Femtoscopically Probing the Freeze-out Configuration in Heavy Ion Collisions

Abstract

Two-particle femtoscopy reveals the space-time substructure of the freeze-out configuration from heavy ion collisions. Detailed fingerprints of bulk collectivity are evident in space-momentum correlations, which have been systematically measured as a function of particle type, three-momentum, and collision conditions. A clear scenario, dominated by hydrodynamic-type flow emerges. Reproducing the strength and features of the femtoscopic signals in models involves important physical quantities like the Equation of State, as well as less fundamental technical details. An interesting approximate "factorization" in the measured systematics suggests that the overall physical freeze-out scale is set by final state chemistry, but the kinematic substructure is largely universal. Referring to previous results from hadron and lepton collisions, we point to the importance of determining whether these "universal" trends persist from the largest to the smallest systems. We review theoretical expectations for heavy ion femtoscopy at the LHC, and point to directions needing further theory and experimental work at RHIC and the LHC.

Symmetry constraints on generalizations of Bjorken flow

“…This is a mild generalization because one could perform an overall boost to restore the Z 2 symmetry. 4 Already in [7,11] the hope was expressed that heavy-ion collisions might be approximately invariant under an O(3) symmetry (or an O(2) subgroup for off-center collisions) generated as explained here. Failure of motivation and insight, in some combination, prevented me from actually writing down the key equation (13) for almost two years.…”

“…This is probably wrong, even for perfectly central collisions, and one might worry that it significantly distorts the subsequent hydrodynamical flows on which much of heavy-ion phenomenology depends. Indications that the absence of initial radial flow could be problematic can be found, for example, in [2,3]; see also [4,5,6]. The question naturally arises, can we estimate in some way deviations from the Bjorken picture, based on the finite size of the colliding nuclei, which lead to non-zero u x ⊥ ?…”

“…10 One can go further and show that if a tensor on R 3,1 respects the SO(3) q symmetry with ζ-weight α, then mapping it with conformal weight α to S 3 × R with conformal weight α results in a tensor on S 3 × R which has vanishing Lie derivative under the Killing vectors that generate the SO(3) that preserves surfaces of constant y 3 . A special case of this general claim is that the energy density on S 3 (ignoring the conformal anomaly of the stress tensor) is˜ = W 4 = W4 τ 4ˆ , which is SO(3)-invariant because bothˆ and…”

Interferometric signatures of the temperature dependence of the specific shear viscosity in heavy-ion collisions

“…To save time and effort, we employ for this pioneering study the shortcut of calculating the HBT radii from the source variances of the emission function [10][11][12][13] for directly emitted pions only, instead of performing a 3-D Gaussian fit to the correlation function including all resonance decays [13][14][15][16]. This approximation, which holds exactly only for Gaussian sources [12], is known to be sufficient to discern qualitative features of the HBT radii and their K T dependences, although it is not accurate enough for quantitative comparisons with experimental data [17].…”

Kinetic freeze-out from an anisotropic fluid in high-energy heavy-ion collisions: particle spectra, Hanbury Brown–Twiss radii, and anisotropic flow

“…5 is much more sensitive to the choice of decoupling parameters ( , ξ ). This is actually somewhat reassuring, since femtoscopic measurements are precisely designed to probe the space-time configuration at decoupling [33].…”