Showing 1–10 of 10 results for author: Werthmann, C

Friction terms in multi-fluid description of heavy-ion collisions

Authors: Clemens Werthmann, Iurii Karpenko, Pasi Huovinen

Abstract: In multi-fluid description of heavy-ion collisions, the primary scatterings and particle production are described in terms of interaction between fluids, so called friction. These friction terms can be derived from kinetic theory, but they are not unique. We compare different approaches to derive the friction terms, introduce a new ``charge transfer" friction, which allows to move charge to the mi… ▽ More In multi-fluid description of heavy-ion collisions, the primary scatterings and particle production are described in terms of interaction between fluids, so called friction. These friction terms can be derived from kinetic theory, but they are not unique. We compare different approaches to derive the friction terms, introduce a new ``charge transfer" friction, which allows to move charge to the midrapidity fireball, and implement them in the MUFFIN model. The charge transfer friction is more consistent with the assumption of three fluids clearly separated in momentum space, and allows better comparisons of the experimental data and underlying equation of state. It also leaves room for entropy generation due to dissipation in individual fluids, and we present the first results obtained using viscous multi-fluid dynamics. △ Less

Submitted 13 November, 2025; originally announced November 2025.

Comments: 14 pages, 6 figures

Adiabatic hydrodynamization and quasinormal modes of nonthermal attractors

Authors: Matisse De Lescluze, Michal P. Heller, Aleksas Mazeliauskas, Bruno Scheihing-Hitschfeld, Clemens Werthmann

Abstract: Nonthermal attractors characterize the emergent self-similar evolution of far-from-equilibrium quantum systems, from nuclear collisions to cold-atom experiments. Within the adiabatic hydrodynamization framework, the approach to the nonthermal attractor can be understood as the decay of excited states of an effective Hamiltonian. We use an exactly solvable case of the Boltzmann equation -- the long… ▽ More Nonthermal attractors characterize the emergent self-similar evolution of far-from-equilibrium quantum systems, from nuclear collisions to cold-atom experiments. Within the adiabatic hydrodynamization framework, the approach to the nonthermal attractor can be understood as the decay of excited states of an effective Hamiltonian. We use an exactly solvable case of the Boltzmann equation -- the longitudinally expanding overoccupied gluon plasma dominated by small-angle elastic collisions -- to map the eigenmodes in adiabatic hydrodynamization to the quasinormal mode spectrum of the nonthermal attractor. If generalized, this equivalence may be used as a discovery tool for new phenomena in out-of-equilibrium systems. A byproduct of our analysis is the analytic prescaling solutions for systems undergoing strong longitudinal expansion. △ Less

Submitted 16 October, 2025; originally announced October 2025.

Comments: 9 pages

Extended applicability domain of viscous anisotropic hydrodynamics in (2+1)-D Bjorken flow with transverse expansion

Authors: Yiyang Peng, Victor E. Ambrus, Clemens Werthmann, Sören Schlichting, Ulrich Heinz, Huichao Song

Abstract: We perform (2{+}1)-D simulations of viscous anisotropic hydrodynamics (VAH) under boost-invariant and conformal conditions. Comparing both VAH and traditional viscous hydrodynamics with kinetic theory in the relaxation-time approximation as the underlying microscopic theory, we show that VAH provides a superior description of the evolution across a wide range of opacity, effectively extending the… ▽ More We perform (2{+}1)-D simulations of viscous anisotropic hydrodynamics (VAH) under boost-invariant and conformal conditions. Comparing both VAH and traditional viscous hydrodynamics with kinetic theory in the relaxation-time approximation as the underlying microscopic theory, we show that VAH provides a superior description of the evolution across a wide range of opacity, effectively extending the boundaries of the applicability of hydrodynamic modeling. Our results demonstrate VAH's potential for describing collective flow in small systems where traditional hydrodynamics faces challenges. △ Less

Submitted 4 September, 2025; originally announced September 2025.

Comments: 11 pages pages, 4 figures

Early time hydrodynamic attractor in a nearly-unitary Fermi gas

Authors: Michal P. Heller, Clemens Werthmann

Abstract: The hydrodynamic attractors paradigm aims to explain the applicability of hydrodynamics after a very short timescale in ultra-relativistic nuclear collisions at RHIC and LHC in terms of the emergence of universal behavior across different initial states. The attractors observed in theoretical models are first driven by a rapid expansion of matter, which later gives way to the decay of exponentiall… ▽ More The hydrodynamic attractors paradigm aims to explain the applicability of hydrodynamics after a very short timescale in ultra-relativistic nuclear collisions at RHIC and LHC in terms of the emergence of universal behavior across different initial states. The attractors observed in theoretical models are first driven by a rapid expansion of matter, which later gives way to the decay of exponentially damped transient excitations into the viscous fluid regime. Recently, a theoretical proposal was made how to realize hydrodynamic attractors in cold atomic gases focusing on the decay of transients and opening prospects for a future experimental realization in table top experiments. Motivated by this, in the present paper we show how the far-from-equilibrium, expansion-driven part of the hydrodynamic attractor emerges in a model of a nearly-unitary Fermi gas. △ Less

Submitted 22 August, 2025; v1 submitted 3 July, 2025; originally announced July 2025.

Comments: 8 pages, 3 figures. v2: Corrected formula for energy density, adapted to theoretical model data for contact and bulk viscosity to extend validity to lower temperatures. Several minor changes

Collective dynamics in heavy and light-ion collisions -- II) Determining the origin of collective behavior in high-energy collisions

Authors: Victor E. Ambrus, Sören Schlichting, Clemens Werthmann

Abstract: Exploiting the first measurements of the same ion species in OO collisons at RHIC and LHC, we propose an observable to distinguish whether collective behavior builds up through a hydrodynamic expansion of a strongly interacting QGP or few final state re-scatterings. Our procedure allows to disentangle the effects of the initial state geometry and the dynamical response mechanism on anisotropic flo… ▽ More Exploiting the first measurements of the same ion species in OO collisons at RHIC and LHC, we propose an observable to distinguish whether collective behavior builds up through a hydrodynamic expansion of a strongly interacting QGP or few final state re-scatterings. Our procedure allows to disentangle the effects of the initial state geometry and the dynamical response mechanism on anisotropic flow. We validate its ability to discriminate between systems with different interaction rates using results from event-by-event simulations in kinetic theory. △ Less

Submitted 20 March, 2025; v1 submitted 29 November, 2024; originally announced November 2024.

Comments: 15 pages, 15 figures. v2: several minor changes, added data files in ancillary folder. v3: several minor changes, data files moved to cited zenodo archive, equivalent to published version

Journal ref: Phys. Rev. D 111 (2025) 054025

System size dependence of pre-equilibrium and applicability of hydrodynamics in heavy-ion collisions

Authors: Victor E. Ambruş, Sören Schlichting, Clemens Werthmann

Abstract: We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description, in order to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics provides a quantitatively accurate description of collective flow when the average inverse Reynolds number $\mathrm{Re}^{-1}$ is sufficiently small and… ▽ More We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description, in order to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics provides a quantitatively accurate description of collective flow when the average inverse Reynolds number $\mathrm{Re}^{-1}$ is sufficiently small and the early pre-equilibrium stage is properly accounted for. By determining the breakdown of hydrodynamics as a function of system size and energy, we find that it is quantitatively accurate in central lead-lead collisions at LHC energies, but should not be used in typical proton-lead or proton-proton collisions, where the development of collective flow can not accurately be described within hydrodynamics. △ Less

Submitted 17 July, 2023; originally announced July 2023.

Comments: 6 pages, 4 figures. Contribution to the HardProbes2023 conference proceedings

Attractors for Flow Observables in 2+1D Bjorken Flow

Authors: Victor E. Ambruş, Sören Schlichting, Clemens Werthmann

Abstract: We examine the capabilities of second-order Israel-Stewart-type hydrodynamics to capture the early-time behaviour of the quark-gluon plasma created in heavy-ion collisions. We point out that at very early times, the dynamics of the fireball is governed by the local 0+1-D Bjorken flow attractor due to the rapid expansion along the longitudinal direction. Discrepancies between hydrodynamics and kine… ▽ More We examine the capabilities of second-order Israel-Stewart-type hydrodynamics to capture the early-time behaviour of the quark-gluon plasma created in heavy-ion collisions. We point out that at very early times, the dynamics of the fireball is governed by the local 0+1-D Bjorken flow attractor due to the rapid expansion along the longitudinal direction. Discrepancies between hydrodynamics and kinetic theory in this far-from-equilibrium regime leads to disagreement at the level of late-time observables, such as elliptic flow. We show that rescaling the initial energy-density profile for hydrodynamics accounts for such discrepancies, restoring agreement with kinetic theory for large opacities (small shear viscosity / large system size / high energy). △ Less

Submitted 21 February, 2023; originally announced February 2023.

Comments: 6 pages, 3 figures. Contribution to the TIM-2022 conference proceedings

Journal ref: AIP Conf.Proc. 3181 (2024) 050004

Opacity dependence of transverse flow, pre-equilibrium and applicability of hydrodynamics in heavy-ion collisions

Authors: Victor E. Ambrus, S. Schlichting, C. Werthmann

Abstract: We evaluate the full opacity dependence of collective flow in high-energy heavy-ion collisions within a microscopic kinetic description based on the Boltzmann equation in the conformal relaxation time approximation. By comparing kinetic theory calculations to hydrodynamic and hybrid simulations for an average initial state, we point out shortcomings and inaccuracies of hydrodynamic models and pres… ▽ More We evaluate the full opacity dependence of collective flow in high-energy heavy-ion collisions within a microscopic kinetic description based on the Boltzmann equation in the conformal relaxation time approximation. By comparing kinetic theory calculations to hydrodynamic and hybrid simulations for an average initial state, we point out shortcomings and inaccuracies of hydrodynamic models and present modified simulation setups to improve them. △ Less

Submitted 11 May, 2023; v1 submitted 25 November, 2022; originally announced November 2022.

Comments: 35 pages, 15 figures. v2: Several minor changes to spelling, notation, affiliations, acknowledgements, references. Equivalent to version published in PRD. Figure 8 had to be scaled down dramatically because otherwise it wouldn't compile on the arXiv server

Establishing the Range of Applicability of Hydrodynamics in High-Energy Collisions

Authors: Victor E. Ambrus, S. Schlichting, C. Werthmann

Abstract: We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description in the conformal relaxation time approximation, in order to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics provides a quantitatively accurate description of collective flow when the average inverse Reynolds numb… ▽ More We simulate the space-time dynamics of high-energy collisions based on a microscopic kinetic description in the conformal relaxation time approximation, in order to determine the range of applicability of an effective description in relativistic viscous hydrodynamics. We find that hydrodynamics provides a quantitatively accurate description of collective flow when the average inverse Reynolds number is sufficiently small and the early pre-equilibrium stage is properly accounted for. We further discuss the implications of our findings for the (in)applicability of hydrodynamics in proton-proton, proton-nucleus and light nucleus collisions. △ Less

Submitted 18 April, 2023; v1 submitted 25 November, 2022; originally announced November 2022.

Comments: 9 pages, 4 figures v2: Several minor changes to spelling, notation, affiliations, acknowledgements, references. Moved Supplemental Material to end of document. Equivalent to version published in PRL

Statistical analysis of initial state and final state response in heavy-ion collisions

Authors: Nicolas Borghini, Marc Borrell, Nina Feld, Hendrik Roch, Sören Schlichting, Clemens Werthmann

Abstract: We develop a general decomposition of an ensemble of initial density profiles in terms of an average state and a basis of modes that represent the event-by-event fluctuations of the initial state. The basis is determined such that the probability distributions of the amplitudes of different modes are uncorrelated. Based on this decomposition, we quantify the different types and probabilities of ev… ▽ More We develop a general decomposition of an ensemble of initial density profiles in terms of an average state and a basis of modes that represent the event-by-event fluctuations of the initial state. The basis is determined such that the probability distributions of the amplitudes of different modes are uncorrelated. Based on this decomposition, we quantify the different types and probabilities of event-by-event fluctuations in Glauber and Saturation models and investigate how the various modes affect different characteristics of the initial state. We perform simulations of the dynamical evolution with KoMPoST and MUSIC to investigate the impact of the modes on final-state observables and their correlations. △ Less

Submitted 10 March, 2023; v1 submitted 2 September, 2022; originally announced September 2022.

Comments: v2: final version: minor additions, 59 pages, 40 figures (depending on the viewer, Figs.22-25 may display incorrectly)

Journal ref: Phys. Rev. C 107 (2023) 034905