​In 2024, researchers working on the NA62 experiment at the CERN Super Proton Synchrotron announced the first ever observations of an ultra-rare particle decay at a statistical significance of five sigma – the gold standard in particle physics research.

Ten years earlier, members of the team who work on NA62 visited ISIS to test their equipment before the experiment began. Those tests enabled the NA62 team to implement ways to protect the electronic components from the stream of particles they would be exposed to, helping ensure the data collected was reliable and that the experiment worked as planned.

As a result, the team saw charged kaons decay into a charged pion and a neutrino-antineutrino pair, which is predicted to occur to less than one in 10 billion kaons by the Standard Model of physics.

“To have this information prior to the start of data taking was crucial and, with these protections in place, NA62 has been acquiring data since 2016, leading to several key measurements - including, most recently, the five-sigma observation of the rarest decay ever measured in the history of particle physics!!" says Dr Angela Romano from the University of Birmingham and a member of the NA62 research team.

Two years in ten hours

The first ISIS experiment the team conducted was to expose their electronic circuit boards to a stream of muons in the RIKEN facility at ISIS.

Within NA62, the electronic components responsible for detecting the decay particles and collecting data are exposed to a stream of particles. Billions of particles are generated every second when the Super Proton Synchrotron's beam of high energy protons strike a beryllium target. 6% of those particles are kaons, and of those, a small fraction will decay. It is those decay particles that NA62 were investigating.

However, the proton beam also generates muons, which can cause problems with the electronics. In what is known as a single event upset, the muons could alter the data collected during the experiment; that is a particular risk for NA62 because the decay events they were hoping to observe are so rare. To help mitigate that risk, the NA62 team needed to understand how the electronics were affected by muons, and how likely single event upsets were to occur.

At ISIS, the components experienced two years of muon exposure in just ten hours.

The data collected helped the researchers understand how the single event upsets affected the electronic components and allowed them to develop an effective monitoring system for single event upsets and a semi-automated procedure for resetting the electronics board. As a result, they could minimise the amount of corrupted data collected.

Vital results

The second test at ISIS exposed the electronics to a stream of neutrons from the VESUVIO instrument. Again, the test simulated the conditions in which the electronics needed to operate at CERN. This turned out to be vitally important. “The neutron tests… showed that our electronics wouldn't have survived. The board would stop working almost immediately," explains Dr Romano.

To shield their electronics from neutrons, the NA62 researchers encased them in a boron-layered cupboard.

Following those tests and modifications, NA62 began collecting data in 2016. In late 2024 the team announced their discovery. The findings could help guide physicists to new physics beyond the Standard Model.

CERN announcement: NA62 experiment at CERN observes ultra-rare particle decay | CERN

Original 2014 ISIS science highlight: ISIS helps CERN experiment test the Standard Model of Particle Physics​