Abstract
Ultraperipheral collisions (UPCs) of heavy ions and protons are the energy frontier for electromagnetic interactions. Both photonuclear and two-photon collisions are studied at collision energies that are far higher than those available elsewhere. In this review, we discuss physics topics that can be addressed with UPCs, including nuclear shadowing, nuclear structure, and searches for physics beyond the Standard Model.
Highlights
Some of the ultraperipheral collision reactions discussed in this review: (a) generic photonuclear interaction with nuclear breakup of the target; (b) incoherent photoproduction, generic to heavy quarks and jets; (c) exclusive photoproduction of a vector meson; (d ) coherent photoproduction of a vector meson, accompanied by nuclear breakup; (e) dilepton production γ γ → + −; ( f ) dilepton production γ γ → + − + γ, including higher-order final-state radiation; (g) light-by-light scattering, with no nuclear breakup; and (h) central exclusive diphoton production with double breakup
Similar physics is accessible in principle at electron–ion colliders, such as EIC, LHeC, and Future Circular Collider (FCC)-eh, while two-photon physics can be studied at e+e− colliders
For Ultraperipheral collisions (UPCs), we focus on heavier ions
Summary
Fermi (22), von Weizsäcker (23), and Williams (24) showed that these perpendicular fields may be treated as a flux of linearly polarized virtual photons; the energy spectrum is given by the Fourier transform of their spatial (along the ion direction) dependence. To allow for reconstruction of exclusive final states, collisions are excluded in which the nuclei interact hadronically This can be done by taking the minimum impact parameter bmin to be 2RA. P0had(b) can be determined with a Glauber calculation (25), which accounts for the nuclear shape and interaction probability In these calculations, the nucleon distribution of heavy nuclei is well described by a Woods–Saxon distribution, whereas a Gaussian form factor is appropriate for lighter nuclei (Z ≤ 6) (13). Pairs: M is the final-state mass for a dilepton state, and Y is the final-state pair rapidity
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