NA62 announces its first search for long-lived particles

Studying rare particle physics processes is like looking for a needle in a haystack. But to find the needle, we first need the haystack – a large amount of statistical data collected from high-luminosity experiments. The NA62 experiment, also known as CERN’s kaon factory, generates this haystack of collision data to enable physicists to study rare particle physics processes and search for weakly interacting new particles. The collaboration recently presented the results of its first search for long-lived new particles at the 42nd International Conference on High Energy Physics in Prague.

“While experiments at the Large Hadron Collider are known for pushing the energy boundaries with proton-proton collisions at the world record energy of 13.6 trillion electron volts, fixed-target experiments like NA62 push the intensity limit at a billion billion (1018) protons per year,” said Jan Jerhot, a postdoctoral fellow at the Max Planck Institute for Physics who led the analysis of the latest NA62 results.

These collisions in NA62, an experiment with a fixed target, lead to up to 10¹² positively charged kaon K⁺ decays per year – a much higher luminosity than that which can be achieved in collider experiments. Fixed-target approaches and collider experiments complement each other in their quest to find new physics beyond the Standard Model. The excellent energy, momentum and time resolutions of the NA62 detector enable the search for the rarest processes in these large data sets.

The NA62 experiment operates in two modes – the standard kaon mode, which mainly studies rare kaon processes such as the conversion of a kaon into a positively charged pion and a neutrino-antineutrino pair (denoted K⁺ → π⁺ ννbar), and a beam dump mode, where the Super Proton Synchrotron’s proton beam is directed into an absorber, allowing the search for new heavy particles with twice the proton intensity compared to the kaon mode. At ICHEP, NA62 shared the preliminary results of its search for a long-lived new physical particle using data obtained with 1.4 x 1017 protons on target from the beam dump operation in 2021.

The collaboration specifically looked for the decay of a particle beyond the Standard Model into two charged hadrons, such as pions and kaons, and the decay of neutral hadrons into photons. These are possible decays of new physical particles in theories beyond the Standard Model and promising candidates to explain elusive dark matter such as axion-like particles, dark photons, and dark Higgs bosons.

Felix Kahlhöfer, professor of theoretical physics at the Karlsruhe Institute of Technology, explains that the recent NA62 results are model-independent and that physicists worldwide can reinterpret these results to constrain many different models beyond the Standard Model. “We obtain information for a whole class of decay channels at the same time, making it possible to distinguish different models of long-lived particles beyond the Standard Model,” he said.

The distance between the NA62 target and the calorimeters is more than 240 meters, making the experiment very well suited to search for long-lived new physics particles that travel a macroscopic distance before decaying. These are particularly difficult to detect for other particle detectors at the LHC such as ATLAS (46 meters long) and CMS (21 meters long), as they may not be able to see the decay of such exotic particles before they leave the detector.

Exploring these unexplored areas can solve some of the problems that the robust Standard Model cannot explain. Although no evidence of a new physical signal was found in this latest analysis, NA62 was able to rule out new mass and interaction strength ranges in theories that go beyond the Standard Model. NA62 physicists plan to examine seven times more data in the beam dump mode of operation by the end of the next NA62 physics run compared to the current analysis mode, and hope to unravel the rarest of the rare events in particle physics.