Proton collisions at CERN’s Large Hadron Collider have produced experimental evidence of an extraordinarily rare particle decay, giving physicists a new way to test the Standard Model at the smallest scales yet explored. Researchers observed a signal consistent with a neutral B meson decaying into a muon and an antimuon—a process predicted to occur only once in every 10 billion such decays.
Evidence of a one-in-10-billion decay
Scientists at CERN reached the result by colliding protons at nearly the speed of light. These collisions produce many different particles, including neutral B mesons. In the data, researchers identified a signal indicating that one of these mesons had decayed into a muon and an antimuon.
Mesons are short-lived particles formed in high-energy interactions. They decay into other particles almost immediately. Muons are similar to the electrons found in atoms, but they are about 200 times heavier. Studying how these particles decay allows physicists to test established theories and search for signs of unknown particles or forces.
The Standard Model of particle physics predicts that this particular transformation should occur only once for every 10 billion neutral B meson decays. In statistical terms, observing 10 billion such mesons would be expected to produce just one decay into a muon and an antimuon on average. The result is the first experimental evidence of this exceptionally rare process. The researchers have published their findings in an arXiv preprint.
“Physicists have been tracking this collapse for more than 40 years,” Marco Santimaria, co-leader of the international analysis team, said in the paper. “Selecting muons from an overwhelming background requires great precision.” CERN also described the result in a press release.
How the LHC made the search possible
The extreme rarity of the decay helps explain why researchers needed decades to obtain evidence for it. An upgrade to the Large Hadron Collider in 2024 improved the experiment’s ability to collect data. New machine-learning techniques also helped researchers separate the potential signal from the enormous background of other particle decays.
As CERN explained, the improved data collection and analysis methods helped “suppress the vast background of the process that would otherwise have overwhelmed the signal from these rare cosmetic particle decays.”
What is the Zeptoverse?
The result is consistent with the Standard Model, but future measurements could reveal deviations that point to new physics. Such experiments probe distances measured in zeptometers, or 10-21 meters—a scale sometimes described as the “Zeptoverse.” A typical atom is about 100,000 million zeptometers across, illustrating how far below ordinary atomic dimensions these investigations reach.
Exploring this scale could eventually help scientists investigate conditions that existed during the first fraction of a second after the Big Bang, when the universe was far smaller and denser than it is today. However, the current result does not establish new physics, and the implications of the Zeptoverse remain uncertain.
Beyond the reach of current experiments
Even smaller distances are associated with the Planck length, approximately 1.6 × 10-35 meters. It is often treated as the smallest meaningful scale in theoretical physics, although scientists do not yet know whether space and time retain their familiar properties at that level. Many theories suggest that existing descriptions of physics would break down there.
For now, what lies beyond the Zeptoverse remains speculative. The rare neutral B meson decay is an important experimental milestone, but further data will be needed to determine whether measurements at these scales reveal anything beyond the Standard Model.
This story was originally published in the Spanish version of WIRED and translated from Spanish.
Source: www.wired.com