Hydrodynamic fluctuations near a critical endpoint and Hanbury-Brown–Twiss interferometry

We analyze the evolution of hydrodynamic fluctuations in a heavy ion collision as the system passes close to the QCD critical point. We introduce two small dimensionless parameters λ and ∆ s to characterize the evolution. λ compares the microscopic relaxation time (away from the critical point) to the expansion rate λ ≡ τ 0 /τ Q , and ∆ s compares the baryon to entropy ratio, n/s, to its critical value, ∆ s ≡ (n/s − n c /s c )/(n c /s c ). We determine how the evolution of critical hydrodynamic fluctuations depends parametrically on λ and ∆ s . Finally, we use this parametric reasoning to estimate the critical fluctuations and correlation length for a heavy ion collision, and to give guidance to the experimental search for the QCD critical point.

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“…See also Refs. [21][22][23][24] for previous studies of critical fluctuations based on stochastic hydrodynamics.…”

Section: A Overview and Goalsmentioning

confidence: 99%

“…and where S ee , S en , S nn are defined in Eq. (22). 6 The bar in xā and Xā indicate that the longitudinal momentum and velocity are appended to the set x a and X a defined in Sect.…”

Section: The Derivation Of Hydro-kinetic Equationsmentioning

confidence: 99%

Transits of the QCD critical point

et al. 2019

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“…Other works also focus on the implementation of local thermal fluctuations in the dynamical fluid-evolution of the system. [10][11][12].A common denominator of all these fluid dynamical models is that at some point the transported fluid fields have to be converted into "real" hadrons. This is usually done via the Cooper-Frye (C-F) freeze-out prescription [31].…”

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confidence: 99%

“…In fluid dynamics one usually assumes that the particle number in a given fluid element sufficiently large so that local fluctuations of the particle number, and hence of the baryon-and energy density, can be neglected. Since this is not strictly the case in heavy ion collisions new fluid * steinheimer@fias.uni-frankfurt.de † vkoch@lbl.gov dynamical approaches, including local (thermal) fluctuations have been proposed [9][10][11][12]. Understanding and propagating fluctuations in fluid dynamics is important since the fluctuations of conserved charges are considered a very sensitive probe for the QCD phase transition and the associated critical point [13,14].…”

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confidence: 99%

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Effect of finite particle number sampling on baryon number fluctuations

Steinheimer

Koch

2017

Phys. Rev. C

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The effects of finite particle number sampling on the net baryon number cumulants, extracted from fluid dynamical simulations, are studied. The commonly used finite particle number sampling procedure introduces an additional Poissonian (or multinomial if global baryon number conservation is enforced) contribution which increases the extracted moments of the baryon number distribution. If this procedure is applied to a fluctuating fluid dynamics framework one severely overestimates the actual cumulants. We show that the sampling of so called test-particles suppresses the additional contribution to the moments by at least one power of the number of test-particles. We demonstrate this method in a numerical fluid dynamics simulation that includes the effects of spinodal decomposition due to a first order phase transition. Furthermore, in the limit where anti-baryons can be ignored, we derive analytic formulas which capture exactly the effect of particle sampling on the baryon number cumulants. These formulas may be used to test the various numerical particle sampling algorithms.The goal of heavy ion collision experiments is to study the properties of very hot and dense matter. It has been proposed that in the most energetic collisions of nuclei at the RHIC and LHC a new state of matter, the so called Quark-Gluon-Plasma, has been created [1][2][3][4][5]. Lattice QCD results predict the transition at vanishing net baryon number density to be a crossover [6][7][8]. Due to the fermion sign problem, these calculations cannot be directly extended to the interesting region of high net baryon density where the cross-over may change to a first order phase transition. One of the main goals of current and future experimental programs is to find experimental signals for a possible first order phase transition and critical point in the phase diagram of the strong interaction, Quantum Chromo Dynamics (QCD).The systems created in these nuclear collisions are very small, rapidly expanding and not always in local thermal and chemical equilibrium. Thus the dynamical evolution of such a collision is far from trivial and sophisticated transport models are being employed. The current stateof-the-art of such models are the fluid dynamical hybrid models. In these models one uses a non-equilibrium initial state for a viscous fluid dynamical evolution, which is followed by a Boltzmann-transport description for the hadronic freeze-out phase. This setup is convenient as the fluid dynamical equations allow for a straight forward inclusion of the Equation of State (EoS). Since the systems created in heavy ion collisions are rather small (on the order of ∼ 10-20 fm) the application of standard fluid dynamics is limited. In fluid dynamics one usually assumes that the particle number in a given fluid element sufficiently large so that local fluctuations of the particle number, and hence of the baryon-and energy density, can be neglected. Since this is not strictly the case in heavy ion collisions new fluid * steinheimer@fias.uni-frankfurt.de † vko...

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A machine learning study to identify spinodal clumping in high energy nuclear collisions

Steinheimer

Pang

Zhou

et al. 2019

J. High Energ. Phys.

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The coordinate and momentum space configurations of the net baryon number in heavy ion collisions that undergo spinodal decomposition, due to a first-order phase transition, are investigated using state-of-the-art machine-learning methods. Coordinate space clumping, which appears in the spinodal decomposition, leaves strong characteristic imprints on the spatial net density distribution in nearly every event. On the other hand, the corresponding momentum distributions do not show clear event-by-event features. However, a small subset of events can be systematically differentiated even if only the momentum space information is available. In such scenarios, observables like the baryon number cumulants signal a spinodal non-equilibrium phase transition. Indeed the thirdorder cumulant, the skewness, does exhibit a peak at the beam energy (E lab = 3 − 4 A GeV), where the transient hot and dense system created in the heavy ion collision reaches the first-order phase transition.

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Hydrodynamic fluctuations near a critical endpoint and Hanbury-Brown–Twiss interferometry

Cited by 18 publications

References 36 publications

Transits of the QCD critical point

Transits of the QCD critical point

Effect of finite particle number sampling on baryon number fluctuations

A machine learning study to identify spinodal clumping in high energy nuclear collisions

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