The NA62 experiment has detected two candidate events for the decay of a positively charged kaon into a pion and a neutrino–antineutrino pair

Are there new, unknown particles that can explain dark matter and other mysteries of the universe? To try to answer this question, particle physicists typically sift through the myriad of particles that are produced in particle collisions. But they also have an indirect but powerful way of looking for new particles, which is to measure 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.

One such process is the transformation, or “decay”, of a positively charged variant of a particle known as kaon into a positively charged pion and a neutrino–antineutrino pair. In a seminar that took place today at CERN, the NA62 collaboration reported two potential instances of this ultra-rare kaon decay. The result, first presented at the International Conference on Kaon Physics, shows the experiment’s potential to make a precise test of the Standard Model.

The Standard Model predicts that the odds of a positively charged kaon decaying into a positively charged pion and a neutrino–antineutrino pair (K+ → π+ ν ν) 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.

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

In 2018, the NA62 team reported finding one candidate event for the K+ → π+ ν ν decay in a dataset recorded in 2016 that comprised about 100 billion K+ decays. In its new study, the collaboration analysed an approximately 10-fold larger dataset recorded in 2017 and spotted two candidate events. By combining this result with the previous result, the team finds that the relative frequency (known as “branching ratio”) of the K+ → π+ ν ν 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.

“This is a great achievement and one we will build upon. Having clearly established our experimental technique, we’ll now explore ways to perfect it using a dataset that we took in 2018,” says spokesperson Cristina Lazzeroni. “The 2018 dataset is twice as large as the 2017 dataset, so it should allow us to find more events and make a more precise test of the Standard Model.”

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For a detailed account of the results, see the recording of the CERN seminar and the EP newsletter article.