The Long Slow Death of the HBT Puzzle

At the onset of the RHIC era femtoscopic source sizes inferred from two-particle correlations at RHIC defied description with hydrodynamic models. This failure, which became known as the HBT puzzle, now appears to be solved. The source of the discrepancy appears to be a conspiracy of several factors, each of which contributed to making the evolution of RHIC collisions more explosive. These included pre-equilibrium flow, using a stiffer equation of state and adding viscosity. Hydrodynamics represents the foundation of RHIC modeling. The success of ideal hydrody-namics in reproducing elliptic flow and spectra inspired the phrase “perfect liquid”, as it would appear that the quark-gluon plasma is strongly interacting with perhaps the lowest ratio of vis-cosity to entropy of any measured substance. However, these models failed to reproduce femto-scopic source sizes inferred from two-pion correlations (a.k.a. HBT measurements, named after Hanbury-Brown and Twiss who pioneered similar techniques with photons [1]). The hydrody-namic models [2, 3, 4] also fared much worse than purely microscopic simulations [5, 6, 7, 8], which did not incorporate soft regions in the equation of state. The failure of hydrodynam