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

The detector has been planned, prototyped, and constructed over nearly a decade by an international collaboration of 250 physicists and engineers from Brazil, Spain, Switzerland, the United Kingdom, and the United States. After a few months of carefully turning on each of the detector subsystems, the moment they had all been waiting for finally arrived.

“It isn’t every day that a detector sees 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 toward this moment and this first data is a very promising start to our search for new physics.”

SBND is the final element that completes Fermilab’s Short-Baseline Neutrino (SBN) Program and will play a critical role in solving a decades-old mystery in particle physics.

The Standard Model is considered the best theory for understanding how the universe works at its most fundamental level. Despite being well-tested, it remains incomplete. Over the past 30 years, multiple experiments have observed anomalies that may hint at the existence of a new type of neutrino.

Neutrinos are abundant but difficult to study because they only interact through gravity and weak nuclear force. They come in three types: muon, electron, and tau. These particles change among these flavors through oscillation.

Scientists have predictions about how many of each type should be present at different distances from a neutrino source. Yet observations from previous experiments disagreed with those predictions.

“That could mean that there's more than the three known neutrino flavors,” explained Fermilab scientist Anne Schukraft. “Unlike the three known kinds of neutrinos, this new type wouldn’t interact through the weak force. The only way we would see them is if measurements don’t add up as expected.”

The SBN Program aims to perform searches for neutrino oscillation and look for evidence pointing to this fourth neutrino. SBND serves as the near detector while ICARUS, which started collecting data in 2021, acts as the far detector. A third detector called MicroBooNE finished recording particle collisions with the same beamline that year.

The SBN Program differs from previous short-baseline measurements because it features both near and far detectors. SBND will measure neutrinos as produced in Fermilab's beam while ICARUS will measure them after potential oscillation.

“Understanding anomalies seen by previous experiments has been a major goal in this field for 25 years,” said Schmitz. “Together SBND and ICARUS will have outstanding ability to test these new neutrinos' existence.”

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

Located close to the beamline, SBND will observe 7,000 interactions per day—more than any other similar detector—allowing researchers to study interactions with unprecedented precision.

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

Beyond studying neutrinos, SBND scientists might observe other phenomena outside of Standard Model predictions due to their proximity to Fermilab's particle beam.

“There could be things outside of Standard Model produced as byproducts that our detector would see,” added Schukraft.

One unanswered question by Standard Model is dark matter; although SBND would only detect lightweight particles potentially providing insight into 'dark sector.'

“So far ‘direct’ dark matter searches haven’t turned anything up,” noted Andrzej Szelc from University Edinburgh & SBND physics co-coordinator.“Theorists devised dark sector models producing lightweight particles within beams which our tests can validate."

These initial findings mark just beginning; collaboration continues operating analyzing millions interactions over next several years.“Seeing these first moments starting long process worked towards," concluded Palamara."This marks new era collaboration.”

—Adapted from an article published by Fermilab.