The NA62 collaboration at CERN has found a candidate event for the ultra-rare decay K+ π+ ν ν, demonstrating the experiment’s potential to test heavily-suppressed corners of the Standard Model (SM).

The SM prediction for the K+ π+ ν ν branching fraction is 0.84 ± 0.03 × 10–10. The very small value arises from the underlying coupling between s and d quarks, which only occurs in loops and is suppressed by the couplings of the quark-mixing CKM matrix. The SM prediction for this process is very clean, so finding even a small deviation would be a strong indicator of new physics.

NA62 was approved a decade ago and builds on a long tradition of kaon experiments at CERN (CERN Courier June 2016 p24). The experiment acts as a kaon factory, producing kaon-rich beams by firing high-energy protons from the Super Proton Synchrotron into a beryllium target and then using advanced Cherenkov and straw trackers to identify and measure the particles (see figure). Following pilot and commissioning runs in 2014 and 2015, the full NA62 detector was installed in 2016 enabling the first analysis of the K+ π+ ν ν channel.

Finding one candidate event from a sample of around 1.2 × 1011 events allowed the NA62 team to put an upper limit on the branching fraction of 14 × 10–10 at a confidence level of 95%. The result, first presented at Moriond in March, is thus compatible with the SM prediction, although the statistical errors are too large to probe beyond-SM physics.

Several candidate K+ π+ ν ν events have been previously reported by the E949 and E787 experiments at Brookhaven National Laboratory in the US, inferring a branching fraction of 1.73 ± 1.1 × 10–10 – again consistent, within large errors, with the SM prediction. Whereas the Brookhaven experiments observed kaon decays at rest in a target, however, NA62 observes them in-flight as they travel through a large vacuum tank and therefore creates a cleaner environment with less background events.

The NA62 collaboration expects to identify more events in the ongoing analysis of a 20-fold-larger dataset recorded in 2017. In mid-April the experiment began its 2018 operations with the aim of running for a record number of 218 days. If the SM prediction is correct, the experiment is expected to see about 20 events with the data collected before the end of this year.

“The K+ π+ ν ν decay is special because, within the SM, it allows one to extract the CKM element |Vtd| with a small theoretical uncertainty,” explains NA62 spokesperson Augusto Ceccucci. “Developing the necessary experimental sensitivity to be able to observe this decay in-flight has involved a long R&D programme over a period of five years, and this effort is now starting to pay off.”