University of Manchester physicists have helped find the first evidence of an extremely rare particle decay at CERN’s Large Hadron Collider, bringing scientists closer to exploring an unknown world where new particles and forces of nature could be hiding.
Physicists have found evidence of a particle decay so rare that it is expected to occur only around once in every 10 billion ‘beauty’ particles.
The result, from the upgraded LHCb experiment at CERN’s Large Hadron Collider (LHC), follows a search spanning more than 40 years and brings physicists closer to exploring the ‘Zeptoverse’ – an unexplored world that exists at distances of a billionth of a trillionth of a metre, or a ‘zeptometre’, where new particles and forces of nature may be hiding.
Researchers from The University of Manchester have played key roles in the LHCb experiment and its major upgrade – which can now collect data at up to five times its previous rate – including developing detector technology that has helped make the new measurement possible.
The finding will allow physicists to test the Standard Model, our current best theory of fundamental particles and their interactions, with greater precision and search for signs of particles or forces that may lie beyond it
Searching for signs of new physics
The LHCb studies particles containing beauty quarks, heavier relatives of the up and down quarks found in the ordinary matter around us.
Beauty quarks are produced in huge numbers in collisions at the LHC but exist for only around one and a half trillionths of a second before decaying.
By making precise measurements of the different ways beauty quarks decay, physicists can detect the influence of new particles that may be far too small to see at the LHC.
The rarer a decay is, the more sensitive it can be to the influence of new particles. These particles could alter how often the decay occurs.
The new result concerns a particularly rare decay known as ‘Bd to mu mu’, in which a particle containing a beauty quark decays into two muons, heavier relatives of electrons.
Scientists predict that this happens only around once in every 10 billion beauty particles. Until now, no single experiment had found solid evidence for the process.
The new measurement reached a statistical significance of 3.2 sigma. This is sufficient for physicists to describe the result as the first evidence of the decay, although further data will be needed to reach the five-sigma threshold required to claim a discovery in particle physics.
Professor Chris Parkes, Head of the Department of Physics and Astronomy at The University of Manchester, led the international LHCb collaboration through the construction of the upgrade and led the collaboration’s approval of the new result.
He said: “The upgraded LHCb was a major undertaking – a fifteen-year design and construction project from a 1900 international member collaboration.
“The UK made the largest nation contribution to the project, pioneering new technologies that have made today’s result possible where we probe nature at a new scale. But this is also just the start, we are planning the construction of an experiment that will exploit the capabilities of the next generation High Luminosity LHC, probing physics at an even smaller scale.”
Professor Parkes, who was made a Fellow of the Royal Society earlier this year for his work on LHCb 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 of the upgraded LHCb experiment is a silicon pixel detector that tracks the paths of particles created in collisions at the LHC with extreme precision.
Known as the Vertex Locator, it sits just five millimetres from the LHC beams and enables scientists to pinpoint where particles are produced and where they decay.
Dr Stefano De Capua, who led the construction of the detectors at The University of Manchester added: “We worked with nearly 90 UK companies to construct this detector and trained the next generation of scientists and engineers on the novel technologies we are using. It is these detectors that we use to locate where the beauty meson was produced and pinpoint where it decayed into two muons.”
Into the ‘Zeptoverse’
The physicists hope measurements such as this could allow them to probe the ‘Zeptoverse’, referring to extraordinarily small distance scales where currently unknown particles and forces may be hiding.
While they know the new measurement agrees with the predictions of the Standard Model, increasingly precise measurements of these rare decays could help physicists search for signs of new physics and reveal where the model might eventually begin to break down.
Professor Conor Fitzpatrick of The University of Manchester and Deputy Physics Coordinator of the international LHCb collaboration, said: “There has been a more than forty year long search by physicists worldwide to observe this decay. It is a golden channel to observe the effects of new physics from beyond our current knowledge.
“The results from this experiment are expanding our understanding of nature at its most fundamental level. We are excited to have finally seen this elusive signal and looking forward to the future discoveries that will come from our data.”
What happens next?
The upgraded experiment is expected to collect a much larger dataset into the early 2030s, allowing physicists to study the decay with increasing precision.
A further planned upgrade could increase the rate of data collection by another factor of ten, enabling more precise measurements of these decays and giving physicists greater sensitivity to undiscovered particles at even shorter distances.
Professor Brian Cox, Particle Physicist at The University of Manchester, who was not involved in the experiment, said: “This is an important result and a beautiful example of how science advances. We upgrade our instruments and we see nature in greater detail. Occasionally we see something we don’t understand, and then progress is made. We cannot know whether this progress will be incremental or genuinely world-changing – that’s science! LHCb is a major scientific success.”
The experiment is funded by the Science and Technology Facilities Council as part of UK Research and Innovation. The UK LHCb collaboration includes researchers from ten UK Universities and the STFC Rutherford Appleton Laboratory.
