Azimuthal femtoscopy in central $p$+Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with ATLAS

Hanbury Brown and Twiss (HBT) radii with respect to the 2nd-order event plane are measured in central $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02\,\TeV$ with the ATLAS detector at the LHC. A total integrated luminosity of $28~\mathrm{nb}^{-1}$ is sampled. The radii and their relative modulation are presented as a function of the magnitude of the flow vector $|\vec{q}_{2}|$ in the side of the calorimeters that the Pb beam faces with pseudorapidity $\eta < -2.5$. Modulations of the transverse HBT radii are observed with the same orientation as in heavy-ion collisions, in which they are attributed to hydrodynamic evolution from an elliptic initial geometry. This modulation is consistent with a hydrodynamic evolution of a short-lived medium.