Values of (2/$\langle N_{\mathrm{part}} \rangle$)\avdndeta for central Pb--Pb and Au--Au collisions as a function of $\sqrt{s_{\mathrm{NN}}}$. The $\sqrt{s_{\mathrm{NN}}}$ dependence of the heavy-ion collision data is well described by the function $s^{0.156}_{\mathrm{NN}}$ (solid line). The shaded band represents the uncertainty on the extracted power-law dependency. The central Pb--Pb measurements from ATLAS at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV and from CMS at $\sqrt{s_{\mathrm{NN}}}=2.76$ and 5.36 TeV have been shifted horizontally for clarity.