Rapidity- and azimuthally-dependent femtoscopy with charged pions in $p$+Pb collisions at $\sqrt{s_\mathrm{NN}} = 5.02$ TeV with ATLAS

Hanbury Brown and Twiss (HBT) radii are measured as a function of centrality, transverse momentum, rapidity, and azimuthal angle with respect to the 2nd-order event plane 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 nb${}^{−1}$ is sampled. The radii are presented as a function of the local density $dN/dy^{*}$ and the rapidity-dependence is shown to depend directly on the density. The radii and their relative azimuthal modulation are presented as a function of the magnitude of the flow vector $|q_2|$ measured at 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 ion-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.