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Observing light-by-light scattering at the Large Hadron Collider

Elastic light-by-light scattering, $γγ\toγγ$, is open to study at the Large Hadron Collider thanks to the large quasi-real photon fluxes available in electromagnetic interactions of protons (p) and lead (Pb) ions. The $γγ\toγγ$ cross sections for diphoton masses $m_{γγ} > 5$ GeV amount to 105 fb, 260 pb, and 370 nb in p-p, p-Pb, and Pb-Pb collisions at nucleon-nucleon center-of-mass energies $\sqrt{s_{NN}}$ = 14 TeV, 8.8 TeV, and 5.5 TeV respectively. Such a measurement has no substantial backgrounds in Pb-Pb collisions where one expects about 70 signal events per run, after typical detector acceptance and reconstruction efficiency selections.