Constraints from $v_2$ fluctuations for the initial state geometry of heavy-ion collisions

Abstract

The ability to accurately compute the series of coefficients $v_n$ characterizing the momentum space anisotropies of particle production in ultrarelativistic heavy ion collisions as a function of centrality is widely regarded as a triumph of fluid dynamics as description of the bulk matter evolution. A key ingredient to fluid dynamical modeling is however the initial spatial distribution of matter as created by a yet not completely understood equilibration process. A measurement directly sensitive to this initial state geometry is therefore of high value for constraining models of pre-equilibrium dynamics. Recently, it has been shown that such a measurement is indeed possible in terms of the event by event probability distribution of the normalized $v_n$ distribution as a function of centrality, which is to high accuracy independent on the details of the subsequent fluid dynamical evolution and hence directly reflects the primary distribution of spatial eccentricities. We present a study of this observable using a variety of Glauber-based models and argue that the experimental data place very tight constraints on the initial distribution of matter and rule out all simple Glauber-based models.

6 pages, 4 figures

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• Event-by-event hydrodynamics $+$ jet energy loss: A solution to the $R_{AA}\otimes v_2$ puzzle
• Linear and cubic response to the initial eccentricity in heavy-ion collisions
• Transverse momentum-flow correlations in relativistic heavy-ion collisions
• Relative flow fluctuations as a probe of initial state fluctuations
• Characterizing flow fluctuations with moments
• Sensitivity of flow harmonics to sub-nucleon scale fluctuations in heavy ion collisions
• Skewness of elliptic flow fluctuations
• ${}^{16}{\mathrm{O}}^{16}\mathrm{O}$ at RHIC and the LHC comparing $\alpha$ clustering vs substructure
• Azimuthal Anisotropy Distributions in High-Energy Collisions
• Study of the sensitivity of observables to hot spot size in heavy ion collisions
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• Baseline predictions of elliptic flow and fluctuations at the RHIC Beam Energy Scan using response coefficients
• Anisotropy fluctuation and correlation in central $\alpha$-clustered ${}^{12}$C+${}^{197}$Au collisions
• Multi-particle correlations, cumulants, and moments sensitive to fluctuations in rare-probe azimuthal anisotropy in heavy ion collisions
• Time dependence of partition into spectators and participants in relativistic heavy-ion collisions
• Impact of Multiplicity Fluctuations on Entropy Scaling Across System Size
• Zeroing in on the initial state -- tomography using bulk, jets and photons