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

Bose-Einstein correlations between identified charged pions are measured for p+Pb collisions at $\sqrt{s_{NN}}$ = 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 average transverse pair momentum (k_T) and rapidity ($y_{\pi \pi }^{\ast }$) as well as collision centrality. Pairs are selected with a rapidity −2 < $y_{\pi \pi }^{\ast }$ < 1 and with an average transverse momentum 0.1 < k_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_T. A correlation with the local single-particle multiplicity dN_ch/dy* 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_ol, which couples radial and longitudinal expansion, is measured as a function of rapidity, and a departure from zero is observed with 4.8 σ combined significance for $y_{\pi \pi }^{\ast }$ > −1 in the most central events. (paper)