Figure. The energy dependence of HBT parameters for central Au+Au (Pb+Pb) collisions at midrapidity and pT = 0.17 GeV/c. The strong similarity between the HBT radii across a wide range of collision energies has been called the "HBT Puzzle."

Hanbury-Brown Twiss (HBT) interferometry of two-pion correlations allows one to study the source size at thermal freeze-out, the point at which particles no longer interact. Predictions of long-lived systems, associated with quark-gluon plasma formation, led to predictions of much larger system sizes at RHIC. However, first HBT studies by STAR showed that the extracted source sizes are comparable to measurements at lower energies. Hydrodynamic models have been able to reproduce dynamic quantities such as elliptic flow and transverse momentum spectra, but their failure to reproduce HBT radii has been dubbed the "HBT puzzle."