Correlation of flow harmonics and mean transverse momentum in 5.02 TeV p+Pb and Pb+Pb collisions and event-plane dependence of HBT radii in high-multiplicity p+Pb collisions with the ATLAS detector

To assess the properties of the quark-gluon plasma formed in heavy ion collisions, correlations between the mean transverse momentum, $p_{\mathrm{T}}$, and the magnitude of the flow harmonics, $v_n$, are measured by the ATLAS experiment at the LHC. The analysis uses data samples of lead-lead collisions at $\sqrt{s_{NN}} = 5.02$~TeV and proton-lead collisions at $\sqrt{s_{NN}} = 5.02$~TeV, corresponding to integrated luminosities of 22 $\mu\mathrm{b}^{-1}$ and 28 $\mathrm{nb}^{-1}$, respectively. The correlations are measured using a modified Pearson coefficient that is independent of multiplicity fluctuations. To suppress any short-range correlations, the $[p_{\mathrm{T}}]$ is measured at mid-rapidity and the flow harmonics are measured at forward rapidity. In Pb+Pb collisions, significant (non-zero) values of the correlation coefficients are observed for all studied harmonics, which show a strong centrality dependence but vary only weakly with the charged particle $p_{\mathrm{T}}$ range used in the measurement. On the other hand, in $p$+Pb collisions, the correlation coefficient measured for the $2^{nd}$ harmonic is found to show only a weak centrality dependence. The predictions of a 3+1D viscous-hydrodynamic model are found to be qualitatively consistent with the data, indicating hydrodynamic origin of these correlations in $p$+Pb collisions. As an independent test for the hydrodynamic description of collectivity observed in $p$+Pb collisions, measurements of two-pion HBT correlations as a function of the angle of the pion pair with respect to the second-order event plane angle are presented. The HBT correlation functions, corrected for event plane resolution, are measured as a function of $q_{out}$, $q_{side}$ and $q_{long}$ in intervals of pair transverse momentum and second-order flow-vector magnitude. The correlation functions are fit using the Bowler-Sinyukov form with the exponential HBT correlation function. The extracted HBT radii, $R_{out}$, $R_{side}$ and $R_{long}$ and a significant out-side cross-term are found to exhibit significant modulation with respect to the second-order event plane, similar to that observed in heavy ion collisions. Results of the measurements and physics implications of the result, will be discussed.