ATLAS observes W-boson pair production from light colliding with light
https://atlas.cern/updates/physics-briefing/observation-w-pair-from-light
In everyday life, two crossing light beams follow the rules of classical electrodynamics and do not deflect, absorb or disrupt one another. However, at the high energies seen in LHC collisions, effects of quantum electrodynamics become important. For a short moment, photons radiated off the incoming proton beams can scatter and transform into a particle–antiparticle pair which appears as light-by-light interactions in the detector. This process was first observed by the ATLAS Collaboration in 2019. Indeed, the Standard Model describes quantum electrodynamics as part of electroweak theory, which not only predicts that force-carrying particles – the W bosons, Z boson and photon – interact with ordinary matter, but also among themselves.
The newly observed process proceeds via a very rare type of phenomenon where two photons collide to directly produce two W bosons of opposite electric charge via a four force-carrier interaction, among others (see Figure 2). Although the ATLAS and CMS Collaborations saw first evidence of this process in data recorded during Run 1 of the LHC (2011–2012), its observation required the substantially larger dataset taken during Run 2 (2015–2018).
This rare process occurs as bunches of high-energy protons skim past each other in “ultra-peripheral collisions”, if only their surrounding electromagnetic fields interact. Quasi-real photons from these fields scatter off one another to produce a pair of W bosons and leave a distinct signature in the ATLAS experiment. As the skimming protons stay intact, the only detectable particles produced in the interaction are the visible decay products of the W bosons – namely, for this measurement, an electron and a muon with opposite electric charge.