LHCb experiment finds first evidence of rare B0 decay
Physicists at the LHCb experiment have found the first evidence of a B0 meson decaying into two muons, an event occurring roughly once in every 10 billion decays, with a statistical significance of 3.2 sigma.
The LHCb experiment at CERN has reported the first evidence of an extremely rare particle decay. The finding offers a new probe into the zeptoverse, a realm of unimaginably small distances where new physics may hide.
A B0 meson, containing a beauty quark, was observed decaying into a pair of muons. This process, known as Bd to mu mu, is predicted to occur only about once in every 10 billion such decays. The detection was achieved with a statistical significance of 3.2 sigma, the first experimental evidence for this specific decay channel. Beauty particles are produced in the Large Hadron Collider's collisions and exist for about 1.5 trillionths of a second before decaying.
Zeptoverse probed by unprecedented precision
The observation allows physicists to probe distances as small as one zeptometer. A zeptometer is a billionth of a trillionth of a meter. This scale, dubbed the zeptoverse, is the smallest ever explored in particle physics. The Large Hadron Collider can probe distances of around 100 zeptometers. Investigating this realm is key for testing the limits of the Standard Model, which cannot explain phenomena like dark matter and dark energy that constitute most of the universe.
Experimental challenges and upgrades
Observing this decay required a detector acting as a 'very powerful microscope'. The LHCb experiment was specifically designed to study particles containing beauty quarks. After more than four decades of searching, technical upgrades enabled the breakthrough. The upgrade itself was a 15-year design and construction project by an international collaboration of about 1,900 members. Researchers at the University of Manchester helped develop advanced detector technology, including a precision silicon detector.
Paula Collins reflected on the long search, saying, "People have been searching for it for more than 40 years. Everyone's just over the moon."
Significance and implications
The result has a significance level of 3.2 sigma. This means there is roughly a 0.3% chance the signal is a random fluctuation. It falls short of the 5 sigma standard required for a formal discovery. The measured rate of the decay aligns closely with predictions from the Standard Model.
The analysis is preliminary and the findings have not yet been peer-reviewed. The research paper was submitted to Physical Review X by Dr. Harry Cliff and colleagues. The paper is titled 'Evidence for the decay B0→μ+μ- and improved measurements of the decay B0s→μ+μ-'. The collaboration also made improved measurements of a related, slightly more common decay.
| Measurement | Branching Fraction (Combined Data) | Standard Model Prediction |
|---|---|---|
| B0 → μ+μ- | 1.14 in 10 billion | ~1.03 in 10 billion |
| B0s → μ+μ- | 3.31 in a billion | 3.66 in a billion |
The B0s measurement is described as the most precise determination to date. Its effective lifetime in this decay was measured at 1.60 picoseconds when combined with earlier data. Harry Cliff, a physicist at the University of Cambridge’s Cavendish Laboratory, described the moment. "These very, very rare decays, they're almost impossible according to the current laws of physics."
Professor Tim Gershon of the University of Warwick said the result shows LHCb’s extraordinary capabilities to probe nature at the fundamental level. The research involved about 20 physicists from the UK, Germany, Italy, Spain, and the Netherlands. Additional observations are needed to meet the five-sigma threshold required for a formal discovery.
