Femtoscopy with identified charged pions in proton-lead collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with ATLAS

Bose-Einstein correlations between identified charged pions are measured for $p+\mathrm{Pb}$ collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV with the ATLAS detector with a total integrated luminosity of $28~\mathrm{nb}^{-1}$. 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 the average transverse momentum ($k_{\mathrm{T}}$) and rapidity ($y^{\star}_{\pi\pi}$) of the pair. 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 of the radii with the local multiplicity is demonstrated. The scaling of the extracted radii with the mean number of participants is also used to compare a selection of initial-geometry models. The cross term $R_\mathrm{ol}$ is measured as a function of rapidity, and a departure from zero is observed with 4.8 $\sigma$ combined significance in the proton-going side of the most central events.