Light interacts with matter all the time. Blue skies, green leaves, and the colors of the rainbow all come about because of the scattering of light. But light doesn't scatter off itself. In fact, under Maxwell's equations, which originally defined electromagnetism, light can't scatter off itself.

As with many other things, however, quantum mechanics showed that what we thought was impossible is possible: a photon can, technically, scatter off another photon. But that was recognized back in the 1930s, and we've not been able to confirm that it actually happens.

Until now, that is.

Researchers from the ATLAS detector at the Large Hadron Collider have found evidence that photons can scatter in cases in which lead atoms barely miss each other. The scattering could provide an independent test of whether there are particles in addition to those predicted by the Standard Model.

In theory...

Scattering is the product of electromagnetic interactions. For photons, this generally occurs because they can interact with charged particles. Since photons themselves aren't charged, it's impossible for them to interact with each other under classical physics.

But we live in a world where quantum mechanics rules, and quantum mechanics provides photons a way to interact. That interaction involves a loop of what are called virtual particles, which appear out of the fabric of space and rapidly disappear again, though they can interact with other particles during their brief existence. In this case, the two photons enter into a cycle in which they interact with a charged virtual particle. The virtual particle is gone in the end, but so are the original two photons. In their place are two new ones, moving in different directions.

This replacement turns out to be more significant than it sounds. The number of possible cycles that can involve the photons is a product of the number of charged particles that exist. So, the more particles out there, the more likely this sort of scattering is. Which means that, if we see a large difference from Standard Model predictions, that's a strong indication that there are additional charged particles that we've yet to discover.

So, we've a strong motivation for trying to observe photon-photon scattering. But, after over 80 years, no obvious route has existed for doing so. The sorts of energies and densities of light needed were beyond the reach of even extremely powerful lasers. Various means of approaching the issue indirectly had been tried, but there was no obvious route to a clear detection.

Theory to practice

This is where theorists provided an extremely useful contribution. Last year, they published a paper suggesting that CERN's Large Hadron Collider could produce the desired effect. That's because, when the collider runs using lead atoms instead of hydrogen, it can produce lots of virtual photons. There was just one catch: the lead atoms couldn't collide.

The secret turned out to be the relatively high charge of the lead atoms, which are completely stripped of electrons before being accelerated in the LHC. This creates a very intense electromagnetic field in their immediate vicinity. These fields effectively act as a collection of virtual photons. As the team from the LHC's ATLAS detector phrase it, "the colliding lead nuclei can be treated as a beam of quasi-real photons with a small virtuality."

Technically, if two lead ions collide, these fields get a brief chance to interact immediately before the collision (extremely brief, given that relativistic effects squash them as flat as a pancake). But the collision and ensuing debris produce photons in abundance, so it would be nearly impossible to identify photon scattering in the mess of debris that followed. Instead, the ATLAS team relies on near misses—cases where the two ions passed in close proximity, allowing their electromagnetic fields to interact.

So, what the team was looking for was a detection in which two photons were picked up by the hardware, but there were no signs of a collision. Conveniently, during the 2015 run of the LHC, the ATLAS team has specifically saved some events in which there was some activity in specific detector hardware but little sign of any particles. The team then identified and got rid of any cases in which electrons or muons were also present, since these could generate photons. The researchers also limited their search to photons in a specific energy range, since photons are hard to produce through processes that normally occur at the LHC's energies. To get rid of additional background events, the authors also set strict time limits so that the photons could not be produced by the decay of some other particle.

These restrictions were so stringent that the authors estimated that the entire 2015 run of lead collisions should only produce 2.6 events that pass their criteria. Instead, they identified 13 events, which is 4.4 standard deviations higher. That's not quite enough for announcing discovery (which requires five standard deviations), but it's extremely suggestive, especially given the low level of background noise in the experiment.

Why not analyze 2016's data? The possibility exists that they simply don't have any. The LHC produces far more collisions than we can record, so it has what are called "triggers:" signatures of interesting events that indicate they're worth saving. Given that this trigger fired when there wasn't a collision, chances are that it wasn't a high priority to maintain once the LHC ramped up to lots of high-energy collisions.

But this paper makes a strong case for making it a priority in future years. By the end of 2018, the people running the LHC expect that they'll be producing twice the number of lead ion collisions, and further upgrades are planned during the 2020s. It's also possible to modify the detector slightly to cover a larger range of energy in the photons. With enough data, we could not only confirm that photon-photon scattering occurs (after an 80+ year wait); we could tell how well its rate matches the predictions of the Standard Model. And thus have another test for physics beyond the Standard Model.

Nature Physics, 2017. DOI: 10.1038/nphys4208 (About DOIs).