The LHC: a new high energy photon collider

19 September 2007

A look at the LHC’s potential for photon-induced interactions.


Photon-induced interactions have traditionally been studied with electron beams in fixed-target experiments and colliders, LEP (electron–positron) and HERA (electron–proton) in particular. However, photon–hadron and photon–photon interactions also occur when the electron beams are replaced by ultra-relativistic beams of other charged particles such as protons or heavy nuclei. In these cases, the maximum photon energies are restricted by the form factor of the projectile, but at the extremely high energies of the LHC they will be higher than at any other existing accelerator: up to a photon energy of around 4 TeV in the photon–proton centre-of-mass frame. Furthermore, since the intensity of the electromagnetic field – the number of photons in the “cloud” surrounding the charge of the beam particle – is proportional to the square of the particle’s charge Z, photonic interactions are enhanced by up to a factor of Z2, or around 104 for heavy ions. Indeed, the fields from heavy ions are strong enough that multiple photons may be exchanged in a single event. Figure 1 shows a schematic view of such an electromagnetic (or ultra-peripheral) nucleus–nucleus collision.

The study of photon-induced interactions at the LHC, as well as at existing hadron colliders such as RHIC at Brookhaven or the Tevatron at Fermilab, is challenging despite the high photon energies and fluxes. The interaction is always electromagnetic with an electron beam and the small contribution from the weak interaction can usually be neglected or easily separated. By contrast, the photonic interactions at hadron colliders must be separated from a dominant QCD background. The low multiplicity and mostly longitudinal kinematics of electromagnetic processes result in an event topology that is different from hadronic interactions. In particular, event triggering is a critical issue that depends much on instrumentation in the very forward direction, close to the beam line. The workshop on Photoproduction at collider energies: from RHIC and HERA to the LHC (held at ECT*-Trento in January), looked at how these issues have been addressed and solved in previous experiments, and considered the perspectives at the LHC. The workshop gathered around 40 physicists, equally divided between theorists and experimentalists.

Much of the workshop focused on the latest advances in the study of low-x parton densities in protons and nuclei probed by photons. Ultra-peripheral collisions at the LHC can probe the physics of parton saturation at Bjorken-x values as low as 10–5. Talks by SLAC’s Stan Brodsky, Mark Strikman of Pennsylvania and Leonid Frankfurt of Tel Aviv highlighted these theoretical aspects. HERA saw its last collisions at the end of June and has been an important machine for the field. Michael Klasen from Grenoble and DESY’s Sergey Levonian gave theoretical and experimental overviews, respectively, of the HERA results. At the Tevatron, the CDF collaboration has recently published its first analysis of two-photon interactions in proton–antiproton collisions. Andrew Hamilton of Geneva presented the results at the workshop. At RHIC, the STAR and PHENIX collaborations have studied ultra-peripheral gold–gold collisions. Yury Gorbunov of Creighton and David Silvermyr from Oak Ridge showed the latest results on vector meson photoproduction.

Looking to the future, Krzysztof Piotrzkowski from UC Louvain presented the group’s comprehensive study of various photon-induced electroweak and beyond-Standard Model processes that can be studied in proton–proton collisions at the LHC. These include associated W-Higgs and single-top photoproduction, as well as two-photon production of W boson pairs. To conclude the series of talks at the workshop, Otto Nachtmann of Heidelberg and Ute Dreyer of Basel covered the theory of anomalous gauge-boson couplings in γ– γ, γ–p and γ–A interactions.

The physics of photon–nucleus interactions in ultra-peripheral collisions is also the focus of a CERN Yellow Report, completed in June. This 230-page document, the joint effort of more than 20 contributors, summarizes results from the SPS at CERN and from RHIC. It examines planning for ultra-peripheral collisions at the ALICE, ATLAS, and CMS experiments at the LHC. The vitality of this research field was also evident in the number of contributions at the Photon 2007 conference held in Paris in July.

The conclusion is that the LHC has much to offer as a photon collider. Photon–hadron and photon–photon processes will reach energies an order of magnitude larger than at previous colliders. They will not only provide valuable information on the strong interaction – in particular of low-x parton densities and non-linear QCD phenomena – but will also open new windows on electroweak processes and physics beyond the Standard Model, which will complement the mainstream studies in proton–proton and nucleus–nucleus collisions.

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