The protons and nuclei accelerated by the LHC are surrounded by strong electric and magnetic fields. These fields can be treated as an equivalent flux of photons, making the LHC the world’s most powerful collider not only for protons and lead ions but also for photon–photon and photon–hadron collisions (CERN Courier October 2007). This is particularly so for beams of multiply charged heavy-ions, where the number of photons is enhanced by almost four orders of magnitude compared with the singly charged protons (the photon flux is proportional to the square of the ion charge).
The ALICE collaboration has recently taken advantage of this effect in a study of coherent photoproduction of J/ψ mesons in lead–lead (PbPb) collisions. The J/ψ is detected through its dimuon decay in the muon arm of the ALICE detector, which also provides the trigger for these events. The relevant collisions typically occur at impact parameters of several tens of femtometres, which is well beyond the range of the strong force, so the nuclei usually remain intact and continue down the beam pipe. The photonuclear origin of the J/ψ is therefore ensured by requiring that the detector is void of other particles, that there is only one positive and one negative muon candidate, and that the J/ψ has very low transverse momentum, etc. The appearance of these events (see figure) stands in sharp contrast to central heavy-ion collisions, where thousands of particles are produced.
These interactions carry an interesting message about the partonic substructure of heavy nuclei. Exclusive photoproduction of heavy vector mesons is believed to be a good probe of the nuclear gluon distribution. The cross-section measured in a heavy-ion collision Pb+Pb → Pb+Pb+J/ψ is a convolution of the equivalent photon spectrum with the photonuclear cross-section for γ+Pb → J/ψ+Pb. The latter process can be modelled as the colourless exchange of two gluons.
At the rapidities (y around 3) studied in ALICE, J/ψ photoproduction is sensitive mainly to the gluon distribution at values of Bjorken-x of about 10–2. Although the experimental error is rather large, the conclusion from ALICE is that the data favour models that include strong modifications to the nuclear gluon distribution, known as nuclear shadowing.