An international collaboration involving Lancaster physicists has presented its latest result on the search for the ultra-rare decay of a charged particle called a “kaon”.

The NA62 team at CERN in Switzerland has reported two potential instances of this ultra-rare kaon decay, showing the experiment’s potential to make a precise test of the Standard Model of Particle Physics.

This worldwide collaboration of research physicists includes UK scientists from the Universities of Lancaster, Birmingham, Bristol and Glasgow, funded by the European Research Council (ERC) and the UK Science and Technology Facilities Council (STFC).

The NA62 experiment produces positively charged kaons (K+) and other particles by hitting a beryllium target with protons from the Super Proton Synchroton accelerator. It then uses several types of detector to identify and measure the K+ kaons and the particles into which they decay.

The researchers are looking for new particles by measuring processes that are both rare and precisely predicted by the Standard Model of particle physics. A slight discrepancy between the Standard Model prediction and a high-precision measurement would be a sign of new particles or phenomena never before observed.

The Standard Model of Particle Physics predicts that the odds of a positively charged kaon decaying into a positively charged pion and a neutrino–antineutrino pair are only about one in ten billion, with an uncertainty of less than ten percent.

Finding a deviation, even if small, from this prediction would indicate new physics beyond the Standard Model.

In 2018, the NA62 team reported finding one positively charged kaon decaying into a positively charged pion and a neutrino–antineutrino pair using a 2016 dataset comprising about 100 billion K+ decays. In this new study, the collaboration analysed an approximately 10-fold larger dataset recorded in 2017 and spotted two such events.

By combining this result with the previous result, the team finds that the relative frequency (known as “branching ratio”) of this decay would be at most 24.4 in 100 billion K+ decays. This combined result is compatible with the Standard Model prediction and allowed the team to put limits on beyond-Standard-Model theories that predict frequencies larger than this upper bound.