Initial state fluctuations and final state correlations in relativistic heavy-ion collisions

Abstract

Abstract. We review the phenomenology and theory of bulk observables in ultrarelativistic heavy-ion collisions, focussing on recent developments involving event-byevent fluctuations in the initial stages of a heavy ion collision, and how they manifest in observed correlations. We first define the relevant observables and show how each measurement is related to underlying theoretical quantities. Then we review the prevailing picture of the various stages of a collision, including the state-of-the-art modeling of the initial stages of a collision and subsequent hydrodynamic evolution, as well as hadronic scattering and freeze-out in the later stages. We then discuss the recent results that have shaped our current understanding and identify the challenges that remain. Finally, we point out open issues and the potential for progress in the field.PACS numbers: 25.75.Ag,24

Event-by-EventHydrodynamics+JetEnergy Loss: A Solution to theRAA⊗vNoronha-Hostler1, Betz2, Noronha3 et al. 2016Phys. Rev. Lett.11761573View full textAdd to dashboardCite

“…It is now understood [54] that the initial spatial anisotropies present in the early stages of nucleus-nucleus collisions are converted to final stage momentum anisotropies (i.e., anisotropic flow) in a way that is consistent with viscous relativistic hydrodynamic simulations performed on an event-by-event basis (see, e.g., the review [47]). Furthermore, once the full information regarding the soft event-by-event v n distributions became available [55] at the LHC, powerful constraints on the initial conditions of the hydrodynamic modeling of the QGP have been obtained [56][57][58][59][60].…”

“…An important detail that has been overlooked so far in model calculations is that the theoretical v hard 2 (p T ) is not the appropriate quantity to be compared with experimental data. In fact, the experimental high p T > 10 GeV flow coe cients v exp n (p T ) are measured via the correlation between soft and hard hadrons in a given centrality class, emphasized in [47,48] …”

Kinetic regime of hydrodynamic fluctuations and long time tails for a Bjorken expansion

“…Thus a fluctuation with wavenumber k is suppressed by k 3 /s. The suppression factor k 3 /s is roughly the inverse of the degrees of freedom inside a box of volume ∆V ∼ (1/k) 3 , which must be a huge number for local thermodynamics to apply. This is why the linear analysis of the hydrodynamic fluctuations is justified.…”

“…The specific test case of Bjorken flow (which is a hydrodynamic model for the longitudinal expansion of a nucleus-nucleus collision [1]) is motivated by the experimental program of ultra-relativistic heavy-ion collisions at RHIC and the LHC. Detailed measurements of two particle correlation functions have provided overwhelming evidence that the evolution of the excited nuclear material is remarkably well described by the hydrodynamics of the Quark Gluon Plasma (QGP) with a small shear viscosity to entropy ratio of order η/s ∼ 2/4π [2,3]. The typical relaxation times of the plasma, while short enough to support hydrodynamics, are not vastly smaller than the inverse expansion rates of the collision.…”

Initial conditions for hydrodynamics from weakly coupled pre-equilibrium evolution

“…Viscous relativistic hydrodynamics provides a remarkably detailed and phenomenologically successful description of the expansion of the Quark Gluon Plasma (QGP) in the ultrarelativistic heavy-ion collisions realized at the BNL Relativistic Heavy Ion Collider (RHIC) and at the CERN Large Hadron Collider (LHC) [1][2][3]. Hydrodynamics is an effective theory based on an assumption that the medium is sufficiently close to local thermal equilibrium that the full stress tensor can be expanded in gradients of the energy and momentum densities [4].…”