NA62 sees first significant evidence of rare process

Physicists look for new physics phenomena in many ways. One is by observing and measuring processes that are predicted to be extremely rare and looking for differences between data and theoretical predictions. The detector – the 62nd experiment located in CERN’s North Area – is designed to observe with high precision one such process, in which a positively charged particle known as a kaon transforms into a positively charged pion and a neutrino-antineutrino pair (denoted by K⁺→π⁺νν). Yesterday, at the , the NA62 collaboration reported recording 17 candidate events for this particular transformation in data they collected in 2018. By combining the data they collected in 2016 and 2017, NA62 can claim the first evidence for this ultra-rare process, with a statistical significance of three-and-a-half sigma (3.5σ).

Colliding particles – into other particle beams or into fixed targets – at sufficiently high energies can produce heavy, unstable particles, like the kaons sought by NA62. These heavy particles transform (or “decay”) almost instantaneously into lighter particles in various combinations. The predicts how often a given particle will undergo all possible transformations. In the case of the kaon, only around one in every ten billion are expected to transform into a pion and a neutrino-antineutrino pair, with an uncertainty of about 10%. It is thus one of the rarest processes that can be observed by physicists.

While CERN is famous for the Large Hadron Collider, other accelerators at the laboratory provide particle beams for smaller but highly specialised experiments. The NA62 detector gets its beam from the . Proton beams from the SPS, with an energy of 450 gigaelectronvolts, slam into a fixed target made of beryllium located upstream of NA62. Nearly a billion secondary particles are produced each second as a result and race towards the detector. Of these particles, around 6% are positively charged kaons. The kaons enter the detector, where a dedicated device identifies them before they undergo transformation into lighter particles. The physicists therefore have to first count the kaons produced and identify which of them transformed into a pion and a neutrino-antineutrino pair. Since neutrinos and their antiparticle counterparts leave no trace in the NA62 detector, their presence has to be deduced by calculating the angles between the parent kaon and the daughter pion and by measuring their speed and direction of motion.

In 2018, the NA62 detector collected data for 217 days, at the expense of around a billion billion (10¹⁸) protons. By sifting through these data, the collaboration was able to identify 17 new events that fit the K⁺→π⁺νν profile, in addition to the and the . Combining these data allowed NA62 to experimentally determine that the rate at which kaons undergo this rare transformation is around one in ten billion, with an uncertainty of about 35%. The experimental value is compatible with the Standard Model’s prediction at the current level of precision.

This is an important milestone for the experiment. NA62 is now on track to reach the threshold of 5σ statistical significance to claim observation of the process. The detector will receive new batches of kaons when the SPS resumes operations in 2021, following the second long shutdown of CERN’s accelerator complex.