Central exclusive production
Central exclusive production is any process of the form
where is any system in the rapidity gap between the two final-state protons, hence called central. The production of is exclusive, because the hadronic final state is fully specified: the scattering protons remain intact and there are no beam remnants due to breaking-up protons.
In hadron collisions other than proton-proton, the incoming and outgoing particles are replaced accordingly.