by Joakim Nystrand and Daniel Tapia Takaki. Published: 28 March 2014

The CERN Large Hadron Collider (LHC) has worked fantastically well in the past few years, going beyond all expectations. It started its physics programme in 2009 colliding protons at 900 GeV, and then reaching into the Tera electron-volt range supplanting the Fermilab Tevatron accelerator as the most powerful hadron-hadron collider. The LHC was also designed to collide heavy-ions, with the idea of exploring a new energy domain beyond that of RHIC at the Brookhaven National Laboratory. In 2010 this task was completed when the LHC collided lead (Pb) beams at 2.76 TeV, an energy which is more than an order of magnitude larger than that at RHIC.

Although the LHC was not primarily designed to study photon-hadron and photon-photon collisions, they occur in both pp and heavy-ion collisions. The beam energies at the LHC are high enough to make the LHC the most energetic photon source ever built. The protons and ions which are accelerated by the LHC themselves carry an electromagnetic field, which can be viewed as a source of photons. That is, a photon generated by one of these hadrons can interact with another photon (or with a hadron) producing all kinds of particles. Such physics processes are called photon-induced reactions, as they are driven by the interacting photon.

The appearance of these events stands in sharp contrast to central heavy-ion collisions, where the overlap between the incoming ions is the largest, and thousands of particles are produced. The relevant collisions typically occur at impact parameters of several tens (or even hundreds) of femtometres – cases when the incoming ions barely overlap, and well beyond the range of the strong force.

J/ψ candidate in an ultra-peripheral Pb-Pb collision. A dimuon pair in otherwise an empty detector.

It is worth pointing out that photon-induced processes have by far the largest cross sections in Pb-Pb collisions at the LHC. The total cross section for breaking up one of the nuclei through a photonuclear process is over 200 barns. In most of these reactions the nucleus just breaks up without any particle production. However, the cross section for having at least one photoproduced charged particle inside the main ALICE tracker device (the Time Projection Chamber) acceptance is still substantial, about 4 b. But both these numbers are dwarfed by the total cross section for producing an e+e- pair from an interaction between two photons. This cross section is about 3 million times larger than that for normal hadronic pp collisions.

A photonuclear interaction that has attracted a lot of interest is exclusive vector meson production. That is, a reaction where only a vector meson is produced in the final state, and nothing else. The large cross section of this process is understood from what is known as Vector Meson Dominance. This means that the photon may fluctuate into a quark-anti-quark pair and, since the photon has spin 1 and negative parity, the fluctuation will most likely be to a Vector Meson. The J/ψ vector meson is one of the particles that is particularly interesting for the heavy-ion community.

Previously, the HERA experiments at DESY, namely, H1 and ZEUS, studied systematically the photo-production of J/ψ and were able to reach 300 GeV in the centre-of-mass of the photon-proton collision. The proton contains a large number of gluons each carrying a very small fraction of the proton momentum.

The interaction between hadrons and gluons is governed by the theory of strong interactions called Quantum Chromo-Dynamics (QCD), although there is not yet any known method to predict the gluon density inside a nucleon or a nucleus. Having a good understanding of the proton gluon density is essential for many physics analysis processes sensitive to the strong interaction that governs the interaction between hadrons and gluons.

There are some theoretical and phenomenological constraints on how the gluon density should behave at high energy but there is not an overall agreement as to what happens when we reach very high energies such as those produced at the LHC. There are several theoretical ideas that can describe what could happen at the LHC photon-hadron energies. Gluon saturation is one of these ideas. J/ψ photoproduction is thought to be sensitive to this in a way that this effect can be easily factorized from other possible mechanics. Nobody knows at what energy gluon saturation phenomena might start to show up in a way that we can distinguish it, but certainly the 1 TeV energy scale is worth studying.

So far, ALICE has studied photon-photon, photon-lead and photon-proton interactions. At the LHC we are not only reaching the highest energies when colliding photons, but also exploring new kinematic regions that have never been explored before. Some of these photons are indeed very energetic, allowing us to produce collisions at 1 Teraelectronvolt for the first time.

The ALICE collaboration has recently taken advantage of this effect in a study of coherent photoproduction of J/ψ mesons in Pb-Pb interactions. The J/ψ is detected through its dimuon decay in the muon arm of the ALICE detector [1,2], which also provides the trigger for these events, or in its dielectron or dimuon decay in the central barrel [3]. At the rapidities (y around 3) studied in the muon arm, J/ψ photoproduction is sensitive mainly to the gluon distribution at values of Bjorken-x of about 10–2, whereas at mid-rapidity on probes x ≈ 10 -3. The result from ALICE is that the data favour models that include strong modifications to the nuclear gluon distribution, known as nuclear shadowing.

During the first running period of the LHC there have been quite a few results on photon-induced collisions. Results with heavy-ion beams have so far come only from ALICE. LHCb has, however, accumulated an impressive statistics of about 100,000 exclusively produced J/ψs in p-p collisions, and CMS have published papers on two-photon interactions in p-p collisions, including a study of W+W- production. In the future, one can expect higher luminosities and thereby probe rarer final states. There are in addition to exclusive vector meson production several things one can look for. These include two-photon production of rare final states, for example γγ→ K0K0, light-by-light scattering, γγ →γγ, and various inclusive photonuclear processes, for example γ +A → jet +X.

Further reading

1. B. Abelev et al. (ALICE Collaboration) Phys.Lett. B718 (2013) 1273-1283

2. Using the LHC as a photon collider, CERN Courier, 6 November 2012 http://cerncourier.com/cws/article/cern/51147

3. A. Abbas et al. (ALICE Collaboration) Eur. J. Phys. C73 2617