Physicists on the upgraded LHCb experiment at CERN reported on Tuesday 3 November the first evidence of a decay expected once in 10 billion beauty particles, ending a search of more than 40 years.
The University of Manchester, whose researchers helped build and lead the upgrade, said that the decay, known as Bd to mu mu, has now been seen at the Large Hadron Collider.
In it, a particle containing a beauty quark decays into two muons, which are heavier relatives of electrons.
Until now, no single experiment had found solid evidence for the process.
The measurement reached a statistical significance of 3.2 sigma. That is enough for physicists to call it the first evidence of the decay, but further data are needed to reach the five-sigma threshold required to claim a discovery in particle physics.
The result agrees with the predictions of the Standard Model, the current best theory of fundamental particles and their interactions.
Rarer decays can be more sensitive to the influence of new particles, which could change how often the decay occurs. Beauty quarks are produced in huge numbers at the collider but last only around one and a half trillionths of a second before decaying.
Physicists hope ever more precise measurements will let them probe what they call the ‘Zeptoverse’, distances of a billionth of a trillionth of a metre where unknown particles and forces may be hiding.
Professor Chris Parkes, Head of the Department of Physics and Astronomy at Manchester, led the LHCb collaboration through construction of the upgrade and its approval of the result. He described the project as fifteen years of design and construction by a collaboration of 1900 members.
He said the UK made the largest national contribution. Parkes added: “This decay is remarkably unlikely, the chance of it happening is roughly like going on a romantic date and, on your very first try, finding the one perfect match for you among everyone on Earth.”
A major UK-built component is the Vertex Locator, a silicon pixel detector that sits five millimetres from the LHC beams. It lets scientists pinpoint where particles are produced and where they decay.
Dr Stefano De Capua, who led construction of the detectors at Manchester, said the team worked with nearly 90 UK companies and trained the next generation of scientists and engineers.
Professor Conor Fitzpatrick, Deputy Physics Coordinator of the LHCb collaboration, said the search had lasted more than forty years and called the decay “a golden channel to observe the effects of new physics”.
Professor Brian Cox, a particle physicist at Manchester who was not involved in the experiment, said: “This is an important result and a beautiful example of how science advances.”
The upgraded detector can collect data at up to five times its previous rate. It is expected to gather a much larger dataset into the early 2030s, allowing the decay to be studied with increasing precision.
A further planned upgrade could raise the rate of data collection by another factor of ten, giving greater sensitivity to undiscovered particles at even shorter distances.
The experiment is funded by the Science and Technology Facilities Council as part of UK Research and Innovation. The UK collaboration includes researchers from ten UK universities and the STFC Rutherford Appleton Laboratory.