Ultra-peripheral collisions are collisions between two nuclei (or more generally two hadrons) without overlap. The impact parameter is thus larger than the sum of the nuclear radii. Purely strong interactions are consequently heavily suppressed in these collisions, which are instead mediated by the electromagnetic field. The electromagentic field of a relativistic, charged particle can be treated as an equivalent flux of photons. In an ultra-peripheral collision, a photon from one of the nuclei may interact with a photon from the field of the other nucleus (a pure QED process), or it might interact with the other, target nucleus directly. Examples of the latter process are exclusive vector meson production, gamma+A --> V+A, and photon-parton interactions for example gamma+gluon --> q+qbar.
Initially, ultra-peripheral collisions were associated with exclusive processes, for example two-photon production of lepton pairs and exclusive vector meson production. In Run 3, the focus has instead shifted to more general photonuclear interactions like the gamma-gluon fusion process mentioned above. The upgrade of the ALICE read-out system to allow for continuous read-out has contributed to this. These interactions provide a new possibility to search for collectivity in small systems. Many of the initially proposed Quark-Gluon Plasma signatures, believed to be unique for central heavy-ion collisions, have subsequently been observed also in proton-proton and proton-nucleus collsions. Inelastic photonuclear interactions will extend these studies to a new system. The results are highly relevant for the future Electron-Ion Collider (EIC) which will be built at Brookhaven National Laboratory.
In this talk I will summarize the latest results on ultra-peripheral collisions with a special focus on their implications for the EIC.
