Physicists on the LHCb experiment at CERN say they have found the first evidence of a particle decay that occurs in roughly one of every ten billion beauty particles. Nobody has managed it before. The search started 42 years ago.
Cambridge, Birmingham and Manchester were among the institutions that put out statements on 3 November, and the coverage has kept rolling since.
The particle is the B0 meson, sometimes written Bd. It is short-lived and made of a beauty quark bound to a lighter antiquark. Most of the time it falls apart in messy ways that produce a spray of other particles. Very rarely it decays into just two muons, which are heavier cousins of the electron.
That last route is the one the team went after. The Standard Model, the theory that describes the known particles and forces, says it should show up about once per ten billion B0 mesons. Finding it means picking out muon pairs that truly came from a B0, and ignoring the many background processes that can imitate one.
Dr Harry Cliff of Cambridge's Cavendish Laboratory led the analysis. "We're knocking on the door of the zeptoverse now," he said.
3.2 sigma.
Sigma is how particle physicists express the odds that a signal is a statistical fluke. Around 3 is where the word "evidence" becomes acceptable. Five is the line for announcing a discovery, so this one falls short of that. Sci.News reports the 3.2 figure includes earlier LHCb data. What makes it notable is that it is the first time a single experiment has seen evidence of this decay.
The paper is titled "Evidence for the decay B0 to mu+ mu- and improved measurements of the decay B0s to mu+ mu-." It is listed as in press at Physical Review X and is on arXiv under 2611.00230. At least one outlet notes the results had not been through peer review when first presented, so treat the status as in progress.
LHCb is not a copy of CERN's bigger detectors. ATLAS and CMS look in all directions around the collision point. LHCb is a forward spectrometer, so it watches along the beam direction, where beauty and charm quarks tend to fly.
The detector has just been through a large rebuild. Professor Chris Parkes of the University of Manchester described it as a 15-year design and construction project involving a collaboration of about 1,900 people. The UK's contribution was the Vertex Locator, a silicon pixel detector that sits only five millimetres from the beams. Being that close lets researchers work out where a particle was produced and where it decayed, and for a decay this faint you need that precision.
The same analysis measured the B0s meson decaying into two muons. This process is more common, though still extraordinarily rare, and it is old news in one sense. LHCb and CMS first observed it together in 2015.
The new figures are the most precise yet. They also agree with theory.
That 2015 dataset already carried early hints of the B0 decay, according to CERN's own account of the work. The hints were faint and nobody could lean on them. A decade on, with a rebuilt detector and far more collisions to sift, the same signal is finally strong enough to be called evidence.
Because agreement is information.
Quantum physics lets heavy particles that nobody has ever seen nudge the rate of rare processes, even though they are far too massive to be produced directly in the collider. If something unknown exists at very short distances, it could push the B0 decay rate up or down. A zeptometre is a billionth of a trillionth of a metre, and "Zeptoverse" is the nickname for whatever might be going on at that scale. Dr Niladri Sahoo of the University of Birmingham, who convenes the LHCb working group on very rare decays, called the result the first single-experiment evidence for this decay.
So far the rate looks like the Standard Model's. That rules out some theories of new physics and leaves others standing, which is useful for anyone deciding where to look next.
LHCb is still taking data with the upgraded detector, and more data means a tighter measurement. Birmingham's statement says the team plans to explore the Zeptoverse in greater detail as that data piles up, with some of the biggest open questions in physics in mind. If the B0 decay keeps matching theory as the statistics grow, the Standard Model collects another win. If the rate starts to drift, the field has something big on its hands.
There is also a bigger argument in the background. Particle physicists are currently weighing what machine should follow the LHC, and precision results like this one feed straight into it, since they show which measurements a successor would need to push further. The UK's Science and Technology Facilities Council, which backed the work, made that connection in its statement.
The results are due to be presented at CERN, and the paper can already be read on arXiv.
