Study of light-by-light scattering with PbPb collisions at 5.02 TeV with CMS

The elastic scattering of two photons in vacuum, γγ → γγ, is a pure quantum mechanical process that proceeds at leading order in the fine structure constant, o(α⁴), via virtual box diagrams containing charged particles. In the standard model (SM), the box diagram involves charged fermions (leptons and quarks) and boson (W^±) loops. Despite its simplicity, light-by-light (LbyL) scattering remains still unobserved today because of its tiny cross section σ_γγ ∝ α o(α⁴) ≈ 3.10^-9. It was demonstrated that the large (quasi) real photon fluxes of the protons and ions accelerated at TeV energies at the CERN Large Hadron Collider (LHC) can be used to detect the elastic γγ scattering. Since, the photon fluxes scales as Z², γγ luminosities are extremely enhanced for ion beams, up to Z⁴ = 5.10⁷ in the case of Pb-Pb collisions