The international ICARUS collaboration announced its first physics results on neutrino oscillation searches in a paper recently posted to the preprint server arXiv. They did not observe muon-neutrino disappearance in the data collected with the neutrino beam at Fermilab. The analysis is unique for its rigorous treatment of uncertainties around detector performance. It also demonstrates the quality of the ICARUS data and advances the development of analysis techniques and software tools.
ICARUS is located at the U.S. Department of Energy’s Fermi National Accelerator Laboratory near Chicago and is the world’s first large liquid-argon neutrino detector. It began operating in 2010 at Italy’s Gran Sasso National Laboratory, governed by INFN, the Italian Institute for Nuclear Physics. In 2014, the detector was moved to CERN to be refurbished and improved and, three years later, it journeyed to its new home at Fermilab to become part of the Short Baseline Neutrino (SBN) Program.
The search for neutrino oscillations
The SBN Program consists of three experiments situated along the lab’s neutrino beam: The Short Baseline Near Detector (SBND) is the closest to the neutrino source at just 110 meters away, followed by MicroBooNE at 470 meters, and finally ICARUS at 600 meters. All three SBN experiments study how mysterious, ultra-lightweight particles called neutrinos change as they move through space and matter.
“With ICARUS fully validated and operating we are entering, in concert with SBND, a new era of neutrino physics in which definitive, world‑leading measurements are finally within reach.”
Carlo Rubbia, 1984 Physics Nobel laureate and ICARUS spokesperson
Neutrinos come in three types, called flavors: electron, muon and tau. In a phenomenon called neutrino oscillation, neutrinos change flavor as they travel. To fully understand this behavior, physicists designed baseline experiments — in which particle detectors are placed at various distances along a neutrino beam, like in the SBN Program — to study it.
One specific phenomenon ICARUS is looking for could be evidence of a postulated fourth flavor of neutrino, the sterile neutrino. In the so-called 3+1 model of neutrino behavior, the sterile neutrino could mix with the three known flavors and cause oscillations that appear as muon‑neutrino disappearance over short distances. So, if ICARUS were to observe muon-neutrino disappearance, it would be evidence supporting the 3+1 sterile-neutrino hypothesis; if not, the collaboration could set limits on the model parameters.
First results from ICARUS
In this first analysis, which used data taken from 2022 to 2023, the collaboration did not observe evidence of muon-neutrino disappearance in ICARUS. But the result represents a first important milestone for the SBN Program. The collaboration demonstrated the ICARUS data’s excellent quality and its suitability for physics analyses, as well as the maturity of the software tools for event selection, fitting and detector simulation.
“These first disappearance results mark a major milestone for ICARUS and the broader Short Baseline Neutrino Program at Fermilab,” said Carlo Rubbia, 1984 Physics Nobel laureate and ICARUS spokesperson. “They demonstrate the exceptional performance and stability of the detector and confirm that we now have the precision analysis tools in place to rigorously explore the sterile‑neutrino hypothesis.”
These results were also vital for developing construction and analysis techniques. The collaboration thoroughly examined uncertainties that arose from the collected data sample itself, which allowed them to properly describe the neutrino flux in the neutrino beam, the neutrino interactions in the liquid argon and the detector performance. They were able to place exclusion limits on the 3+1 sterile-neutrino model with 90% confidence.
Future combined analyses with other SBN detectors will be vital to reduce the uncertainties further. Collaborators say it will be essential to work with SBND to reduce uncertainties and conduct a robust two-detector analysis.
“With ICARUS fully validated and operating we are entering, in concert with SBND, a new era of neutrino physics in which definitive, world‑leading measurements are finally within reach,” said Rubbia.
Preparing for DUNE
ICARUS, as well as SBND and MicroBooNE, is a liquid‑argon time projection chamber detector. Neutrinos are notoriously hard to detect, but when they occasionally interact with argon atoms in a liquid-argon TPC, they cause ionization, creating electrons. A high voltage drifts the electrons to wire planes inside the tank, resulting in a distinctive, precise signal that yields important information about the neutrino interaction and allows for a 3D reconstruction of the particles’ trajectories.
This same technology will be used for the forthcoming Deep Underground Neutrino Experiment, DUNE, led by Fermilab. Currently under construction at the Long Baseline Neutrino Facility at Fermilab in Batavia, Illinois, and in Lead, South Dakota, DUNE will be the world’s most comprehensive neutrino experiment. It will consist of liquid-argon TPC detectors that use technology pioneered by ICARUS — but more than 20 times larger.
ICARUS is supported by the U.S. Department of Energy Office of Science, the Italian National Institute for Nuclear Physics (INFN) and CERN, the European Organization for Nuclear Research.
Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics and accelerator research. Fermi Forward Discovery Group manages Fermilab for the U.S. Department of Energy Office of Science. Visit Fermilab’s website at www.fnal.gov and follow us on social media.
The Underground Construction Association has awarded the Long-Baseline Neutrino Facility/Deep Underground Neutrino Experiment at the Sanford Underground Research Facility located in South Dakota, the prestigious 2026 Project of the Year Award. Hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory, DUNE is a cutting-edge neutrino experiment comprised of three massive caverns located a mile below the surface. The underground space will house massive detectors and an entire laboratory system dedicated to neutrino research.
“The dedicated engineering teams who designed, excavated and constructed the colossal caverns in South Dakota, completed the project successfully and with an impeccable safety record,” said Fermilab Director Norbert Holtkamp. “Congratulations to the design and construction teams who achieved this important milestone. Construction of a project like this has never been done before in the U.S. They have my deep appreciation as we move to the next phase of making the underground laboratory a reality.”
LBNF won the award for a project in the $100M – $500M category. The construction teams of the LBNF/DUNE project included engineers from Arup, Delve Underground, Fermilab, Kiewit-Alberici Joint Venture, SURF and Thyssen Mining, Inc. Together, they pushed the limits of geotechnical engineering with the formation of two massive caverns, each measuring 65-feet wide, 92-feet tall, and 495-feet long (20 meters x 28 meters x 150 meters). The three caverns of the new research facility span an underground area close to the size of eight soccer fields.
“The dedicated engineering teams who designed, excavated and constructed the colossal caverns in South Dakota, completed the project successfully and with an impeccable safety record.”
Fermilab Director Norbert Holtkamp
For Mike Headley, the executive director of the South Dakota Science and Technology Authority and laboratory director at SURF, this award was made possible thanks to more than two decades of visionary leadership, generous philanthropy and dedicated labor.
“I think this UCA award is tremendous in that it recognizes the monumental scale of this project,” Headley said. “Building something like this on the surface would be challenging enough; it’s so impressive that through the cooperation of multiple partners, we have this enormous accomplishment nearly a mile underground,” Headley said of the project’s safety record; SURF logged more than one million hours during construction without a lost-time incident.
“The UCA Project of the Year Awards are presented to a project team or group that demonstrates insight and understanding of underground construction or a significant project, which may include practices, developing concepts, theories or technologies to overcome unusual problems within a project, resulting in little to no outstanding issues.”
Excavation work at the far site began in early 2019 and was completed in February 2024. During that time, the underground spaces were prepared for the DUNE project with the restoration and expansion of historic rock-handling systems that removed over 800,000 tons of rock from approximately 5,000 feet (1,520 meters) below ground. The rock traveled up the renovated mile-deep Ross shaft at SURF, continuing along an above-ground three-quarter-mile-long conveyor to a large former mining area called the Open Cut. LBNF consists of three long cut-out caverns; two of the caverns will house two far detector modules each, placed end-to-end, while the third will house cryogenics equipment and other utilities that keep the powerful detectors running.
Excavation successfully concluded with the stellar safety record of 1,135,105 hours worked without any lost-time injury.
DUNE scientists will study the behavior of mysterious particles known as neutrinos to solve some of the biggest questions about our universe. Why is our universe composed of matter? How does an exploding star create a black hole? Are neutrinos connected to dark matter or other undiscovered particles? The project is the largest neutrino collaboration in history and consists of more than 1,500 scientists and engineers from over 35 countries.
Fermi National Accelerator Laboratory is America’s premier national laboratory for particle physics and accelerator research. Fermi Forward Discovery Group manages Fermilab for the U.S. Department of Energy Office of Science. Visit Fermilab’s website at www.fnal.gov and follow us on social media.
When Steven Gardiner first embarked on his scientific career, his research centered on the practical applications of nuclear physics, such as prototyping neutron detectors for counterterrorism and improving simulations used by nuclear engineers to design reactors. Fundamental research on neutrino physics was far from his mind until he considered PhD programs and heard about the Deep Underground Neutrino Experiment, hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory.
After speaking to his adviser, Bob Svoboda at University of California, Davis, about the exciting science potential of DUNE, Gardiner recalled, “I became a neutrino person and never looked back.” He joined Fermilab in 2018, bringing a unique background in neutron simulations to the study of some of the most elusive particles in the universe.
This career track recently earned Gardiner a Department of Energy Early Career Award, which provides funding to explore the low-energy research potential of DUNE. While the experiment is primarily designed for high-energy beam physics, Gardiner is advocating for its use as an unusual kind of telescope. Rather than collecting rays of light to study the universe, DUNE will be sensitive to ghostly low-energy neutrinos coming from outer space, including those from the Sun, supernovae, black holes and possibly dark matter.
“Even though DUNE is designed for beam physics, you can go way lower in energy, and it still performs,” Gardiner said. He believes his specific background allows him to make a “unique contribution due to my career trajectory,” taking him from neutrons to neutrinos.
The technical heart of this research involves upgrading a computer simulation code called MARLEY, or Model of Argon Reaction Low Energy Yields. Because neutrinos interact so weakly with ordinary matter, they are not directly visible in detectors. Instead, physicists must look for extremely rare collisions between neutrinos and atomic nuclei. Like a subatomic version of a car crash investigation, the tracks left by particles coming out of each collision provide clues about what originally happened. To put these puzzle pieces back together, scientists rely on detailed simulations of the collision physics, which is where MARLEY becomes essential. Results from these simulations will help researchers tell the difference between uninteresting noise, low-energy cosmic neutrinos and potential signals from undiscovered new particles. The ultimate goal of this work is to push the boundaries of physics. “My hope is that as a result of this project, we are thinking about new physics beyond the Standard Model,” Gardiner said.
He also emphasized science opportunities going far beyond the microscopic world. “Neutrinos reveal the inner workings of stars to us, whether it’s the core of our own Sun or the first moments of a supernova explosion,” Gardiner said. By refining the models used to interpret neutrino interactions, he is helping to open a wider view of the distant universe. “DUNE is a new window to the cosmos,” he said. “It gives us a new way of seeing into a hidden world where all kinds of crazy stuff might be going on.”
The DOE funding ensures that low-energy neutrino research may expand our understanding of the fundamental building blocks of the universe through the unparalleled sensitivity of the DUNE detectors.
Fermi National Accelerator Laboratory is America’s national laboratory for particle physics and accelerator research. Fermi Forward Discovery Group manages Fermilab for the U.S. Department of Energy Office of Science. Visit Fermilab’s website at www.fnal.gov and follow us on social media.