In 1924 Enrico Fermi travelled to Leiden, the Netherlands, to visit Paul Ehrenfest on a three-month fellowship from the International Education Board, which had been founded the year before by John D Rockefeller Jr for the "promotion and advancement of education throughout the world". Fermi was just 23 years old. During his stay in Leiden, he developed a method to calculate the interactions of charged particles with matter, which he called "äquivalente strahlung". This was extended to the relativistic case in the 1930s by Carl Friedrich von Weizsäcker and E J Williams, who realized that charged particles in cosmic rays could deliver the highest-energy (virtual) photon beams to study pair production of the positron. Today, the technique is known as the equivalent photon approximation, and it provides a powerful tool for fundamental physics.

In an echo of the insight of von Weizsäcker and Williams, a growing community of physicists is looking to CERN's forthcoming Large Hadron Collider (LHC) as a new source of very-high-energy photon-hadron interactions. When the LHC stores colliding lead beams at a centre of mass energy of 5.4 TeV per nucleon, for example, lead ions in grazing collisions will be bombarded with photons of energy extending to hundreds of tera-electron-volts in their rest frame. Similarly, photon-photon invariant masses in the range of 100 GeV will be created in this way.

Such collisions are known as ultraperipheral, and have the general feature of an absence of particles produced along at least one beam direction (Bjorken's "rapidity gap"), because the photon is neutral. In many cases the produced system can be measured in a rather clean way in any of the LHC's detectors. The strong field of these "quasi-real photons" has found many interesting applications in atomic, nuclear and particle physics.

In October of Fermi's centenary year, 2001, a workshop in Erice, Sicily, focused on ultraperipheral collisions. It was followed by a second meeting at CERN last March. The Erice workshop - Electromagnetic Probes of Fundamental Physics - was organized by Bill Marciano and Sebastian White from the US Brookhaven National Laboratory, and featured sessions on the muon g-2, ultraperipheral collisions at Brookhaven's Relativistic Heavy-Ion Collider (RHIC) and the LHC, as well as topics in strong field research.

Francis Farley of Yale University, US, gave a review of 44 years of g-2, while David Hertzog of Illinois, US, presented the latest results from Brookhaven. Andreas Höcker of Orsay, France, Simon Eidelman of Novosibirsk, Russia, and Marciano discussed the theoretical side, with an emphasis on hadronic corrections. Since the meeting, higher-precision results on g-2 have been announced (CERN Courier September). Teams from Brookhaven, Novosibirsk and the ALEPH experiment at CERN's large electron-position collider presented results on the related analyses of electron-positron annihilation to hadrons and semi-leptonic tau lepton decays. An intriguing discrepancy between the electron-positron and tau analyses is now the focus of the interpretation of g-2 as a potential indicator of physics beyond the Standard Model. New data from the Belle, BaBar and Kloe experiments at Japan's KEK laboratory, SLAC in the US and Frascati in Italy are eagerly awaited.

Ilya Ginzburg and Valery Serbo of Novosibirsk presented the related topic of future photon-photon colliders. The spontaneous breakdown of the vacuum by intense lasers was discussed by Adrian Melissinos of Rochester University, US, while Andreas Ringwald of Germany's DESY laboratory talked about the science that will be accessible with the X-ray free-electron lasers being developed by the DESY-led TESLA project. The sources and interactions of ultra-high-energy cosmic rays were the subject of talks from Alex Kusenko of UCLA and Steve Reucroft of Boston's Northeastern University, US.

Carlos Bertulani of Michigan State University, US, and Spencer Klein of Berkeley, US, along with Kai Hencken of Basel University, Switzerland, and Gerhard Baur of Jülich, Germany, reviewed the theoretical background of ultraperipheral collisions, while also pointing out the opportunities in both photon-photon and photon-hadron physics.

After presentations on recent experimental results from RHIC by Jim Thomas and White of Brookhaven, and Pablo Yepes of Rice University, US, the discussion turned to a prioritized list of the opportunities at RHIC and the LHC. These are summarized in an Erice white paper on hot topics in ultraperipheral collisions, which is included in the workshop proceedings (see Further reading).

The white paper was adopted as the point of departure for an exploratory meeting on ultraperipheral relativistic heavy-ion collisions that was held at CERN in March. This meeting, organized by Hencken and Yepes, was well attended not only by theoreticians, but also by experimentalists from the LHC collaborations interested in ultraperipheral collisions.

SLAC's Stan Brodsky gave the introductory talk, followed by Baur. Both presented a range of interesting applications from a theoretical point of view for photon-photon and photon-hadron collisions at RHIC and the LHC. Leonid Frankfurt of Tel Aviv University, Israel, discussed photoproduction of vector mesons on protons and ions. The first results from the STAR experiment at RHIC for coherent rho-meson production were presented by Falk Meissner of Berkeley. Joakim Nystrand of Lund University, Sweden, showed that interesting interference phenomena, due to the fact that each ion can either be the source of the photon or the target, can be studied in these collisions. Highlights from the rich photon-hadron programme at DESY's HERA electron-proton collider were presented by Giuseppe Iacobucci of the INFN in Bologna, Italy.

The strong electromagnetic excitation of heavy ions in ultraperipheral collisions is largely due to the strong source of low-energy photons. As they subsequently decay, heavy-ion fragments can be used for luminosity monitoring by detecting the neutrons in a zero-degree calorimeter, as discussed by White, Igor Pshenichnov of Moscow's Institute for Nuclear Research and Vladimir Korotkikh from Moscow State University, Russia.

Physics opportunities

A range of physics opportunities was discussed at the CERN workshop. Berkeley's Ramona Vogt pointed out the possibility of deducing the gluon structure function within the nuclear medium using heavy-quark production in photon-gluon fusion. Krzysztof Piotrzkowski from Louvaine-la-Neuve, Belgium, described how the possibility of tagging protons in a forward detector after photon emission permits the study of photon-photon collisions at invariant masses beyond the Z mass. This would allow researchers to look at the electromagnetic coupling of W bosons and also to search for new physics. Brookhaven's Anthony Baltz and Hencken, and Frank Krauss of Cambridge University, UK, discussed aspects of lepton pair production. These include bound-free pair production and the production of muonium.

Studies on detecting ultraperipheral collisions within the LHC detectors were presented by Serguei Sadovsky of Protvino, Russia, for ALICE and Yepes for CMS, both of whom emphasized the importance of having a trigger for these events. Daniel Brandt of CERN discussed the possibilities of different ion beams at the LHC, where electromagnetic processes can limit the maximum beam luminosity.

One tangible outcome of the workshop is the formation of a working group on ultraperipheral collisions, paying particular attention to the potential for this physics at the LHC. Another meeting is scheduled to take place at CERN on 11-12 October. Its goal will be to start work on a report giving full details of the physics of ultraperipheral heavy-ion collisions accessible within the LHC detectors as currently conceived. With such an intense source of photons, the conclusion of the workshops was that the future is bright for ultraperipheral collisions at the LHC.

Further reading

Fermi's original work on the equivalent photon approximation:
See e-print hep-th/0205086 at http://www.arxiv.org/.
Hot topics in ultraperipheral collisions is available as e-print hep-ex/0201034 at http://www.arxiv.org/.
For CERN workshop details see http://quasar.unibas.ch/upc/.