Heavy ion collisions, particularly lead-on-lead, produce very large photon fluxes. The quantummechanical processes induced by these photons can be efficiently studied in the clean environment of ultraperipheral collisions. In this presentation, highlights of the first observation of the 𝜏 lepton production in ultraperipheral Pb-Pb collision data recorded at √SNN = 5.02 TeV by the CMS experiment at the LHC will be discussed. This measurement paves the way for the determination of the anomalous electromagnetic moments of the 𝜏 lepton, which is poorly constrained so far. I will also present the first combination measurements of light-by-light scattering using Pb-Pb data recorded by the ATLAS and CMS experiments. An accurate determination of the integrated luminosity is of absolute importance to carry out precise measurements of the cross-section of a given process at colliders. In the last part of the talk, I will briefly discuss luminosity measurements for heavy-ion collisions.
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