The unreasonable effectiveness of hydrodynamics in heavy ion collisions

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Event-by-event hydrodynamic simulations of AA and pA collisions involve initial energy densities with large spatial gradients. This is associated with the presence of large Knudsen numbers (Kn≈1) at early times, which may lead one to question the validity of the hydrodynamic approach in these rapidly evolving, largely inhomogeneous systems. A new procedure to smooth out the initial energy densities is employed to show that the initial spatial eccentricities, εn, are remarkably robust with respect to variations in the underlying scale of initial energy density spatial gradients, λ. For sNN=2.76 TeV LHC initial conditions generated by the MCKLN code, εn (across centralities) remains nearly constant if the fluctuation scale varies by an order of magnitude, i.e., when λ varies from 0.1 to 1 fm. Given that the local Knudsen number Kn≈1/λ, the robustness of the initial eccentricities with respect to changes in the fluctuation scale suggests that the vn's cannot be used to distinguish between events with large Kn from events where Kn is in the hydrodynamic regime. We use the 2+1 Lagrangian hydrodynamic code v-USPhydro to show that this is indeed the case: anisotropic flow coefficients computed within event-by-event viscous hydrodynamics are only sensitive to long wavelength scales of order 1/ΛQCD≈1 fm and are incredibly robust with respect to variations in the initial local Knudsen number. This robustness can be used to justify the somewhat unreasonable effectiveness of the nearly perfect fluid paradigm in heavy ion collisions.

Original languageEnglish (US)
Pages (from-to)890-893
Number of pages4
JournalNuclear Physics A
StatePublished - Dec 1 2016
Externally publishedYes


  • anisotropic flow harmonics
  • Relativistic hydrodynamics
  • smoothing scale

ASJC Scopus subject areas

  • Nuclear and High Energy Physics

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