The factorisation ratio $r_{2}$, as a function of $p_{\rm T}^{\, a}$ in bins of $p_{\rm T}^{t}$ for 0--5$\%$, 20--30$\%$ and 40--50$\%$ centralities in Pb--Pb collisions at $\sqrt{s_{_{\rm NN}}} = 2.76$~TeV, is presented (solid circles). CMS measurements are presented by open square~. Hydrodynamic calculations with MC-Glauber initial conditions and $\eta/s =$ 0.08~ and with MC-KLN initial conditions and $\eta/s =$ 0.20~, with Trento initial conditions and temperature dependent $\eta/s$~ and AMPT initial conditions and $\eta/s =$ 0.08~ are shown in green dot-dash, orange dashed curves, and magenta and grey shaded areas, respectively. | |