Observing Light-by-Light Scattering at the Large Hadron Collider

Elastic light-by-light scattering (gamma gamma -> gamma gamma) is open to study at the Large Hadron Collider thanks to the large quasireal photon fluxes available in electromagnetic interactions of protons (p) and lead (Pb) ions. The gamma gamma -> gamma gamma cross sections for diphoton masses m(gamma gamma) > 5 GeV amount to 12 fb, 26 pb, and 35 nb in p-p, (p)-Pb, and Pb-Pb collisions at nucleon-nucleon center-of-mass energies root s(NN) = 14, 8.8, and 5.5 TeV, respectively. Such a measurement has no substantial background in Pb-Pb collisions where one expects about 20 signal events per run, after typical detector acceptance and reconstruction efficiency selections.