The Large Hadron Collider has been slamming protons together for years. But recently, a proton collision produced an improbable result that is the first step toward exploring the so-called “zeptoverse”—a scale of reality many, many times smaller than an atom. This would be the smallest scale of the physical world ever explored and may defy our expectations of how physics works.

While crashing protons together at nearly the speed of light, scientists at CERN found evidence of one of the most improbable decays of matter—so unlikely that it’s a phenomenon physicists have been hoping to observe for more than four decades. The collision produced numerous particles, including some known as neutral B mesons, and scientists found signals indicating that one of these had decayed into a muon and an antimuon.

Mesons decay all the time. These tiny particles, smaller than atoms, arise from collisions between other particles and exist only briefly before transforming into other particles. Muons, on the other hand, are similar to the electrons found in atoms, but about 200 times heavier. Scientists study these particles and their decays to test the predictions of current physics and search for clues to unknown particles.

The standard model—the sort of bible of particle physics —predicts that for every 10 billion decays of neutral B mesons, this transformation will occur once. In other words, even if scientists were to observe 10 billion of these particles decaying, they would expect to find, on average, only a single muon and antimuon.

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