Coherent gamma-gamma and gamma-A interactions in very peripheral collisions at relativistic ion colliders

TL;DR

This work surveys the physics of very peripheral relativistic heavy-ion collisions, where coherent electromagnetic fields generate abundant quasireal photons enabling photon-photon and photon-nucleus interactions. It develops and applies the equivalent photon approach (EPA) in impact-parameter space, incorporating elastic/inelastic nuclear form factors and strong-interaction effects, to yield γγ luminosities and accessible invariant-mass ranges up to ~100 GeV at the LHC. The authors review γγ phenomenology across hadron colliders, including lepton-pair production, meson and resonance production, vector-m meson diffraction, and potential new-physics signals (SUSY, Higgs, extra dimensions, monopoles), as well as photon-nucleus channels and diffractive backgrounds. They also discuss experimental strategies, triggers, and backgrounds for isolating very peripheral γγ and γ-hadron events, highlighting STAR results at RHIC and the prospects for LHC detectors like CMS and ALICE. Overall, the paper establishes peripheral heavy-ion collisions as a powerful laboratory for high-energy QCD, electroweak processes, and beyond-Standard-Model searches, with vector-meson factories and photon-induced processes offering rich, testable predictions at current and upcoming colliders.

Abstract

Due to coherence, there are strong electromagnetic fields of short duration in very peripheral collisions. They give rise to photon-photon and photon-nucleus collisions with a high flux up to an invariant mass region hitherto unexplored experimentally. After a general survey of the field equivalent photon numbers and photon-photon luminosities, especially for relativistic heavy ion collisions, are discussed. Special care needs to be taken to include the effects of the strong interaction and nuclear size in this case. Photon-photon and photon-hadron physics at various invariant mass scales are then discussed. The maximum equivalent photon energy in the lab-system (collider frame) are typically of the order of 3 GeV for RHIC and 100 GeV for LHC. Diffractive processes are an important background process. Lepton-pair, especially electron-positron pair production is copious. Due to the strong fields there will be new phenomena, like multiple e+e- pair production. The experimental techniques to select gamma-gamma-processes are finally discussed together with important background processes.

Figures (41)

• Figure 1: A fast moving nucleus with charge $Ze$ is surrounded by a strong electromagnetic field. This can be viewed as a cloud of virtual photons. These photons can often be considered as real. They are called " equivalent" or " quasireal photons". In the collision of two ions these quasireal photons can collide with each other and with the other nucleus. For very peripheral collisions with impact parameters $b>2R$, this is useful for photon-photon as well as photon-nucleus collisions.
• Figure 2: The production of a muon pair at the ISR with (a) and without the production of hadrons (b) is shown. Typical characteristics of very peripheral collisions are seen: low particle multiplicity and a small sum of transverse momenta. Reproduced from Fig. 2, p. 244 of Vannucci80 with kind permission of Springer-Verlag and the author.
• Figure 3: In the collision of either leptons with hadrons or hadrons with hadrons photon-photon (a) and photon-hadron collisions (b) can be studied. The principal diagrams are shown schematically here. In the collisions of hadrons additional effects need to be taken into account: inelastic photon emission processes (c), "initial state interaction" (d) and (e), as well as final state interaction (f).
• Figure 4: The general form of a two-photon process is shown. The two incoming particles $A_1$ and $A_2$ either stay in their ground states or undergo a transition to exited states $A_1$ and $A_2'$, while each emitting a photon. The two photon fuse to a final state $X_f$. Also shown are the momenta of all particles involved
• Figure 5: In the semiclassical picture, which is valid in the case of heavy ion collisions, initial state interactions between the two ions take place if the impact parameter between the two ions is smaller than $R_{min}=R_1+R_2$. Final state interaction can occur if the individual $b_i$ are smaller than $R_i$.