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. This discovery is the first time an experiment has obtained evidence of this phenomenon, the details of which the researchers have made available in a preprint on arXiv.

“Physicists have been chasing this decay for more than 40 years,” Marco Santimaria, who co-led the international analysis team, said in a press release from CERN. “Extreme precision is required to pick the muons out of the overwhelming background.”

The extraordinary rarity of this process explains why it’s taken decades of research to arrive at this discovery. But upgrades to the Large Hadron Collider in 2024 also made observing this phenomenon possible. The revamp enabled faster data collection, and new machine-learning techniques allowed researchers “to suppress the vast background of processes that would have otherwise swamped the signals from these rare beauty-particle decays,” as CERN described it in its statement.

This decay opens a new avenue for investigating the limits of what scientists have been able to observe. For now, the results are consistent with current physics predictions, but future measurements could reveal phenomena unique to the zeptoverse, which describes the scale where distances are measured in zeptometers (equivalent to one quadrillionth of a meter). To put this unit into perspective, a typical atom is about 100,000 million zeptometers in diameter. That’s also about the difference in size between you and one of your skin cells.

Beyond the zeptoverse lie even smaller scales, such as the Planck length, which is about 60 trillion times smaller than a zeptometer and the smallest theoretically possible unit in physics. Scientists still don’t know whether anything truly exists at that level, but it’s generally believed that physics as we know it completely breaks down on that scale. However, if scientists could gain access to this micro world, it could help us understand the first infinitesimal fraction of a second after the Big Bang, when the whole universe was such a size.

But, for now, much of what might happen there remains in the realm of speculation.

This story originally appeared on WIRED en Español and has been translated from Spanish.

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