Sensitivity of flow harmonics to subnucleon scale fluctuations in heavy ion collisions

Abstract

In this paper a new procedure to smooth out the initial energy densities of hydrodynamics is employed to show that the initial spatial eccentricities εm,n, which drive the final state flow harmonics vn, are remarkably robust with respect to variations of the underlying scale of initial energy density spatial gradients, λ, in nucleus-nucleus collisions. For √ s = 2.76 TeV Pb+Pb collisions, the εm,n's (across centrality classes) change by less than 10% if the scale of fluctuations is varied from 0.1 to 1 fm. We show, using the 2+1 Lagrangian hydrodynamic code, v-USPhydro, that this robustness is transferred to the final vn's computed within event by event viscous hydrodynamics. This indicates that the flow harmonics in nucleus-nucleus collisions are not particularly sensitive to the underlying microscopic sub-nucleon physics below the confinement scale. On the other hand, the eccentricities of top 1% high multiplicity √ s = 5.02 TeV p+Pb collisions are found to be very sensitive to sub-nucleonic scale fluctuations, which should be contrasted with the robustness found in peripheral Pb+Pb collisions with the same multiplicity.

Event-by-EventHydrodynamics+JetEnergy Loss: A Solution to theRAA⊗vNoronha-Hostler1, Betz2, Noronha3 et al. 2016Phys. Rev. Lett.Self Cite11661573View full textAdd to dashboardCite

“…, which is in accordance with the fact that MCKLN gives smaller values for " 3 in comparison to MCGlauber's (see, for instance, [63]). One can see in Fig.…”

“…The soft sector is modeled via event-by-event hydrodynamic simulations performed using the v-USPhydro code [61][62][63][64] while the hard sector is described using the energy loss framework developed in [36,65,66]. We show that the positive contribution from low p T v 2 fluctuations overwhelms the small, negative contributions from event plane fluctuations, which in turn leads to an overall enhancement of high p T elliptic flow in comparison to previous calculations.…”

“…Hydrodynamic evolution. The expanding QGP is modeled through event-by-event simulations performed using the 2+1 (i.e., boost invariant) viscous relativistic hydrodynamics code called v-USPhydro [61][62][63][64]. vUSPhydro accurately [68] solves the energy-momentum conservation equations and the equations of motion for the dissipative currents using the Lagrangian formulation of hydrodynamics encoded in the Smoothed Particle Hydrodynamics (SPH) algorithm [69,70].…”

Initial conditions for hydrodynamics from weakly coupled pre-equilibrium evolution

“…In momentum space this corresponds to suppressing the large k 4 See ref. [14] for a current discussion of the observables that are influenced by this arbitrary regulator.…”

“…In most simulations the prethermal evolution is either completely neglected [10], or modelled in a way that does not contain the correct physics to produce hydrodynamic flow [6,11,12]. In addition, in some models (such as the successful IP-glasma model [12] motivated by parton saturation) the initial conditions contain strong gradients which limit the effectiveness of the hydrodynamic derivative expansion [13,14]. Different hydrodynamic codes regulate these extreme initial conditions in different ad hoc ways, e.g.…”