First neutrinos detected at Fermilab’s short-range detector

Scientists working on the Short Baseline Proximity Detector (SBND) at Fermi National Accelerator Laboratory have identified the detector’s first neutrino interactions.

The SBND collaboration planned, prototyped and built the detector for nearly a decade. After a few months of carefully commissioning each of the detector’s subsystems, the long-awaited moment finally arrived.

“It’s not every day that a detector detects its first neutrinos,” said David Schmitz, co-spokesperson for the SBND collaboration and associate professor of physics at the University of Chicago. “We’ve all spent years working to get to this moment, and these first data are a very promising start to our search for new physics.”

The SBND detector is the final element of Fermilab’s Short-Baseline Neutrino (SBN) program and will play a critical role in solving a decades-old mystery in particle physics. Developing the SBND detector to this point has been an international effort. The detector was built by an international collaboration of 250 physicists and engineers from Brazil, Spain, Switzerland, the United Kingdom, and the United States.

The Standard Model is the best theory of how the universe works at its most fundamental level. It’s the gold standard that particle physicists use to calculate everything from high-intensity particle collisions in particle accelerators to very rare decays. But while it’s a well-tested theory, the Standard Model is incomplete. And over the past 30 years, numerous experiments have observed anomalies that could indicate the existence of a new type of neutrino.

Neutrinos are the second most abundant particle in the universe. Despite their abundance, they are extremely difficult to study because they only interact with gravity and the weak nuclear force, meaning they almost never show up in a detector.

Neutrinos come in three types or flavors: muon, electron, and tau. The strangest thing about these particles is that they vary from one flavor to the next, oscillating from muon to electron to tau.

Scientists have a pretty good idea of ​​how many neutrinos of each type should be present at different distances from a neutrino source. However, observations from some previous neutrino experiments contradict these predictions.

“This could mean that there are other types of neutrinos than the three known ones,” says Anne Schukraft, a scientist at Fermilab. “Unlike the three known types of neutrinos, this new type of neutrino would not interact through the weak force. The only way to see them would be if the measurement of the number of muon, electron, and tau neutrinos did not add up as they should.”

Fermilab’s Short-Bias Neutrino Program will investigate neutrino oscillations and look for evidence that might indicate the presence of this fourth neutrino. SBND is the Short-Bias Neutrino Program’s near detector, while ICARUS, which began collecting data in 2021, is the far detector. A third detector called MicroBooNE finished recording particle collisions with the same neutrino beamline that same year.

Fermilab’s short-baseline neutrino program differs from previous short-baseline measurements with accelerator-made neutrinos because it includes both a near and a far detector. SBND will measure neutrinos as they were produced in the Fermilab beam, and ICARUS will measure them after they have potentially oscillated. So, where previous experiments had to make assumptions about the original composition of the neutrino beam, the SBN program will know definitively.

“Understanding the anomalies observed in previous experiments has been a major goal in this field over the past 25 years,” Schmitz said. “Together, SBND and ICARUS will have the unique ability to test for the existence of these new neutrinos.”

Beyond the hunt for new neutrinos

In addition to the search for a fourth neutrino alongside ICARUS, SBND has its own exciting physics program.

Because of its proximity to the neutrino beam, the SBND detector will detect 7,000 interactions per day, more neutrinos than any other detector of its type. The large data sample will allow researchers to study neutrino interactions with unprecedented precision. The physics of these interactions is an important part of future experiments that will use liquid argon to detect neutrinos, such as the Long-Distance Deep Underground Neutrino Detection Experiment, known as DUNE.

Every time a neutrino collides with the nucleus of an atom, the interaction sends a shower of particles through the detector. Physicists must take into account all the particles produced in this interaction, whether visible or invisible, to deduce the properties of these ghostly neutrinos.

It’s relatively easy to model what happens with simple nuclei, such as helium and hydrogen, but SBND, like many modern neutrino experiments, uses argon to trap neutrinos. The nucleus of an argon atom is made up of 40 nucleons, making interactions with argon more complex and harder to understand.

“We’re going to collect ten times more data on how neutrinos interact with argon than all previous experiments combined,” said Ornella Palamara, a Fermilab scientist and co-spokesperson for SBND. “So the analyses we do will be very important for DUNE as well.”

But neutrinos won’t be the only particles SBND scientists will be keeping a close eye on. With the detector located so close to the particle beam, it’s possible the collaboration could uncover other surprises.

“There could be things outside the Standard Model that have nothing to do with neutrinos but are produced as a byproduct of the beam that the detector would be able to see,” Schukraft said.

One of the biggest questions the Standard Model has no answer to is dark matter. Although SBND is only sensitive to light particles, these theoretical particles could provide a first glimpse of a “dark sector.”

“So far, direct searches for dark matter to detect massive particles have yielded nothing,” said Andrzej Szelc, SBND physics coordinator and professor at the University of Edinburgh. “Theorists have devised a whole series of models of dark sectors of light dark particles that could be produced in a neutrino beam, and SBND will be able to test whether these models are true.”

These neutrino signatures are just the beginning of SBND’s work. The collaboration will continue operating the detector and analyzing the millions of neutrino interactions collected over the coming years.

“The observation of these first neutrinos marks the beginning of a long process that we have been working on for years,” Palamara said. “This moment marks the beginning of a new era for our collaboration.”