First observation of ultra-rare particle decay could reveal new physics

Scientists at CERN have discovered an ultra-rare particle decay process, opening a new avenue to uncover physics beyond our understanding of how the building blocks of matter interact.

The NA62 collaboration presented at a CERN EP seminar the first experimental observation of the ultra-rare decay of the charged kaon into a charged pion and a neutrino-antineutrino pair (K⁺ → π⁺νṽ).

This is an extremely rare phenomenon: the Standard Model (SM) of particle physics, which explains how particles interact, predicts that less than one in 10 billion kaons decays in this way. The NA62 experiment was designed and built specifically to measure this kaon decay.

Cristina Lazzeroni, professor of particle physics at the University of Birmingham, said: “With this measurement, K⁺ → π⁺νṽ becomes the rarest decay ever established at the discovery level – the famous 5 sigma. This challenging analysis is the result of excellent teamwork, and I am extremely proud of this new result.”

Kaons are produced by a high-intensity proton beam from CERN’s Super Proton Synchrotron (SPS) colliding with a stationary target. This creates a secondary particle beam of nearly a billion particles per second that enter the NA62 detector, of which about 6% are charged kaons. The detector precisely identifies and measures each kaon and its decay products, except for neutrinos, which appear as missing energy.

Professor Giuseppe Ruggiero, from the University of Florence, added: “This is the culmination of a long project that began more than a decade ago. It involves looking for effects in nature that have probabilities of occurring of the order of 10^-11 “This is an exciting and challenging project. After rigorous and meticulous work, we have achieved an amazing reward for our efforts and delivered a long-awaited result.”

The new result is based on the combination of data collected by the NA62 experiment in 2021-22 and a previously published result based on the 2016-2018 dataset. The 2021-2022 dataset was collected following a series of upgrades to the NA62 configuration, enabling operation at 30% higher beam intensity with several new and improved detectors.

Hardware upgrades combined with refined analysis techniques have enabled candidate signal collection at a 50% higher rate than before, while adding new tools to remove backgrounds.

A group of scientists from the University of Birmingham, currently led by Professor Yevgeny Gudzovsky, joined the NA62 experiment during the design phase in 2007, playing a central role in the collaboration.

Professor Goudzovski commented: “Attracting top talent and providing leadership positions for early-career researchers has always been a priority for the group. We are proud that the current NA62 Physics Coordinator and the current K⁺ → π⁺“The members of νṽ Measurement are former PhD students from Birmingham. It is a privilege to work with and lead such an energetic and constructive team.”

The research team studies the K⁺ → π⁺The decay of νṽ is very sensitive to new physics beyond the Standard Model description. This makes the decay one of the most interesting processes to search for evidence of new physics.

The fraction of kaons that decay into one pion and two neutrinos is estimated to be about 13 in 100 billion. This figure is consistent with the Standard Model predictions, but is about 50% higher.

This could be due to new particles that increase the probability of this decay, but more data is needed to confirm this hypothesis. The NA62 experiment is currently collecting data and scientists hope to confirm or deny the presence of new physics in this decay in the coming years.