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

Elastic light-by-light scattering (gamma-gamma-->gamma-gamma) can be observed 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 gamma-gamma-->gamma-gamma cross sections for diphoton masses m_gammagamma > 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 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 20 signal events per run, after typical detector acceptance and reconstruction efficiency selections.