ALICE measurements of coherents $\rho^0$ photoproduction in Pb--Pb ultraperipheral collisions

The powerful photon fluxes of relativistic nuclei provide the possibility to study photonuclear and two-photon interactions in ultra-peripheral collisions (UPC), where the nuclei do not overlap and no strong nuclear interactions occur. Within the Vector Meson Dominance Model (VDM), the $\rho^0$ contribution dominates the QCD part of the photon structure function. The $\gamma+\textrm{A} \rightarrow \rho^0+\textrm{A}$ process in heavy-ion UPC is an excellent tool to test the black disk regime, where the target nucleus appears like a black disk and the total $\rho^0+\textrm{A}$ cross section reaches its limit. RHIC and first LHC results have deviated from some Glauber+VDM calculations, which thus call for new data. The ALICE Collaboration reports the first measurements of coherent $\rho^0$ photoproduction accompanied by electromagnetic dissociation (EMD) with data taken at $\sqrt{s_{\rm{NN}}} =5.02\,\textrm{TeV}$. The rapidity-dependent cross section of coherent $\rho^0$ photoproduction is measured and it is compared with theoretical models. In addition, a resonance-like structure around $1.7\,\textrm{GeV}/\it{c}^2$ is observed.