Deep inside the Large Hadron Collider, a particle called the Bd, built from a beauty quark and a down antiquark, has briefly shown a faint signature of an event so rare that it happens roughly once in ten billion decays. Physicists have pursued it for more than forty years, and now the LHCb experiment at CERN offers evidence that it occurs, though not yet proof. The result matches the Standard Model, the long-standing account of the fundamental particles, and in doing so it quietly shrinks the space where undiscovered particles could still hide.
CERN's LHCb Finds Evidence of Ultra-Rare Bd Decay After Four Decades of Searching
A decay that happens once in roughly 10 billion, chased for over forty years
Mark
So what exactly did LHCb find?
Mimi
Evidence that the Bd particle decays into a muon and an antimuon. It happens about once in 10 billion decays, and people have been looking for over forty years.
Mark
Why does something so rare matter?
Mimi
Quantum effects let heavy particles, even ones the LHC can't make directly, nudge the rate of the decay. A precise measurement can reveal them indirectly.
Luke
But the result matches the Standard Model. So nothing new has shown up.
Mimi
Right. It's consistent with the theory, and that narrows the possibilities for new particles beyond it.
Luke
And 'evidence' is a specific word here, isn't it?
Mimi
Yes. The combined significance is 3.2 standard deviations. Evidence starts at three. Observation needs five.
Mark
There was an earlier hint in 2015, wasn't there?
Mimi
Yes, from LHCb and CMS together. Later analyses with data through 2018 couldn't confirm it.
Luke
So this could still fade, in principle. The source doesn't say what happens if it doesn't reach five.
Mimi
True. What it does say is that more data is coming: the upgraded detector already collects at five times the old rate, and further upgrades could add another factor of ten.
Mark
And machine learning helped?
Mimi
It helped separate the signal from background, alongside better particle identification.
O Pulso
- A decay predicted to be about thirty times rarer than an already elusive cousin has surfaced as a faint signal in LHCb's collision data.
- At 3.2 standard deviations, the result clears the threshold for evidence but falls short of the 5 needed to call it a discovery.
- Upgraded detectors running at five times the earlier data rate, paired with machine learning, are pulling a whisper out of overwhelming background noise.
- The measurement agrees with the Standard Model, tightening the limits on new physics, while the next phase of data-taking and LHC upgrades in 2030 and beyond could decide whether the signal crosses the discovery line.
Deep inside the Large Hadron Collider, a particle called the Bd, built from a beauty quark and a down antiquark, has briefly shown a faint signature of an event so rare that it happens roughly once in ten billion decays. Physicists have pursued it for more than forty years, and now the LHCb experiment at CERN offers evidence that it occurs, though not yet proof. The result matches the Standard Model, the long-standing account of the fundamental particles, and in doing so it quietly shrinks the space where undiscovered particles could still hide.
At a seminar at CERN, physicists from the LHCb experiment announced evidence for a decay so rare that it occurs about once in every ten billion decays. The particle involved is the Bd, which contains a beauty quark and a down antiquark, and the team found signs that it can break apart into a muon and an antimuon. Muons are like heavier cousins of the electron, and the observed rate agrees with the Standard Model while leaving less room for unknown particles.
Physicists prize decays like this because quantum effects allow unseen particles to leave faint fingerprints on their rates. A rare process measured precisely enough might reveal a gap between theory and experiment, hinting at something new even when the particles themselves are too heavy to create directly. Bd to muon-antimuon belongs to a group known as golden channels for exactly this reason.
The hunt has a long history. In 2015, LHCb and CMS confirmed the related Bs decay and saw an early hint of the Bd version, but later data through 2018 failed to strengthen it. What changed was the detector. After a major upgrade, LHCb began recording data in 2024 at five times its earlier rate, and machine learning helped separate the faint signal from the flood of other collision events. Marco Santimaria, who co-led the 19-person analysis team, described the extreme precision the search demanded.
The combined 2011 to 2025 data reached 3.2 standard deviations, enough to count as evidence but not the 5 needed for observation. If the signal grows, this would be the rarest beauty decay ever seen. More data is coming with the High Luminosity LHC in 2030 and a later LHCb upgrade, which together could multiply the collection rate tenfold. Harry Cliff called the result incredibly exciting, and the next thing to watch is whether the signal climbs toward that final threshold.
At a seminar at CERN, physicists from the LHCb experiment announced that they had found evidence of a particle decay so rare it occurs roughly once in every 10 billion decays. The search for it has run for more than 40 years. It is the first time a single experiment has gathered enough evidence for the event.
The particle at the center of the result is the Bd, which contains a beauty quark and a down antiquark. The team found evidence that it can decay into a muon and an antimuon, written as μ+μ-. Muons resemble electrons but are heavier. The measurement agrees with the Standard Model, the leading theory of fundamental particles and their interactions, and it narrows the room for new particles beyond that theory.
Physicists care about decays like this because quantum effects let other particles influence how they happen. Even particles too massive to be produced directly in the Large Hadron Collider can leave subtle traces in the rate of such a decay. Measure a rare process precisely enough, and any gap between the result and the theoretical prediction could be indirect evidence of something new. The rarer the decay, the more sensitive it tends to be to those hidden influences. Bd → μ+μ- belongs to a group long regarded as 'golden channels' for this kind of search. In the process, the beauty quark and an antiquark of opposite charge annihilate, and their energy becomes a muon and its antiparticle.
A related decay involving a different beauty particle, the Bs, was already known. In 2015, a joint analysis by LHCb and CMS reported the first confirmed observation of Bs → μ+μ-, along with early evidence for the much rarer Bd decay. Later analyses using data through 2018 could not confirm that early hint. The gap in rarity is large: the Bs decay occurs about three times per billion beauty particle decays, while the Bd version is predicted to be about 30 times less frequent.
What changed is the detector. After a major upgrade, LHCb began taking data in 2024 at five times the rate it managed between 2010 and 2018. The new detector is also better at identifying different types of particles. The team added machine learning techniques to separate the faint signal from the vast number of other processes in the collisions. Without those advances, the events would have been buried in background. Marco Santimaria, who co-led the 19-scientist analysis team, said physicists have chased this decay for over forty years and that extreme precision is needed to pick the muons out of the overwhelming background.
The result is expressed in standard deviations, a measure of how likely a signal is to be a random fluctuation. Three standard deviations count as evidence, a level at which random chance would produce a signal at least that extreme about 0.3% of the time. Data from 2024 and 2025 gave 3.0 standard deviations for Bd → μ+μ-. Combined with data from 2011 to 2018, the significance rose to 3.2, above the accepted threshold for evidence. The same analysis produced the most precise measurements yet of the Bs → μ+μ- decay.
This is not yet a confirmed observation, which requires five standard deviations. If future data reach that level, Bd → μ+μ- would become the rarest beauty particle decay ever observed.
More data is coming. The High Luminosity LHC is scheduled to start operating in 2030, and another major LHCb upgrade is planned for the middle of the following decade. Together they could raise the experiment's data collection rate by another factor of ten. Harry Cliff, LHCb's Rare Decays Convener and Analysis Co-lead, called the evidence incredibly exciting and said the future upgrade will let the experiment observe many more of these decays, with greater sensitivity to undiscovered particles. The next marker to watch is whether the signal climbs from 3.2 to 5 standard deviations.
Citações Notáveis
Physicists have been chasing this decay for more than forty years; extreme precision is required to pick the muons out of the overwhelming background.— Marco Santimaria, co-lead of the analysis team
The future LHCb upgrade will allow us to observe many more of these decays, providing even greater sensitivity to undiscovered particles.— Harry Cliff, LHCb Rare Decays Convener and Analysis Co-lead
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