  Two-pion Hanbury-Brown-Twiss (HBT) correlations for central Pb-Pb collisions
at the Large-Hadron-Collider (LHC) energy of $\sqrt{s_{NN}}=2.76$ TeV are
investigated for the first time with the microscopic transport model UrQMD
(Ultra-relativistic Quantum Molecular Dynamics). The transverse momentum
dependence of the Pratt-Bertsch HBT radii is extracted from a three dimensional
Gaussian fit to the correlator in the longitudinal co-moving system (LCMS).
Qualitative agreement with the ALICE data is obtained, however $R_{out}$ is
overpredicted by nearly 50%. The LHC results are also compared to data from the
STAR experiment at RHIC. For both energies we find that the calculated
$R_O/R_S$ ratio is always larger than data, indicating that the emission in the
model is less explosive than observed in the data.
