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The first measurement of the coherent photoproduction of $\rho^{0}$ vectormesons in ultra-peripheral Xe-Xe collisions at $\sqrt{s_{\mathrm{\rm NN}}}=5.44$ TeV is presented. This result, together with previous HERA $\gamma$p dataand $\gamma$-Pb measurements from ALICE, describes the atomic number ($A$)dependence of this process, which is particularly sensitive to nuclearshadowing effects and to the approach to the black-disc limit of QCD at asemi-hard scale. The cross section of the ${\rm Xe}+{\rm Xe} \to \rho^{0} +{\rm Xe}+{\rm Xe}$ process, measured at midrapidity through the decay channel$\rho^{0}\to\pi^+\pi^-$, is found to be ${\rm d}\sigma/{\rm d}y=131.5\pm 5.6{\rm (stat.)} ^{+17.5}_{-16.9} {\rm (syst.)}$ mb. The ratio of the continuum toresonant contributions for the production of pion pairs is also measured. Inaddition, the fraction of events accompanied by electromagnetic dissociation ofeither one or both colliding nuclei is reported. The dependence on $A$ of crosssection for the coherent $\rho^{0}$ photoproduction at a centre-of-mass energyper nucleon of the $\gamma A$ system of $W_{\gamma A,n}= 65$ GeV is found to beconsistent with a power-law behaviour $\sigma (\gamma A\rightarrow \rho^{0} A)\propto A^{\alpha}$ with a slope $\alpha = 0.96 \pm 0.02 {\rm (syst.)}$. Thisslope signals important shadowing effects, but it is still far from thebehaviour expected in the black-disc limit.

physics letters b

First measurement of coherent $\rho^{0}$ photoproduction in  ultra-peripheral Xe-Xe collisions at $\mathbf{\sqrt{s_{\rm  {\scriptscriptstyle \mathbf{NN}}}} = 5.44}$ TeV