Extremely rare decay process identified

Results of the NA62 collaboration are a milestone in the search for new physics

25 August 2020

It's like looking for a needle in a haystack: In order to put the Standard Model of particle physics to the test, physicists observe and measure extremely rare decay processes of certain particles. They want to find out whether there are differences between their measurements and the theoretical predictions of the Standard Model.

The NA62 experiment at CERN is intended to observe with high precision such a process in which a positively charged particle known as kaon transforms into a positively charged pion and a neutrino-antineutrino pair (designated 𝐾+ → 𝜋+𝜈𝜈̅). At the 40th International Conference on High Energy Physics, CERN researcher Radoslav Marchevski, who has already investigated this decay as a NA62 PhD student at the Johannes Gutenberg University Mainz (JGU), now reported 17 candidate events for this extremely rare process in the latest NA62 data collected in 2018. By combining the data they collected in 2016 and 2017, the NA62 collaboration can claim the first evidence for this ultra-rare process, with a statistical significance of 3.5σ.

One of the rarest processes physicists can observe

The NA62 detector gets its particle beam from the Super Proton Synchrotron (SPS) at CERN. Here, proton beams with an energy of 400 gigaelectronvolts collide with a fixed target made of beryllium. This way, nearly a billion secondary particles are produced each second, which fly towards the detector. Of these particles, about 6 percent are positively charged kaons. In the detector they are identified by a dedicated device before they transform into lighter particles. "The art is to first count the kaons produced and then identify which of them have transformed into a pion and a neutrino-antineutrino pair," explains Dr. Rainer Wanke, physicist at the PRISMA+ Cluster of Excellence and head of the Mainz NA62 participation at CERN. The Standard Model of particle physics predicts that only about one in every ten billion kaons will undergo this special decay process. It is thus one of the rarest processes that physicists can observe, and therefore one of the most intriguing ones.

In 2018, the NA62 detector collected data for 217 days. From this data, it was possible to identify the 17 new events that fit the profile of the sought-after kaon decay. Three such potential decays were previously observed in data from 2016 and 2017. "As a result, the rate at which kaons undergo this rare transformation is slightly more than one in ten billion when combining these data, but with an uncertainty of 35 percent," says Rainer Wanke. "Therefore, at the current level of precision, the experimental value is compatible with the Standard Model's prediction."

The next goal is to increase the significance from 3.5σ to 5σ when the SPS at CERN resumes operations in 2021. "The standard deviation σ is a measure of how save we can be that it is really the process we are looking for," explains Rainer Wanke. "Above a threshold of 5σ, we would actually claim to have discovered this new process with certainty". The next step would then be to investigate whether an even larger data set also shows a significant deviation from the standard model. "That would be our dream!"

The NA62 collaboration is comprised of about 140 scientists from 13 European and North American countries. From Germany, a group of JGUs PRISMA+ Cluster of Excellence is involved, which is responsible for the detector's myonveto system. This system is designed to distinguish pions, which are produced during the decay under investigation, from muons, which occur very frequently during kaon decay. It was developed and built in Mainz over a construction period of six years and was transported to CERN in 2015 in a heavy-duty transporter. With a footprint of three by three meters and a height of one meter, the Mainz detector weighs 40 tons.

The particle detector and the participation of the Mainz scientists in the NA62 experiment is financially supported by the Federal Ministry of Education and Research (BMBF) and supported by PRISMA+.