Neutrinos are extremely elusive elementary particles. Day and night, 60 billion of them stream from the Sun through every square centimeter of the Earth every second, which is transparent to them. After the first theoretical prediction of their existence, decades passed before they were actually detected. These experiments are usually extremely large to account for the very weak interaction of neutrinos with matter. Scientists at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg have now succeeded in detecting antineutrinos from the reactor of a nuclear power plant using the CONUS+ experiment, with a detector mass of just 3 kg.
Originally based at the Brokdorf nuclear power plant, the CONUS experiment was relocated to the Leibstadt nuclear power plant (KKL) in Switzerland in the summer of 2023. Improvements to the 1 kg germanium semiconductor detectors, as well as the excellent measurement conditions at KKL, made it possible for the first time to measure what is known as Coherent Elastic Neutrino-Nucleus Scattering (CEvNS). In this process, neutrinos do not scatter off the individual components of the atomic nuclei in the detector, but rather coherently with the entire nucleus. This significantly increases the probability of a very small but observable nuclear recoil. This recoil caused by neutrino scattering is comparable to a ping-pong ball bouncing off a car, with the detection being the changing motion of the car. In the case of CONUS+, the scattering partners are the atomic nuclei of the germanium. Observing this effect requires low-energy neutrinos, such as those produced in large numbers in nuclear reactors.
The effect was predicted as early as 1974, but was first confirmed in 2017 by the COHERENT experiment at a particle accelerator. The CONUS+ experiment has now successfully observed the effect at full coherence and lower energies in a reactor for the first time, as described in a recent Nature research article. The compact CONUS+ setup is located 20.7 m from the reactor core (see Fig. 1). At this position, more than 10 trillion neutrinos flow through every square centimeter of surface every second. After approximately 119 days of measurement between autumn 2023 and summer 2024, the researchers were able to extract an excess of 395±106 neutrino signals from the CONUS+ data, after subtracting all background and interfering signals (see Fig. 2). This value is in very good agreement with theoretical calculations, within the measurement uncertainty. "We have thus successfully confirmed the sensitivity of the CONUS+ experiment and its ability to detect antineutrino scattering from atomic nuclei," explains Dr. Christian Buck, one of the authors of the study. He also emphasizes the potential development of small, mobile neutrino detectors to monitor reactor heat output or isotope concentration as possible future applications of the CEvNS technique presented here.
The CEvNS measurement provides unique insights into fundamental physical processes within the Standard Model of particle physics, the current theory describing the structure of our universe. Compared to other experiments, the measurements with CONUS+ allow for a reduced dependence on nuclear physics aspects, thereby improving the sensitivity to new physics beyond the Standard Model. For this reason, CONUS+ was already equipped with improved and larger detectors in autumn 2024. With the resulting measurement accuracy, even better results are expected. "The techniques and methods used in CONUS+ have excellent potential for fundamental new discoveries," emphasizes Prof. Lindner, initiator of the project and also an author of the study. "The groundbreaking CONUS+ results could therefore mark the starting point for a new field in neutrino research."
