Over the past decade, physicists have seen growing signs that this particular transition may not match expectations.

What did CMS measure?

CMS analysed decays of beauty particles into particles containing strange quarks, accompanied by muon–antimuon pairs. The team examined six distinct decay types and measured how often each occurs, a quantity called the branching fraction — in plain English, the probability that a particle decays one specific way rather than another.

Two things make this analysis notable:

• It is the first time all six decay types were analysed simultaneously, allowing background noise and sources of uncertainty to be handled consistently across the board.
• The data came from Run 3, the LHC's most recent operating period, using a new trigger system that captured a much larger sample than earlier studies, achieving the smallest statistical uncertainty yet for these six branching fractions.

The individual differences between prediction and measurement were small for each decay type. But the differences pointed the same direction across all six. That consistency is intriguing. It could mean something. It could also be a statistical fluke that more data will wash out.

What did LHCb find?

LHCb focused on a single decay from that set of six: a neutral beauty meson — a particle containing an antibeauty quark — transforming into an excited kaon, which contains a strange quark (B⁰→K*μ⁺μ⁻).

Using data from Runs 1 and 2 of the LHC, the collaboration measured a property called P₅′, which describes the angles at which the decay products fly apart. Their result showed a small difference from the Standard Model prediction.

Crucially, LHCb used a new model-independent analysis, meaning the measurement makes no theoretical assumptions about what happens inside the decay. That independence adds credibility, though the word "small" still applies to the deviation itself.

What does this change for physics?

For now, nothing is settled — and that's the honest read. The tensions are real enough to motivate work, not firm enough to announce a discovery. The burden of proof in particle physics is brutally high, and history is littered with anomalies that faded as data accumulated.

Next, LHCb plans to fold Run 3 data into this measurement and its other b→sμ⁺μ⁻ analyses, which should sharpen the picture considerably. CMS, meanwhile, has already demonstrated with Run 3 data how much precision its new trigger can deliver.

The stakes are straightforward. The Standard Model works astonishingly well, but physicists know it is incomplete. If these hints hold up as measurements get more detailed, they could become the clearest signposts yet toward particles nobody has seen.

One decay in ten million. Two experiments. A decade of nudges in the same direction. The next few years of LHC data will either build on those nudges — or bury them.