Coherent γγ and γA interactions in very peripheral collisions at relativistic ion 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 γγ-processes are finally discussed together with important background processes.

Production of QED pairs at small impact parameter in relativistic heavy ion collisions

“…2.7 of Ref. [16] and references therein). One difficulty in this approach is the correct choice of the cutoff parameter present in the expression for the photon spectrum.…”

Searching for Axionlike Particles with Ultraperipheral Heavy-Ion Collisions

“…(See Ref. [3][4][5] for reviews.) Such exclusive processes are characterized by a lack of additional detector activity and a large rapidity gap between the produced particles and outgoing beams.…”

CMS Physics Technical Design Report: Addendum on High Density QCD with Heavy Ions

“…In ultraperipheral collisions (UPCs) of heavy-ions at impact parameters larger than twice the nuclear radius, where no nucleon-nucleon collisions occur, a strong flux of photons is generated that can be used for high-energy photoproduction (γ A, γ γ ) studies, as shown schematically in figure 1.19 [130][131][132]. The equivalent photon spectrum flux in UPCs increases with Z 2 /E γ and, thus, for Pb there is a Z 2 ∼ 7000 enhancement factor compared to electron or proton beams.…”

“…At the LHC, the strong electromagnetic field due to the coherent action of the Z = 82 proton charges of the lead nucleus accelerated at TeV energies generates an equivalent flux of photons which can be used for high-energy photoproduction studies (see e.g. [130][131][132]). The equivalent (Weiszäcker-Williams) photon spectrum of UPCs has a ∝ Z 2 /E γ dependence and, thus, for a Pb beam, a Z 2 ≈ 7000 flux enhancement factor is expected compared to electron or proton beams.…”