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The very first observation of an ultra-rare particle decay process will open a new path to find physics beyond the Standard Model of particle physics.

Credit: CERN

On 24 September 2024, the NA62 collaboration announced the first experimental observation of the ultra-rare decay of the charged kaon into a charged pion and a neutrino-antineutrino pair.

The decay had a significance of 5 sigma (the criterion used in particle physics to declare a discovery), measuring the decay rate to a 25% relative precision.

The NA62 collaboration had a significant UK contribution, with researchers from the universities of Birmingham, Bristol, Glasgow and Lancaster.

Rarest decay ever

This makes the 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.

Dr Karim Massri, the NA62 Physics Coordinator from Lancaster University yesterday welcomed the announcement:

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.

A combination of data

The new result is based on the combination of data taken by the NA62 experiment in 2021 and 2022 and a previously published result based on the 2016 to 2018 dataset.

The 2021 and 2022 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.

The result of excellent teamwork

Professor Cristina Lazzeroni at the University of Birmingham helped ensure those upgrades took place and yesterday said of the latest news:

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 (the famous 5 sigma) has come about thanks to the result of excellent teamwork including many UK colleagues alongside other collaborators in CERN.

Designed to specifically measure the decay

The NA62 experiment has been designed and constructed specifically to measure the ultra-rare kaon decay. 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 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 that show up as missing energy.

Delivered a long-awaited result

Professor Giuseppe Ruggiero at the University of Florence and the current NA62 spokesperson yesterday said:

This is the culmination of a decade of work.

After rigorous and painstaking work we have got a stunning reward to our effort and delivered a long-awaited result.

A ‘golden mode’

Why are physicists looking for processes so rare that they barely occur? Indeed, this is the key point of studying the 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 SM description. This makes the decay a ‘golden mode’, one of the most interesting processes to search for evidence of new physics.

The measured branching ration of the decay is consistent with the SM expectation but is about 50% larger. One reason for this could be the presence of new particles that enhance the probability of the process, but more data is needed to test this hypothesis.

With the data-taking ongoing, NA62 will be able to either:

• confirm the existence of contributions to the decay from new physics
• place strong constraints on the size of any such possible contributions

Attracting top talent and providing opportunities

Professor Evgueni Goudzovski from the University of Birmingham and the current leader of the NA62-UK consortium yesterday 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.

Attracting top talent and providing opportunities and mentorship for early-career researchers has been our priority. It is a privilege to work in and lead such an energetic and constructive team.

Searching for new physics beyond the Standard Model

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

Professor David Britton, team leader of the Glasgow NA62 group and Project Leader of the GridPP Project that delivers key computing infrastructure support for the project yesterday said:

This amazing achievement of observing such an extraordinarily rare decay has relied critically on the long-term support of the GridPP computing infrastructure in the UK by STFC. That infrastructure, led by researchers at the University of Glasgow, has enabled the generation of the billions of simulated events over the last decade that were used to understand the detector, the backgrounds, and the expected signature in the data. This work enabled and under-pins the physics result announced today.

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

Further information

Watch the background video from CERN on NA62.

Building on a long tradition of research into matter-antimatter asymmetry carried out in experiments NA31 and NA48, the NA62 experiment at CERN is examining the decay of kaons, and one type in particular, where a kaon decays into a pion and two neutrinos.

The Standard Model, the theory that underpins particle physics, gives a precise prediction of the frequency of this decay type.

It’s a very rare occurrence, but looking closely at these rare processes is essential for exploring physics beyond the Standard Model. The technique that NA62 uses, observing decay in flight, has never been done anywhere before.

The data used in the research was taken between 2016 and 2018 and 2021 and 2022 at the CERN’s Prevessin site, in France. The research involves over 200 scientists from 31 institutions.

Read about the NA62 experiment.

UK group collaboration

The UK group joined the NA62 experiment at the design phase in 2007 and has played central roles in the collaboration.

The experiment has been designed, constructed and operated with a significant UK contribution.

The UK group consist of researchers from the universities of:

• Birmingham
• Bristol
• Glasgow
• Lancaster
• Liverpool (during the NA62 construction phase)

The UK group has been funded by:

• STFC, which is part of UK Research and Innovation
• Royal Society
• European Research Council
• European Commission

STFC funded two Ernest Rutherford Fellowships and has provided continuous funding for the NA62 experiment. They are currently supporting three postdoctoral research fellows and five doctoral students across the participating groups.