite net-baryon density. When we take into account how hadron masses depend on lattice spacing and quark mass, the coefficients evaluated using the p4 action are equal to those of hadron resonance gas at low temperature. Thus the parametrised equation of state can be smoothly connected to the hadron resonance gas equation of state. We see that the equation of state using Taylor coefficients up to second order is realistic only at low densities, and that at densities corresponding to s/n_B > 40, the expansion converges by the sixth order term.

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arXiv2013

Hydrodynamics at RHIC and LHC: What have we learned?

Fluid dynamical description of elementary particle collisions has a long history dating back to the works of Landau and Fermi. Nevertheless, it is during the last 10-15 years when fluid dynamics has become the standard tool to describe the evolution of matter created in ultrarelativistic heavy ion collisions. In this review I briefly describe the hydrodynamical models, what we have learned when analyzing the RHIC and LHC data using these models, and what the latest developments and challenges are.

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arXiv2013

Dynamical freeze-out criterion in event-by-event hydrodynamics

In hydrodynamical modelling of ultrarelativistic heavy-ion collisions the freeze-out is typically assumed to take place on a surface of constant temperature or energy density. In this work we apply a dynamical freeze-out criterion, which compares the hydrodynamical expansion rate with the pion scattering rate, to an event-by-event ideal hydrodynamics at the full RHIC collision energy. We present hadron spectra and elliptic flow calculated using (2+1)-dimensional ideal hydrodynamics, and show the differences between constant temperature and dynamical freeze-out criteria. We find that when the freeze-out ratio is fixed to one, the different freeze-out criteria lead to slightly different spectra and v_2(p_T) in the event-by-event calculations.

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arXiv2013

Integrated Dynamical Approach to Relativistic Heavy Ion Collisions

We review integrated dynamical approaches to describe heavy ion reaction as a whole at ultrarelativistic energies. Since final observables result from all the history of the reaction, it is important to describe all the stages of the reaction to obtain the properties of the quark gluon plasma from experimental data. As an example of these approaches, we develop an integrated dynamical model, which is composed of a fully (3+1) dimensional ideal hydrodynamic model with the state-of-the-art equation of state based on lattice QCD, and subsequent hadronic cascade in the late stage. Initial conditions are obtained employing Monte Carlo versions of the Kharzeev-Levin-Nardi model (MC-KLN) or the Glauber model (MC-Glauber). Using this integrated model, we first simulate relativistic heavy ion collisions at the RHIC and LHC energies starting from conventional smooth initial conditions. We next utilise each Monte Carlo samples of initial conditions on an event-by-event basis and perform event-by-event dynamical simulations to accumulate a large number of minimum bias events. A special attention is paid to performing the flow analysis as in experiments toward consistent comparison of theoretical results with experimental data.

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arXiv2012

Particlization in hybrid models

In hybrid models, which combine hydrodynamical and transport approaches to describe different stages of heavy-ion collisions, conversion of fluid to individual particles, particlization, is a non-trivial technical problem. We describe in detail how to find the particlization hypersurface in a 3+1 dimensional model, and how to sample the particle distributions evaluated using the Cooper-Frye procedure to create an ensemble of particles as an initial state for the transport stage. We also discuss the role and magnitude of the negative contributions in the Cooper-Frye procedure.

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arXiv2012

Dynamical freeze-out in event-by-event hydrodynamics

In hydrodynamical modeling of the ultrarelativistic heavy-ion collisions the freeze-out is typically performed at a constant temperature or density. In this work we apply a dynamical freeze-out criterion, which compares the hydrodynamical expansion rate with the pion scattering rate. Recently many calculations have been done using event-by-event hydrodynamics where the initial density profile fluctuates from event to event. In these event-by-event calculations the expansion rate fluctuates strongly as well, and thus it is interesting to check how the dynamical freeze-out changes hadron distributions with respect to the constant temperature freeze-out. We present hadron spectra and elliptic flow calculated using (2+1)-dimensional ideal hydrodynamics, and show the differences between constant temperature and dynamical freeze-out criteria. We find that the differences caused by different freeze-out criteria are small in all studied cases.

hep-ph↗

arXiv2012

Equation of state at non-zero baryon density based on lattice QCD

We employ the lattice QCD data on Taylor expansion coefficients to extend our previous parametrization of the equation of state to finite baryon density. When we take into account lattice spacing and quark mass dependence of the hadron masses, the Taylor coefficients at low temperature are equal to those of hadron resonance gas. Parametrized lattice equation of state can thus be smoothly connected to the hadron resonance gas equation of state at low temperatures.

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arXiv2011

Effect of temperature-dependent eta/s on flow anisotropies

We investigate the effects of a temperature-dependent shear viscosity over entropy density ratio eta/s on the flow anisotropy coefficients v_2 and v_4 in ultrarelativistic heavy-ion collisions at RHIC and LHC. We find that v_4 is more sensitive to the viscosity at low temperatures than v_2. At RHIC v_2 is mostly affected by the viscosity around the phase transition, but the larger the collision energy, the more the quark-gluon plasma viscosity affects v_2.

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arXiv2011

Quark Polarization in a Viscous Quark-Gluon Plasma

Quarks produced in the early stage of non-central heavy-ion collisions could develop a global spin polarization along the opposite direction of the reaction plane due to the spin-orbital coupling via parton interaction in a medium that has finite longitudinal flow shear along the direction of the impact parameter. We study how such polarization evolves via multiple scattering in a viscous quark-gluon plasma with an initial laminar flow. The final polarization is found to be sensitive to the viscosity and the initial shear of local longitudinal flow.

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arXiv2011

Radial and elliptic flow in Pb+Pb collisions at the Large Hadron Collider from viscous hydrodynamic

A comprehensive viscous hydrodynamic fit of spectra and elliptic flow for charged hadrons and identified pions and protons from Au+Au collisions of all centralities measured at the Relativistic Heavy Ion Collider is performed and used as the basis for predicting the analogous observables for Pb+Pb collisions at the Large Hadron Collider at sqrt(s)=2.76 and 5.5 A TeV. Comparison with recent measurements of the elliptic flow of charged hadrons by the ALICE experiment shows that the model slightly over-predicts the data if the same (constant) specific shear viscosity eta/s is assumed at both collision energies. In spite of differences in our assumptions for the equation of state, the freeze-out temperature, the chemical composition at freeze-out, and the starting time for the hydrodynamic evolution, our results agree remarkably well with those of Luzum [M. Luzum, Phys. Rev. C 83, 044911 (2011)], indicating robustness of the hydrodynamic model extrapolations. Future measurements of the centrality and transverse momentum dependence of spectra and elliptic flow for identified hadrons predicted here will further test the model and shed light on possible variations of the quark-gluon transport coefficients between RHIC and LHC energies.

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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

Equation of state at finite baryon density based on lattice QCD

We employ the lattice QCD data on Taylor expansion coefficients to extend our previous parametrization of the equation of state to finite baryon density. When we take into account lattice spacing and quark mass dependence of the hadron masses, the Taylor coefficients at low temperature are equal to those of hadron resonance gas. Thus the equation of state is smoothly connected to the hadron resonance gas equation of state at low temperatures. We also show how the elliptic flow is affected by this equation of state at the maximum SPS energy.

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