Femtoscopy in $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV $p$-Pb collisions with ATLAS

Bose-Einstein correlations between identified charged pions are measured for $p$+Pb collisions at $\sqrt{s_\mathrm{NN}}=5.02$ TeV with the ATLAS detector using a total integrated luminosity of 28 nb$^{-1}$. Pions are identified using ionisation energy loss measured in the pixel detector. Two-particle correlation functions and the extracted source radii are presented as a function of collision centrality as well as average transverse pair momentum ($k_\mathrm{T}$), rapidity ($y^\star_{\pi\pi}$), and azimuthal angle with respect to the second-order event plane. Pairs are selected with a rapidity $-2< y^\star_{\pi\pi}<1 $ and with an average transverse momentum $0.1< k_\mathrm{T} <0.8$ GeV. The effect on the two-particle correlation function from jet fragmentation is studied, and a new method for constraining its contributions to the measured correlations is described. The measured source sizes are substantially larger in more central collisions and are observed to decrease with increasing pair $k_\mathrm{T}$. A correlation with the local single-particle multiplicity $dN/dy^\star_{\pi\pi}$ is demonstrated. The cross term $R_\mathrm{ol}$, which couples radial and longitudinal expansion, is measured as a function of rapidity, and a departure from zero is observed with 5.1$\sigma$ combined significance for $ y^\star_{\pi\pi} > -1$ in the most central events. The azimuthal modulation of the radii in central events is observed to be consistent with that predicted by hydrodynamics and observed in A+A collisions.