In a blow to the highly-popular theory of supersymmetry, scientists at the Large Hadron Collider (LHC) at CERN have spotted one of the rarest particle decays ever seen in nature.
Supersymmetry builds on the Standard Model of physics to include gravity and explain dark matter and dark energy.
Because it predicts new phenomena, it can be tested; and a good place to start, says the team, is through the decay of a Bs particle (composed of a beauty quark and a strange anti-quark) into two muons, or very heavy electrons.
And it's this decay that's now been observed for the first time through the LHC beauty (LHCb) experiment, a gigantic particle detector at one of the collision points on the 27 km LHC collider.
The LHC accelerates protons to almost the speed of light and creates collisions, each of which produces a shower of particles, occasionally including a Bs particle.
The Bs particle is unstable, and decays almost immediately - but lives just long enough to be observed by the LHCb detector. Very rarely - about one in 300 million times - it can decay into two muons.
And the team of physicists has now analysed the enormous number of collisions recorded by LHCb, and spotted a handful of likely candidates. The observation is a perfect fit with the Standard Model, but fails to support supersymmetry.
"An observation of this very rare decay is a key result that is putting our supersymmetry theory colleagues in a spin," says Professor Val Gibson of Cambridge University.