Scientists working with proton collisions at CERN have found evidence for an exceptionally rare transformation of matter: a neutral B meson decaying into a muon and an antimuon. The result offers a new way to test particle physics at extraordinarily small distances, a frontier sometimes called the “zeptoverse.”
The finding concerns a process so uncommon that the Standard Model predicts it should occur, on average, only once for every 10 billion neutral B-meson decays. Researchers have pursued the signal for more than four decades, because such rare events can provide unusually sensitive tests of the framework that describes known fundamental particles and forces.
A decay that is difficult to see
Neutral B mesons are short-lived particles produced in high-energy collisions. They decay into other particles in many possible ways. In the newly reported channel, the meson produces two muons: a muon and its antimatter counterpart, the antimuon. Muons resemble electrons but are about 200 times heavier.
The challenge is not simply producing B mesons. Particle collisions create enormous numbers of tracks and secondary particles, and the desired muon pair must be separated from a vast background of other processes. The international analysis team reported evidence for the decay in a preprint made available through arXiv.
Marco Santimaria, a co-leader of the analysis team, said in a CERN release that extreme precision was needed to identify the muons amid that background. The rarity of the signal explains why it has remained out of reach despite decades of collider experiments.
Why rare processes are powerful
Particle physicists use rare decays as stress tests. The Standard Model supplies a precise expectation for how often a process should occur. If measurements eventually differ from that prediction, the gap could point to particles or interactions that are not part of the current theory.
That does not mean this evidence is, by itself, proof of new physics. Its immediate significance is that researchers can now begin examining a decay channel that had long been a target. Further data will be required to sharpen the measured rate and compare it more precisely with theoretical expectations.
The approach is complementary to direct searches for unfamiliar particles. Instead of requiring a new particle to be created and observed openly in a detector, precision measurements can reveal its influence indirectly through minute changes in known processes. A rare B-meson decay is therefore a probe of physics that may operate at energy or distance scales beyond direct experimental reach.
Entering the zeptoverse
A zeptometer is one trillionth of a trillionth of a meter, or 10-21 meters. The term “zeptoverse” is used to describe the physical regime at these exceptionally small scales. The reported decay gives researchers a route to explore that regime through the quantum effects encoded in the decay’s probability.
The scale is far smaller than an atom. It is also a reminder that particle physics does not observe tiny distances in the same way a microscope views a physical object. Instead, scientists infer what can happen at those scales from collision products, their energies and the statistical behavior of their decays.
New capabilities at the collider
The observation became possible following upgrades to the Large Hadron Collider in 2024. The changes enabled faster data collection, increasing the number of relevant collisions available for study. The analysis also used machine-learning techniques to suppress background events and improve the ability to isolate the sought-after signal.
That combination matters. An event expected only once in billions of decays demands both a large sample and an analysis able to reject look-alike signals without discarding the genuine ones. Modern collider discoveries increasingly depend on progress in instrumentation, computing and statistical methods alongside advances in theoretical physics.
What comes next
The result is evidence, not a final precision measurement. Continued operation of the LHC should provide a larger dataset and allow researchers to refine the decay’s properties. The central question will be whether the observed behavior continues to align with the Standard Model or develops a measurable discrepancy.
The decay involves a neutral B meson transforming into a muon and an antimuon.
The Standard Model predicts roughly one such event per 10 billion neutral B-meson decays.
LHC upgrades and machine-learning-based background suppression helped make the analysis possible.
Additional data will be needed for a more precise comparison with theory.
For now, the result is a landmark in precision particle physics: a long-sought decay channel has moved from theoretical expectation to experimental evidence. That gives physicists a new handle on some of nature’s smallest accessible scales—and another demanding test for the theory that has guided the field for decades.
This article is based on reporting by Wired. Read the original article.
Originally published on wired.com
