At the Large Hadron Collider, physicists accelerate protons to nearly the speed of light and smash them together. Usually, the protons themselves take part in such collisions, but sometimes they pass close by without touching, and it is their electric fields that interact. These fields can be thought of as clouds of photons, particles of light, that surround each proton. When two such photons collide, they can produce a pair of W bosons, the particles responsible for the weak force that governs the decays of atomic nuclei.
In the new analysis, scientists from the CMS collaboration selected events where the collision leaves only an electron and a muon, the decay products of the W bosons, with no other particles around the point of origin. Such a "clean" signature indicates that the W bosons were born from photon fusion, not from a direct proton hit. In total, enough events were collected to reliably measure the probability of this process.
The measured cross section for W-pair production matched the predictions of the Standard Model, the theory that describes all known particles and forces except gravity. This agreement allowed physicists to set tight limits on the existence of new interactions that could manifest in such processes. If nature deviated from the Standard Model, it would show up as a change in the probability of producing W bosons.
Now scientists have another tool to test fundamental laws. Measuring the photon-fusion production of W bosons is like using a precise ruler to check whether the space of known physics is curved. And so far, the ruler shows: everything is straight.
The process of photon fusion into a pair of W bosons is so rare that only a few such events occur for every billion proton collisions. To catch them, the CMS detector recorded data for three consecutive years.