Physics of Ultra-Peripheral Nuclear Collisions

TL;DR

Ultra-peripheral collisions (UPCs) at high-energy hadron and heavy-ion colliders enable photon-induced and two-photon processes by treating the fields of fast nuclei as a flux of quasi-real photons via the Weizsäcker-Williams method, allowing photon-nucleus and photon-photon interactions at energies unattainable elsewhere. The paper reviews the formalism for the photon flux, distinguishes photonuclear and two-photon channels, and details photoproduction and two-photon production of mesons, heavy quark pairs, and lepton pairs, including interference effects, multi-photon dynamics, and connections to gluon densities and nuclear shadowing. It also discusses experimental signatures, cross-section calculations (Glauber/ GVMD approaches), triggering strategies, and background considerations, outlining both current capabilities and limitations for UPC studies. With the LHC’s energy frontier, UPCs promise direct probes of nuclear gluon distributions, tests of QED in strong fields, and access to new physics channels, albeit alongside machine-operations challenges such as e+e- pair production and beam losses that must be managed in heavy-ion running.

Abstract

Moving highly-charged ions carry strong electromagnetic fields that act as a field of photons. In collisions at large impact parameters, hadronic interactions are not possible, and the ions interact through photon-ion and photon-photon collisions known as {\it ultra-peripheral collisions} (UPC). Hadron colliders like the Relativistic Heavy Ion Collider (RHIC), the Tevatron and the Large Hadron Collider (LHC) produce photonuclear and two-photon interactions at luminosities and energies beyond that accessible elsewhere; the LHC will reach a $γp$ energy ten times that of the Hadron-Electron Ring Accelerator (HERA). Reactions as diverse as the production of anti-hydrogen, photoproduction of the $ρ^0$, transmutation of lead into bismuth and excitation of collective nuclear resonances have already been studied. At the LHC, UPCs can study many types of `new physics.'

Figures (9)

• Figure 1: (a) One-photon and (b) two-photon processes in heavy ion collisions. (c) Geometrical representation of the photon fluxes at a point outside nuclei 1 and 2, in a collision with impact parameter $b$. The electric field of the photons at that point are also shown. (d) Feynman diagram for $q \overline{q}$ production through photon-gluon fusion to leading order. (e,f) Example of higher order corrections to pair-production: (e) Coulomb distortion, and (f) production of multiple pairs. (g) The dominant diagram for $Au + Au \rightarrow Au^{*} + Au^{*} + \rho^{0}$ and (h) for $Au + Au \rightarrow Au^{*} + Au^{*} + e^{+}e^{-}$ or a meson $X$. The dotted lines in panels (g) and (h) show how the mutual Coulomb nuclear excitation factorizes from the particle production.
• Figure 2: Highly energetic charged particles have Lorentz contracted electric fields. The interaction of these fields can be replaced by the interaction of real (or quasi-real) photons.
• Figure 3: (a) S-factors ($S_{17}$) for the $^{7}$Be(p,$\gamma$)$^{8}$B reaction. The GSI data was obtained using the Coulomb dissociation method Sch04. The other data are from direct capture measurements HammJ. (b) Cross sections for the excitation of the GDR (1-phonon) and the DGDR (2-phonon) in $^{208}$Pb projectiles incident on different targets. The dashed curves are theoretical calculations.
• Figure 4: Rapidity (a) and invariant mass (b) distributions for coherent $\rho^{0}$ production in Au+Au interactions accompanied by mutual Coulomb breakup at $\sqrt{s} =$ 200 A GeV, by the STAR collaboration. The dashed curves in b) corresponds to a relativistic Breit-Wigner function and a Söding interference term; the solid curve is the sum of the two. The dash-dotted curve describes the background from incoherent interactions Meissner03.
• Figure 5: Rapidity distributions for exclusive $J/\psi$ and $\Upsilon$ production in nucleus-nucleus and proton-proton collisions. Adapted from KN04 and KN99.