Physicists Spot Clues to an Exceptionally Rare Particle Decay

WHO:

ScienceAlert - The LHCb Collaboration reported evidence of the decay; Niladri Sahoo, a University of Birmingham particle physicist and convener of the LHCb Very Rare Decays working group, called the result “a powerful new test of the Standard Model and a window into potential new physics.”

ScienceAlert - Harry Cliff, a University of Cambridge particle physicist and leader of the analysis team, said: “Seeing clear evidence for this rarest decay for the first time was thrilling,” and, “We’re knocking on the door of the zeptoverse now. I can’t wait to see what we find as we explore deeper.”

Phys.org - Physicists at CERN reported evidence of the decay B⁰→ µ⁺µ⁻.

Wired Science - Particle physicists reported a first glimpse into the “zeptoverse.”

WHAT:

ScienceAlert - LHCb reported evidence for the exceptionally rare decay of a B d meson into a muon and an anti-muon. The decay is estimated to occur once in every ten billion decays of the relevant beauty particle.

ScienceAlert - The evidence has a significance of 3.2 sigma. A 3-sigma result corresponds to a 0.3 percent chance that the finding is a fluke; 5 sigma is described as the “gold standard” for confirmation.

ScienceAlert - The analysis combines LHCb data from 2024 and 2025 with earlier data collected between 2011 and 2018. The research is a preprint on arXiv and has not been peer-reviewed.

Phys.org - The decay was first theorized in 1984; its evidence tests predictions of the Standard Model.

ScienceAlert - Researchers said their results are consistent with Standard Model expectations and constrain possible new-physics contributions at energy scales beyond direct production at the LHC.

Wired Science - The decay offers a way to research physical scales described as 100 billion times smaller than an atom.

WHERE:

ScienceAlert - The experiment was conducted at LHCb, part of CERN’s Large Hadron Collider, which is located underground.

WHY:

ScienceAlert - Studying rare decays can test the Standard Model and may provide indirect clues to new physics, including questions about dark matter and the matter–antimatter asymmetry.

HOW:

ScienceAlert - LHCb produces beauty quarks by colliding protons at near-light speed. Beauty quarks and down antiquarks combine into mesons through the strong nuclear force.

ScienceAlert - A recent LHCb upgrade increased its collision-detection rate; machine learning improved the signal-to-noise ratio. Its VELO sub-detector sits less than a finger’s width from the particle beam.

ScienceAlert - B d mesons contain a beauty quark and a down antiquark, decay in about 1.5 trillionths of a second, and only rarely produce a muon and antimuon. Future upgrades in the 2030s are expected to increase collision numbers and data collection by an order of magnitude.

(UPDATED November 6, 2026 07:02 UTC)

CERN physicists have found first evidence of an exceptionally rare particle decay, predicted decades ago. The decay occurs about once in ten billion cases, testing the Standard Model’s predictions. Researchers say the result offers clues to possible new physics, but confirmation remains ahead.