Neutrinos, fundamental particles integral to the early universe, are essential for understanding the fundamental laws of nature, including mass acquisition and matter-antimatter asymmetry. Despite their abundance, neutrinos are notoriously difficult to detect due to their minimal interaction with matter, earning them the nickname "ghost particles." Historically, physicists have mostly studied low-energy neutrinos generated in specialized facilities.
The FASER International Collaboration, featuring researchers from the University of Bern's Laboratory for High Energy Physics (LHEP), has measured the interaction rates of electron neutrinos and muon neutrinos-two neutrino subtypes-with atomic nuclei at the highest recorded energy level of 1 teraelectronvolt (TeV). This measurement, performed using the FASERv detector within the FASER experiment, marks the first observation of electron neutrinos in an LHC experiment. "This research result is of great importance because the study of neutrinos at such high energies offers the possibility of gaining deeper insights into the fundamental laws of nature, studying rare processes and possibly discovering new physical phenomena," said Akitaka Ariga, particle physicist and head of the FASER group at LHEP. The findings were published in Physical Review Letters.
State-of-the-Art Forward Detection Technology
The FASERv neutrino detector, which observes high-energy neutrinos produced by proton-proton collisions in the LHC, is positioned 480 meters underground from the collision point. It consists of alternating tungsten plates and emulsion films that can detect particle tracks with nanometer precision. This advanced 1.1-tonne detector has been operational since 2022. "In this study, we analyzed a portion of the data obtained by the FASERv detector in 2022, amounting to 2% of the total data collected so far, so we still have a long way to go," explained Ariga, who leads the FASERv project.
High-Energy Neutrinos: A Gateway to New Physics?
In the ongoing FASER experiment, the number of detected neutrinos is expected to increase significantly in the coming years. This increase will help address questions about the differences between the three neutrino subtypes and potential unknown forces. The tau neutrino, the third subtype, is particularly challenging to produce and detect at low energies. "The high energy of the FASER experiment makes it possible to generate and study tau neutrinos more efficiently. Little is known about these neutrinos and they could provide new physical insights," remarked Ariga. The FASER experiment will continue data collection until the end of 2025.
Future experiments, including the follow-up FASERv2, aim to collect data volumes 10,000 times larger, significantly expanding these investigations. Addressing questions like "Why does the universe consist mainly of matter and very little antimatter?" or "What is dark matter?" may eventually require the discovery of unknown forces or new particles. "Perhaps we will find 'undiscovered physics' with the high-energy neutrinos," added Ariga.
University of Bern Expertise at CERN and Fermilab
CERN, home to the world's most powerful particle accelerator, the LHC, is a leading center for particle physics research. The University of Bern has been a key player at CERN, being a founding member of the ATLAS project, the largest particle detector at the LHC, and contributing to its ongoing development. Ariga's research group has been involved with FASER from its inception.
In addition to its work at CERN, the University of Bern participates in the Deep Underground Neutrino Experiment (DUNE) at Fermilab near Chicago, a major international effort involving over 1,000 researchers from more than 30 countries, which aims to generate the world's most intense neutrino beam.
Related Links
University of Bern, Laboratory for High Energy Physics
Understanding Time and Space
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