Physicists have long wondered what happens at scales so tiny that even the language of particle physics strains to describe them. Now, an extraordinarily rare event recorded at the Large Hadron Collider has given researchers their first experimental handle on the zeptoverse—a domain of reality roughly 100 billion times smaller than an atom.
The observation, made by scientists at CERN, involves a decay of matter so improbable that the Standard Model predicts it will occur only once in every 10 billion decays of a neutral B meson. The collaboration’s findings have been posted as a preprint on arXiv.
A needle in a haystack of particles
Inside the collider, protons are smashed together at nearly the speed of light. These collisions generate a spray of short-lived particles, among them neutral B mesons. These mesons exist only briefly before transforming into other particles. In this case, the detectors picked up signals consistent with a neutral B meson decaying into a muon and an antimuon—a muon being a heavier cousin of the electron, about 200 times more massive.
Such decays are not exotic in themselves; mesons decay constantly. What makes this one special is its rarity. To spot a single instance amid the overwhelming background of other particle processes, the analysis team needed extreme precision. “Physicists have been chasing this decay for more than 40 years,” said Marco Santimaria, who co-led the international analysis team, in a CERN press release. He noted that separating the muons from the background required extraordinary accuracy.
The decades-long hunt was finally made possible by upgrades to the Large Hadron Collider completed in 2024. Those improvements allowed faster data collection, while new machine-learning techniques helped suppress the vast background of competing processes that would otherwise have swamped the faint signal.
Why the zeptoverse matters
The result is consistent with the predictions of the Standard Model, the theoretical framework that describes the known fundamental particles and their interactions. But its importance lies less in confirming existing theory than in opening a new window on physics at unexplored distances. The zeptoverse refers to scales measured in zeptometers—one quadrillionth of a meter. For comparison, a typical atom is about 100,000 million zeptometers across, roughly the same ratio as a human being to one of their skin cells.
Beyond the zeptoverse lies the Planck length, about 60 trillion times smaller still. It is widely thought that known physics breaks down at that scale, and whether anything meaningful exists there remains unknown. Yet if researchers could probe such infinitesimal realms, they might gain insight into the universe’s first fraction of a second after the Big Bang, when all of space was compressed to a comparable size.
For now, the observed decay agrees with expectations. Future measurements of this and similar processes could reveal deviations from the Standard Model—signs of new particles or forces that only manifest at zeptometer distances. The collaboration’s work is a first step toward that goal, demonstrating that such rare events can be isolated and studied.
The findings do not yet challenge known physics, but they mark the beginning of a new kind of exploration. As Santimaria and his colleagues have shown, the zeptoverse is no longer entirely beyond reach.
