Physicists at CERN announced a significant breakthrough in the search for one of the rarest particle decays, having detected evidence of an event expected to occur only once in approximately 10 billion beauty particle decays. This finding was revealed during a seminar at CERN and marks the first instance where a single experiment has produced enough evidence to support this extraordinarily rare decay.
The research team, working with the LHCb experiment, identified evidence that the Bd particle, which consists of a beauty quark and a down antiquark, can decay into a pair of muons (μ+μ–). Muons are similar to electrons but are significantly heavier. The results align with predictions from the Standard Model, a key theory in particle physics, and help refine the understanding of potential new particles not yet observed.
Physicists are particularly interested in these rare decays because quantum effects may influence how they occur. Even particles that are too massive to be created directly in the Large Hadron Collider (LHC) could leave subtle traces in the decay rates, making them valuable for exploring physics beyond the Standard Model. Rarer decays are more sensitive to these hidden influences, allowing researchers to look for discrepancies between theoretical predictions and experimental results.
The specific decay studied, Bd → μ+μ–, is considered a ‘golden channel’ for probing new physics. During this process, a beauty quark and its corresponding antiquark annihilate each other, transforming their energy into a muon and its antiparticle. Although a related process, Bs → μ+μ–, was confirmed in 2015, evidence for the Bd decay had not been consistently established prior to this latest analysis.
Improvements to the LHCb experiment played a critical role in this discovery. Following a significant upgrade, the experiment began collecting data at a rate five times higher than that achieved between 2010 and 2018. Enhanced capabilities also allowed better identification of various particle types, facilitating the detection of rare signals amidst the overwhelming background noise typical of particle collisions.
Marco Santimaria, co-lead of the international analysis team, emphasized the challenge, stating, “Extreme precision is required to pick the muons out of the overwhelming background.” The researchers used a statistical measure called standard deviations to evaluate their findings. They identified evidence of the Bd decay at a significance of 3.0 standard deviations, which increased to 3.2 when combined with prior data, surpassing the threshold of evidence commonly recognized in the field.
However, to officially confirm the observation, a significance of five standard deviations is necessary. Future measurements aiming for this threshold could make Bd → μ+μ– the rarest beauty particle decay recorded.
Imminent upgrades to the LHC, particularly the High Luminosity LHC slated to begin operations in 2030, along with further enhancements to the LHCb experiment, are anticipated to increase data collection rates by an additional factor of ten. These advancements would enable physicists to detect more rare decays and study their properties with greater precision, potentially shedding light on particles that remain elusive under current experimental conditions.
Harry Cliff, LHCb’s Rare Decays Convener, remarked on the excitement surrounding the findings, noting, “Seeing evidence of this rare decay is incredibly exciting, opening up new possibilities to probe beyond the direct reach of the LHC.” With over four decades of pursuit, the new evidence offers a promising route for exploring the fundamental laws of particle physics.
