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 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. And, after a few-months-long process of carefully turning on each of the detector subsystems, the moment they’d 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. ”
A long-standing mystery
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 the best theory for how the universe works at its most fundamental level. It is the gold standard that particle physicists use to calculate everything from high-intensity particle collisions in particle accelerators to very rare decays.
But despite being a well-tested theory, the Standard Model is incomplete. And over the past 30 years, multiple experiments have observed anomalies that may hint at the existence of a new type of neutrino.
Neutrinos are the second most abundant particle in the universe. Despite being so abundant, they’re incredibly difficult to study because they only interact through gravity and the weak nuclear force, meaning they hardly ever show up in a detector.
Neutrinos come in three types, or flavors: muon, electron and tau. Perhaps the strangest thing about these particles is that they change among these flavors, oscillating from muon to electron to tau.
Scientists have a pretty good idea of how many of each type of neutrino should be present at different distances from a neutrino source. Yet observations from a few previous neutrino 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 of neutrino wouldn’t interact through the weak force. The only way we would see them is if the measurement of the number of muon, electron and tau neutrinos is not adding up like it should.”
The Short Baseline Neutrino Program at Fermilab will perform searches for neutrino oscillation and look for evidence that could point to this fourth neutrino. SBND is the near detector for the Short Baseline Neutrino Program while ICARUS, which started 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.
The Short Baseline Neutrino Program at Fermilab differs from previous short-baseline measurements with accelerator-made neutrinos because it features both a near detector and far detector. SBND will measure the neutrinos as they were produced in the Fermilab beam and ICARUS will measure the neutrinos after they’ve potentially oscillated. So, where previous experiments had to make assumptions about the original composition of the neutrino beam, the SBN Program will definitively know.
—Prof. Jacqueline Stewart
