CERN's LHCb experiment finds evidence of a one-in-10-billion particle decay

Overview

Updated 2 hours ago

On 3 November 2026, physicists on the LHCb experiment at CERN reported evidence that a particle called the B_d meson can decay into two muons. The Standard Model predicts this happens about once in every 10 billion beauty-particle decays. Combined with earlier data, the signal reached 3.2 sigma.

Three sigma is the threshold physicists use for 'evidence'; five sigma is needed to call something a discovery. The result matches the Standard Model, so it rules out some proposed new particles rather than finding one. The upgraded detector and a planned high-luminosity collider could turn the hint into a discovery.

Why it matters

If it holds to 5 sigma, it becomes the rarest beauty-particle decay seen and a sharper probe for particles the LHC cannot produce directly.

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Key Indicators

3.2σ

Combined signal significance

Statistical significance of the B_d → μ+μ− signal when 2024–2025 data is combined with 2011–2018 data.

5σ

Discovery threshold

The standard particle physics uses to declare an observation rather than evidence.

~1 in 10 billion

Predicted decay rate

Expected share of beauty-particle decays that produce the B_d → μ+μ− final state.

5×

Data rate after 2024 upgrade

How much faster the upgraded LHCb detector collects data than it did from 2010 to 2018.

Voices

Curated perspectives — historical figures and your fellow readers.

People Involved

Marco Santimaria

Co-leader of the 19-person LHCb analysis team

LHCb researcher at CERN

Harry Cliff

LHCb Rare Decays Convener and analysis co-lead

LHCb researcher at CERN

Organizations Involved

CERN (European Organization for Nuclear Research)

International research laboratory

Operates the Large Hadron Collider and hosts the LHCb experiment

The intergovernmental laboratory near Geneva that runs the Large Hadron Collider.

LHCb collaboration

Particle physics experiment

Reported the 3.2-sigma B_d → μ+μ− evidence

One of the four main experiments at the Large Hadron Collider, built to study particles containing beauty and charm quarks.

CMS Collaboration

International particle physics experiment

Co-reported the 2015 B_s → μ+μ− observation

A general-purpose detector at the Large Hadron Collider that shared the 2015 rare-decay analysis with LHCb.

Timeline

2015 November 2026

5 events Latest: Yesterday

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November 2026
ScienceDaily reports the result
Latest Media coverage

ScienceDaily publishes its account of the 40-year search and the 3.2-sigma evidence.
Yesterday
CERN seminar reports 3.2-sigma evidence
Announcement

LHCb combines 2024–2025 data with 2011–2018 data to reach 3.2 sigma for B_d → μ+μ−.
7 days ago
2024
LHCb detector upgrade goes online
Infrastructure

The upgraded detector collects data at five times the earlier rate and better separates particle types.
2024
2018
B_d evidence fails to hold
Research result

Updates using data collected through 2018 do not confirm the earlier B_d → μ+μ− evidence.
2018
2015
First observation of B_s → μ+μ−
Research result

A joint LHCb and CMS analysis confirms the B_s decay and reports initial evidence for the rarer B_d version.
2015

Scenarios

LHCb reaches 5-sigma discovery of B_d → μ+μ−

Possible Resolves by End of 2035

Discussed by: LHCb collaboration and CERN

More data from the upgraded detector, plus the High-Luminosity LHC due to start in 2030, could push the combined significance past 5 sigma. That would make B_d → μ+μ− the rarest beauty-particle decay ever observed and a precision test of the Standard Model.

Signal fades as more data arrives

Unlikely Resolves by End of 2030

Discussed by: Particle physicists commenting on the result

A 3.2-sigma result is evidence, not proof. Additional data could pull the significance down, as happened after the 2015 B_d hint. The collaboration would then report a tighter upper limit rather than an observation.

Precision measurements diverge from the Standard Model

Uncertain Resolves by End of 2035

Discussed by: Physics World and LHCb researchers

The same analysis produced the most precise B_s → μ+μ− measurements and an effective-lifetime figure of 1.6 trillionths of a second. Future precision could reveal a deviation from the Standard Model, which would point to new particles.

Historical Context

2 moments from history that rhyme with this story — and how they unfolded.

July 2012

Higgs boson discovery (2012)

CERN's ATLAS and CMS experiments announced a new particle at about 5 sigma, the statistical bar particle physics uses to claim a discovery. The result completed the Standard Model's predicted particle set.

Then

The announcement was treated as a discovery because the signal cleared 5 sigma, not 3.

Now

It fixed 5 sigma as the field's public standard for calling something real.

Why this matters now

Explains why LHCb's 3.2-sigma result is called evidence, not a discovery, and what has to happen next.

2015

First observation of B_s → μ+μ− (2015)

LHCb and CMS combined data to confirm that the B_s meson decays into two muons, roughly three times per billion beauty-particle decays. The same analysis reported initial evidence for the rarer B_d decay.

Then

The B_s observation stood; the B_d evidence did not survive later data.

Now

The B_s channel became a benchmark for rare-decay searches and precision tests.

Why this matters now

The B_d result reported in 2026 is the same search that produced the unconfirmed 2015 hint.