NA62 collaboration confirms one-in-10-billion kaon decay

On September 24, the NA62 collaboration working at CERN announced the ‘unequivocal confirmation’ of a rare and significant interaction: the rare decay of a positively charged particle, a kaon, into a positively charged pion and a neutrino-antineutrino pair.

While previous experiments hinted at this process, this is the first time it has been confirmed with a statistical significance of five standard deviations. The decay, represented as , is incredibly rare—according to the Standard Model, fewer than one in 10 billion positively charged kaons is expected to decay in this manner.

(image: the NA62 fixed-target experiment at CERN) 

The Canadian NA62 group at TRIUMF and the University of British Columbia including Doug Bryman, TRIUMF Affiliate Scientist and UBC J.B. Warren Chair, Toshio Numao (TRIUMF Emeritus Scientist), and Bob Velghe (TRIUMF Research Associate) and until recently, Vincent Wong (TRIUMF Postdoctoral Researcher) has been participating in the NA62 measurements of the ultra-rare decay and other processes since 2015. Contributions to data analysis and algorithm development have included background and particle identification studies based on machine learning. The group’s contributions also include hardware aspects and instrumentation related to the liquid krypton calorimeter and the silicon pixel gigatracker (GTK). Training of highly qualified personnel has included several postdoctoral researchers, and students including 16 Masters of Data Science projects. 

"The NA62 collaboration has confirmed the existence of the one in ten billion decay process  by observing 51 events matching its signature,” said Velghe. “I’m excited to be part of this significant measurement, which adds constraints on possible beyond the Standard Model scenarios. “

Led by Bryman, the TRIUMF/UBC rare decay group was among the leaders in the precursor experiments assessing rare kaon decay, which were performed at Brookhaven National Laboratory. Those experiments produced the initial observation of the 1 in 10 billion branching fraction, which has now been confirmed and improved substantially by NA62.

“It’s satisfying to participate in a definitive observation of this ultra-rare reaction, which will lead to further detailed examination of theoretical predictions,” said Bryman.