First observation of neutrinos with prototype at the ultimate neutrino observatory DUNE

Neutrinos are fundamental particles that played an important role in the early phase of the universe. They are the key to learning more about the fundamental laws of nature, such as the question of why there is more matter than antimatter in the universe. The Deep Underground Neutrino Experiment (DUNE) in the USA aims to unlock the secrets of these elementary particles. DUNE is a leading-edge, international experiment at the Fermi National Accelerator Laboratory (Fermilab) particle physics research center near Chicago, involving more than 1,400 researchers from over 200 institutions worldwide. As part of the research agreement between the University of Bern and Fermilab, researchers from the Laboratory for High Energy Physics (LHEP) and the Albert Einstein Center for Fundamental Physics (AEC) at the University of Bern are also involved in DUNE. The collaboration has successfully detected the first neutrinos at Fermilab using the “2x2” prototype of the “ND-LAr” detector, which was developed and built at the University of Bern.

Building the world's ultimate neutrino observatory

DUNE is split between two sites, Fermilab near Chicago and the Sanford Underground Research Facility in South Dakota. A neutrino beam generated at Fermilab first passes through a so-called, “near detector” to detect neutrinos close to the source. The beam then travels 1,300 kilometers through the ground to several giant particle detectors in South Dakota. By taking measurements near and far from the source, researchers hope to learn how the particles change as they travel, a phenomenon known as neutrino oscillation. Researchers will also use DUNE to study the neutrinos' counterparts, called antineutrinos. DUNE is still under construction, but it will be the world's most extensive, cutting-edge neutrino research experiment.

The researchers at the University of Bern are contributing the main component of the near detector. This “ND-LAr”, as the special detector is called, employs an innovative liquid argon technology that was conceived and developed in Bern. The “ND-LAr” detector has a pixel readout system with millimeter resolution, which enables highly precise 3D images of neutrino interactions on a large scale. "What is particularly fascinating about this research project is that we are taking very high-resolution 3D images of a particle that very rarely interacts with the argon in the detector. It is nice that we can now actually see how these particles interact with a detector technology developed in the laboratory here at the University of Bern" says Michele Weber, director of the Laboratory for High Energy Physics (LHEP) and head of the DUNE group in Bern. The “ND-LAr” will consist of 35 liquid argon modules in a 5x7 array that will detect and analyze the neutrinos. The large number of modules is needed to separate and observe many overlapping interactions of the enormous neutrino flux close to it’s source. To test the functionality of the detector, a smaller prototype of the “ND-LAr” - known as the “2x2” prototype because it consists of only four smaller liquid argon modules arranged in a square - has been built using the same technology.

Successful detection of neutrinos

In preparation for the final installation of the full “ND-LAr”, the “2x2” prototype was tested in a neutrino beamline at Fermilab. In the process, the first neutrinos were successfully detected. This milestone proves the innovative technology’s efficacy and validates the design of the “ND-LAr”. "It is great to see how small tests in our laboratory in Bern have now led to an advanced technology with which we can measure neutrinos. Not only has the technology evolved, but also the project itself. A small experiment has turned into a significant collaboration with many other researchers who are at the forefront of particle physics," says Livio Calivers, who built the “2x2” prototype as part of his doctoral thesis at Fermilab. Testing the prototype of the “ND-LAr” was necessary to ensure that the innovative design and technology would also work on a large scale and that the requirements of the “ND-LAr” could be met. Detectors based on liquid argon technology, such as the detector used in the MicroBooNE experiment, which was also conducted at Fermilab and in which the University of Bern played a key role, have already been used in neutrino research. However, a detector like the “ND-LAr”, which can show individual 3D images from an intense stream of neutrinos and antineutrinos, has never been built or tested. "The successful test of the prototype is an important milestone that illustrates the potential of this technology and will pave the way for the construction of the 'ND-LAr'," Weber added.

Next steps in answering fundamental questions about the universe

Detecting neutrinos with the “ND-LAr” prototype represents a significant advance in the DUNE experiment and brings science closer to new insights in neutrino research. "The successful detection of neutrinos using the ‘2x2’ prototype allows us to finalize the design of ‘ND-LAr’ and then start building the near detector. At the same time, liquid argon modules of the size that will be used in the final detector are currently being tested at the University of Bern," Weber explains. After maintenance work, the neutrino beam will go back into operation at Fermilab in the fall of 2024. The prototype will then record data for several months, producing about 10,000 images of neutrinos per day. "This data will be used for many doctoral theses and scientific publications and will form the basis for the commissioning of the ‘ND-LAr’ in 2030," Calivers concludes.

The participation of the University of Bern and Switzerland in the DUNE experiment is funded by the University of Bern and the State Secretariat for Education, Research and Innovation, as well as by competitively acquired funds from the Swiss National Science Foundation and the EU.