CERN Accelerating science

Colliding photons at the LHC!

by Panos Charitos

The interaction of photons with matter such as Compton and Thompson scattering are well-known at higher photon energies. What about the scattering events between photons? In classical Maxwell theory photons do not interact. In contrast, in quantum theory they can interact via quantal fluctuations.

During the early years of quantum electrodynamics (QED), Heisenberg and his student Euler realised that photons may scatter off of each other through a quantum-loop process involving virtual electron and positron pairs. This phenomenon of light-by-light scattering (γγ → γγ), albeit very rare, breaks the linearity of Maxwell’s equations and is one of the oldest predictions of QED.  It was realised in the early history of Quantum Electrodynamics (QED) that this effect is related to the polarisation of the vacuum.

The cross section for elastic γγ → γγ scattering is so small that till recently it was beyond the experimental reach. However, at the LHC is accessible thanks to the large electromagnetic field strengths generated as two ultra-relativistic lead ions collide.

In ‘ultra-peripheral collision’ (UPC) events, with impact parameters larger than twice the radius of the nuclei, the strong interaction becomes less significant and the electromagnetic interaction becomes more important. The electromagnetic field of the ion increases with the proton number (Z). For example, for a lead (Pb) nucleus with Z = 82 the field can be up to 1025 V m−1.

The final-state signature of interest is the exclusive production of two photons, Pb + Pb (γ γ ) → Pb(∗)+ Pb(∗)γ γ , where a possible EM excitation of the outgoing ions is denoted by (∗). Hence, the expected signature is two photons and no further activity in the central detector, since the Pb(∗) ions escape into the LHC beam pipe. The resulting signature – two photons in an otherwise empty detector – is almost the diametric opposite of the tremendously complicated events typically expected from lead nuclei collisions.

As bunches of lead ions are accelerated, an enormous flux of surrounding photons is generated. When ions meet at the centre of the ATLAS detector, very few collide, yet their surrounding photons can interact and scatter off one another.

Using 480 µb−1 of lead–lead collision data recorded at a centre-of-mass energy per nucleon pair of 5.02 TeV by the ATLAS detector, evidence was found for light-by-light scattering. Candidate diphoton events were recorded in the Pb + Pb run in 2015 using a dedicated trigger for events with moderate activity in the calorimeter but little additional activity in the entire detector. A total of 13 candidate events were observed with an expected background of 2.6 ± 0.7 events.

“This measurement has been of great interest to the heavy-ion and high-energy physics communities for several years, as calculations from several groups showed that we might achieve a significant signal by studying lead-ion collisions in Run 2,” says Peter Steinberg (Brookhaven National Laboratory), ATLAS Heavy Ion Physics Group Convener.

ATLAS physicists will continue to study light-by-light scattering during the upcoming LHC heavy-ion run, scheduled for 2018. More data will further improve the precision of the result and may open a new window to studies of new physics. In addition, the study of ultra-peripheral collisions should play a greater role in the LHC heavy-ion programme, as collision rates further increase in Run 3 and beyond.