Sensitivity of flow harmonics to sub-nucleon scale fluctuations in heavy ion collisions

TL;DR

This study demonstrates that flow harmonics in nucleus-nucleus collisions are largely insensitive to sub-nucleon scale fluctuations, whereas high-multiplicity p+Pb collisions show significant sensitivity, indicating different underlying physics.

Contribution

The paper introduces a smoothing procedure showing that initial spatial eccentricities and resulting flow harmonics are robust against variations in the fluctuation scale in heavy ion collisions.

Findings

Flow harmonics are stable against fluctuation scale changes in Pb+Pb collisions.

High-multiplicity p+Pb collisions are sensitive to sub-nucleonic fluctuations.

Robustness suggests flow harmonics are not influenced by sub-nucleon physics below confinement scale.

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 $\varepsilon_{m,n}$, which drive the final state flow harmonics $v_n$, are remarkably robust with respect to variations of the underlying scale of initial energy density spatial gradients, $\lambda$, in nucleus-nucleus collisions. For $\sqrt{s}=2.76$ TeV Pb+Pb collisions, the $\varepsilon_{m,n}$'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 $v_n$'s computed within event by event viscous hydrodynamics. This indicates that the flow harmonics in nucleus-nucleus collisions are not yet particularly sensitive to the underlying microscopic sub-nucleon physics below the confinement scale. On the other hand, the eccentricities of top $1\%$ high multiplicity $\sqrt{s}=5.02$ 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.