Neutrinos, fundamental particles from the early universe, are key to understanding more about the fundamental laws of nature, including the mystery of why there is more matter than antimatter in the universe. DUNE, an international experiment based at Fermilab near Chicago, involves over 1,400 researchers from more than 200 institutions worldwide and aims to unlock these secrets.

The University of Bern's Laboratory for High Energy Physics (LHEP) and the Albert Einstein Center for Fundamental Physics (AEC) are integral to this endeavor. Researchers from these institutions have successfully detected the first neutrinos using the "2x2" prototype of the "ND-LAr" detector, which was developed and built at the University of Bern.

Building the Ultimate Neutrino Observatory
DUNE operates across two locations: Fermilab and the Sanford Underground Research Facility in South Dakota. A neutrino beam generated at Fermilab first passes through a "near detector" to capture neutrinos close to the source. The beam then travels 1,300 kilometers underground to reach giant detectors in South Dakota. By comparing measurements taken near and far from the source, scientists hope to understand neutrino oscillation - a phenomenon where neutrinos change types as they travel. DUNE will also study antineutrinos, the counterparts of neutrinos. Once complete, DUNE will be the most extensive and advanced neutrino research experiment in the world.

The University of Bern is contributing a critical component to the near detector. The "ND-LAr" uses innovative liquid argon technology conceived in Bern. This detector features a pixel readout system with millimeter precision, allowing for high-resolution 3D images of neutrino interactions.

"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," said 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, designed to detect and analyze neutrinos. A large number of modules is necessary to observe and separate many overlapping interactions from the intense neutrino flux close to the source. To test the detector's functionality, a smaller prototype known as the "2x2" prototype, consisting of four smaller liquid argon modules, was built using the same technology.

Successful Detection of Neutrinos
In preparation for the full installation of the "ND-LAr," the "2x2" prototype was tested in a neutrino beamline at Fermilab, successfully detecting the first neutrinos. This milestone validates the innovative technology and 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," noted Livio Calivers, who built the "2x2" prototype as part of his doctoral thesis at Fermilab.

Testing the "ND-LAr" prototype was essential to ensure that the innovative design and technology would function on a large scale and meet the required standards. Detectors using liquid argon technology have been used in neutrino research before, but never on the scale or with the precision of the "ND-LAr." "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'," added Weber.

Next Steps in Unlocking the Universe's Mysteries
The detection of neutrinos with the "ND-LAr" prototype marks a major advancement in the DUNE experiment, bringing scientists closer to new discoveries 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 explained.

Following maintenance work, the neutrino beam at Fermilab will resume operation in the fall of 2024. The prototype will then record data for several months, producing approximately 10,000 images of neutrinos daily. "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," concluded Calivers.

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