Scientists at CERN, including researchers from Lancaster University, have reported the very first observation of an ultra-rare particle decay process, opening a new path to find physics beyond the Standard Model of particle physics.

Today the NA62 collaboration, with a significant UK contribution represented by researchers from the Universities of Lancaster, Birmingham, Bristol and Glasgow, have announced the first experimental observation of the ultra-rare decay of the charged kaon into a charged pion and a neutrino-antineutrino pair (𝐾⁺ → 𝜋⁺𝜈𝜈̅ ) with a significance of 5 sigma (the criterion used in particle physics to declare a discovery), measuring the decay rate to a 25% relative precision.

This makes 𝐾⁺ → 𝜋⁺𝜈𝜈̅ decay the rarest decay ever established experimentally at the 5 sigma level. The Standard Model of particle physics predicts that less than one in 10 billion kaons will decay in this way.

“The rarer is the decay, the more is sensitive to possible effects of new physics,” said Dr Karim Massri, Lecturer in Physics at Lancaster University and the NA62 Physics Coordinator supervising all the physics analyses based on the NA62 data.

“This is because we know that new physics must give small effects at the accessible energies, otherwise we would have discovered it already. Finding hints of new physics still requires more data, but this result is a leap forward and further strengthens an already strong interest in the future results from NA62.”

The new result is based on the combination of data taken by the NA62 experiment in 2021–22 and a previously published result based on the 2016-18 dataset. The 2021-22 dataset was collected following a suite of upgrades to the NA62 setup, allowing operation at 30% higher beam intensity with several new and improved detectors. The hardware upgrades combined with refined analysis techniques allowed collection of signal candidates at a 50% higher rate than before, while adding new tools to suppress backgrounds.

Professor Cristina Lazzeroni of the University of Birmingham helped ensure those upgrades took place.

She said: “During my mandate as NA62 Spokesperson in 2019–2022, I secured the prolongation of NA62 beyond 2018, and I led the detector upgrade. On the back of that extension of the experiment, I am extremely proud of the new result. This measurement of the rarest decay established at discovery level (5 sigma) has come about thanks to the result of excellent teamwork including many UK colleagues alongside other collaborators in CERN.”

The NA62 experiment has been designed and constructed specifically to measure the ultra-rare kaon decay 𝐾⁺ → 𝜋⁺𝜈𝜈̅. Kaons are produced by a high-intensity proton beam provided by the CERN Super Proton Synchrotron (SPS), colliding with a stationary target. This creates a beam of secondary particles with almost a billion (1,000,000,000) particles per second flying into the NA62 detector, about 6% of which are charged kaons. The detector identifies and measures precisely each kaon and its decay products, except neutrinos which show up as missing energy.

“This is the culmination of a decade of work. Looking for effects in nature that happen about once over ten billion trials is fascinating and challenging” - says Professor Giuseppe Ruggiero of the University of Florence, the current NA62 spokesperson - “After rigorous and painstaking work we have got a stunning reward to our effort and delivered a long-awaited result”.

Why are physicists looking for processes so rare that they barely occur? Indeed, this is the key point of studying the 𝐾⁺ → 𝜋⁺𝜈𝜈̅ decay: thanks to the extreme suppression, and the fact that is very precisely predicted theoretically, the decay rate is very sensitive to new physics beyond the Standard Model description. This makes the 𝐾⁺ → 𝜋⁺𝜈𝜈̅ decay a “golden mode”, one of the most interesting processes to search for evidence of new physics.

The findings are consistent with the Standard Model expectation but is about 50% larger. One reason for this could be the presence of new particles which enhance the probability of the process, but more data is needed to test this hypothesis. With data-collection ongoing, NA62 will be able to confirm the existence of contributions to the decay from new physics, or otherwise place strong constraints on the size of any such possible contributions.

Lancaster University has a team of five researchers from its Department of Physics working on NA62.

Professor Roger Jones, Head of Physics at Lancaster and also working on NA62 said: “This is fantastic achievement and illustrates the importance of looking at very precise measurements using very large volumes of data to explore the edges of physics; this route complements the use of the very highest energy collisions in the attempt to make new particles directly.”

Professor Evgueni Goudzovski from University of Birmingham and the current leader of the NA62-UK consortium said:

“The UK group has led the physics exploitation of the NA62 experiment over the past decade, providing leadership at all levels including senior spokesperson and physics coordinator roles.”

Professor Mark Thomson, particle physicist and Executive Chair of the UK’s Science and Technology Facilities Council (STFC), welcomed the results:

"The NA62 experiment is another brilliant example of colleagues from the many nations coming together at CERN to explore the mysteries of the universe.

"This latest announcement, provides an exciting example of how measuring ultra-rare processes can provide a window for searching for new physics beyond the Standard Model."

The UK participants in this research have been funded by STFC which is part of UK Research and Innovation, as well as by the Royal Society and the European Research Council (ERC).

The findings were presented by the NA62 collaboration at a CERN EP seminar on 24^th Sept 2024.