### The RHIC HBT Puzzle

The RHIC HBT Puzzle appeared at the Quark Matter 2001 conference in Stonybrook. At that meeting HBT source radii measurements were reported by the RHIC experiments with the R_out/R_side ratio being very close to unity – contrary to long-held expectations of this ratio being considerably larger than one for a decaying QGP. At the basis of this expectation were hydrodynamic calculations with an equation of state incorporating a strong 1st order phase transition. One of the effects of this EoS is a long-lived mixed phase which slows down the expansion and breakup of the system and thus leads to a long lifetime of the emitting source (which manifests itself in that large R_out/R_side ratio, see .e.g. here). Even the most sophisticated dynamical models of the day, the hydro+micro models, were unable to reproduce the basic features seen in the data. This was demonstrated for the first time in the following paper, which became the poster-child of the RHIC HBT Puzzle:

**Pion interferometry at RHIC: Probing a thermalized quark gluon plasma? **

These findings and the discrepancy between the data and dynamical model calculations based on hydrodynamics or hybrid calculations were subsequently confirmed by other groups:

**Collective flow and two pion correlations from a relativistic hydrodynamic model with early chemical freezeout****Two RHIC puzzles: Early thermalization and the HBT problem**

Interestingly, a purely microscopic transport model, AMPT, has been the most successful in describing the HBT data – it apparently allows for particular space-momentum correlations which are hard to achieve in a hydrodynamic approach (note that the AMPT model itself remains somewhat controversial, in particular due to its mixing of constituent and current quark degrees of freedom in the deconfined phase):

**Partonic effects on pion interferometry at RHIC**

There have been many papers written on the RHIC HBT puzzle and various claims made of solving it within hydrodynamic or blastwave models (which the author of this post does not subscribe to, given that all of these claims still exhibit around 50% deviation from the data or result in unphysical parameters for the blastwave), so the puzzle still remains one of the least understood features of the bulk dynamics at RHIC. A good discussion on the current status and possible ways of resolving the puzzle can be found in this recent write-up: