In the ATLAS detector at the Large Hadron Collider, protons collide billions of times per second. From this stream, physicists fished out eight events in which the collision produced two Z particles and one photon. The Z particle is a heavy carrier of the weak force; it lives less than a billionth of a second and decays into a pair of charged particles. In this case, each Z particle decayed into an electron or a muon, while the photon remained in the detector as a distinct clump of energy.

To isolate these rare events, physicists selected collisions where the Z decays left four charged particles (electrons or muons) and one photon. Such a signature is like four tracks from debris and one flash of light. Over four years of collider operation, from 2015 to 2018, only eight such patterns accumulated. The background from other processes that could mimic a similar signature was estimated at less than one event. This means the signal is real.

The measured rate of producing two Z particles and a photon matches the calculation from the Standard Model, the most precise theory of elementary particles. The agreement between the number and the calculation strengthens confidence in the model. But it is precisely in such rare processes that deviations could appear if new particles or forces exist. So far, no deviations are visible, but physicists now have a tool to keep looking.