In 2017, protons at the Large Hadron Collider collided with an energy of 5.02 TeV. The CMS detector recorded these collisions, and among billions of events, physicists hunted for a rare occurrence: the production of a single top quark without its antiparticle. Usually, top quarks appear in pairs — quark and antiquark together, like two sides of the same coin. But sometimes nature makes an exception and produces only one side.

To catch such an event, scientists looked for events with a single electron or muon and several jets of particles, one of which revealed its origin from a b-quark. With 302 inverse picobarns of accumulated luminosity (think of it as the volume of collected data), they were able to measure the probability of the process. It turned out that a single top quark is produced with a cross section of 17.6 picobarns, while a single top antiquark is produced with only 6.6 picobarns. The difference is nearly threefold: nature clearly prefers quarks over antiquarks, a long-known asymmetry tied to the workings of the weak interaction.

From these numbers, physicists extracted a quantity that shows how readily the top quark participates in weak decays. The result is 0.92 with an uncertainty of about 0.1. This agrees beautifully with predictions from the Standard Model, calculated to high precision. So far, everything is going according to plan, but more precise measurements at other energies could reveal discrepancies that point to new particles or forces.

The solo dance of the top quark at 5.02 TeV is now recorded for the first time. Next steps include more data and different collision energies to check whether this dance ever strays from the rhythm prescribed by theory.