LHCb evidence of rare B-meson decay into two muons, Digi Mart USA News: The LHCb Collaboration has found evidence of a one-in-ten-billion decay of a beauty particle, according to a seminar held at CERN and reported by CERN. The team is the first to observe this decay using data from a single experiment.
The decay involves a particle containing a beauty quark and a down antiquark, known as Bd, decaying into a pair of muons (μ⁺μ⁻). The result is consistent with theoretical predictions and places limits on possible new particles beyond the Standard Model.
Quantum physics describes how particle decays are influenced by other particles. Measuring this beauty-quark decay precisely could therefore reveal signs of new particles that are too heavy to be produced directly in Large Hadron Collider collisions.
In general, the more unlikely a decay, the more sensitive it is to the influence of new particles.
The Bd → μ⁺μ⁻ decay belongs to a group of decays so rare that they have long been regarded as “golden channels” in the search for new physics. In these decays, a beauty quark and an oppositely charged antiquark annihilate, producing a muon, a particle similar to an electron but heavier, and its antiparticle.
In 2015, a joint analysis by the LHCb and CMS Collaborations produced the first observation of Bs → μ⁺μ⁻ and a claim of first evidence for Bd → μ⁺μ⁻. However, later updates using data collected through 2018 did not confirm the evidence for Bd → μ⁺μ⁻.
The Bs → μ⁺μ⁻ decay occurs only about three times in every billion beauty-particle decays, while Bd → μ⁺μ⁻ is even rarer, with a predicted rate about thirty times lower.
In 2024, a major upgrade of the LHCb experiment allowed data to be collected at five times the rate achieved between 2010 and 2018. The upgrade also improved the detector’s ability to distinguish between types of particles.
Combined with machine-learning techniques, these improvements allowed the team to suppress the large background of processes that would otherwise have overwhelmed the signals from these rare decays.
“Physicists have been chasing this decay for more than forty years,” said Marco Santimaria, who co-led the 19-person international analysis team. “Extreme precision is required to pick the muons out of the overwhelming background.”
To claim evidence of a new phenomenon, a result needs a significance level of 3 standard deviations, corresponding to about a 0.3% chance that the signal occurred randomly. Data taken by the LHCb experiment from 2024 to 2025 revealed evidence of the Bd → μ⁺μ⁻ decay at a level of 3.0 standard deviations.
Combining this data with that collected between 2011 and 2018 raised the significance to 3.2 standard deviations, beyond the commonly accepted threshold. In the same analysis, the team also made the most precise measurements to date of the previously observed Bs → μ⁺μ⁻ decay.
Further research will seek to confirm the Bd → μ⁺μ⁻ decay by reaching a significance level of 5 standard deviations. If confirmed, it would be the rarest beauty-particle decay observed to date.
The start-up of the High-Luminosity LHC in 2030 and a planned LHCb upgrade in the middle of the next decade could increase the rate of data collection by another order of magnitude. Harry Cliff, LHCb’s Rare Decays Convener and Analysis Co-lead, said, “Seeing evidence of this rare decay is incredibly exciting, opening up new possibilities to probe beyond the direct reach of the LHC.
The future LHCb upgrade will allow us to observe many more of these decays, providing even greater sensitivity to undiscovered particles.”
