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Measuring light-by-light scattering at the LHC and FCC

Elastic light-by-light scattering, $γγ\toγγ$, can be measured in electromagnetic interactions of lead (Pb) ions at the Large Hadron Collider (LHC) and Future Circular Collider (FCC), using the large (quasi)real photon fluxes available in ultraperipheral collisions. The $γγ\toγγ$ cross sections for diphoton masses m$_{γγ}>$ 5 GeV in pp, pPb, and PbPb collisions at LHC ($\sqrt{\rm s_{_{NN}}}$ = 5.5, 8.8, 14 TeV) and FCC ($\sqrt{\rm s_{_{NN}}}$ = 39, 63, 100 TeV) center-of-mass energies are presented. The measurement has controllable backgrounds in PbPb collisions, and one expects about 70 and 2500 signal events per year at the LHC and FCC respectively, after typical detector acceptance and reconstruction efficiency selections.