First neutrinos detected at Fermilab short-baseline detector

Tuesday, Sep 10, 2024 • College of Science :

The Short-Baseline Near Detector and ICARUS are the near and far detectors, respectively, in the Short-Baseline Neutrino Program. Graphic courtesy of Fermilab.

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

The SBND collaboration has been planning, prototyping, and constructing the detector for nearly a decade. 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.

The Neutrino Group at the University of Texas at Arlington, under the leadership of physics professors Andrew Brandt and Jaehoon Yu, physics associate professor Jonathan Assadi, and physics assistant professor Raquel Castillo Fernández, is playing a critical role in advancing research within the SBND experiment. Also contributing to UTA’s efforts on the project are postdoctoral researcher Leo Aliaga Soplín, graduate students Shweta Yadav and Manuel Dall’Olio, and former UTA postdoc Gabriela Vitti Stenico.

Their efforts focus on the search for sterile neutrinos and exploring other Beyond the Standard Model physics, including potential candidates for dark matter. Additionally, the UTA group leads the effort in the SBND trigger system, which is essential for identifying neutrino events.

“This is an exciting milestone. We have worked for years to get to this point and now we can focus on moving the project forward as we search for new physics,” Castillo Fernández said. “The existence of sterile neutrinos has captivated the scientific community for decades, and with this incredible detector, we are on the verge of breakthroughs that could redefine our understanding of the Universe. The excitement and potential ahead are truly inspiring.”

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. Getting SBND 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 for how the universe works at its most fundamental level. It is the gold standard 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,” Fermilab scientist Anne Schukraft said. “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.

“Understanding the anomalies seen by previous experiments has been a major goal in the field for the last 25 years,” said David Schmitz, co-spokesperson for the SBND collaboration and associate professor of physics at the University of Chicago. “Together SBND and ICARUS will have outstanding ability to test the existence of these new neutrinos.”

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

Because it is located so close to the neutrino beam, SBND will see 7,000 interactions per day, more neutrinos than any other detector of its kind. The large data sample will allow researchers to study neutrino interactions with unprecedented precision. The physics of these interactions is an important element of future experiments that will use liquid argon to detect neutrinos, such as the long-baseline Deep Underground Neutrino Experiment, known as DUNE.

But neutrinos won’t be the only particles SBND scientists will keep an eye out for. With the detector located so close to the particle beam, it’s possible that the collaboration could see other surprises.

“There could be things, outside of 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.

The Short-Baseline Near Detector international collaboration is hosted by the U.S. Department of Energy’s Fermi National Accelerator Laboratory. The collaboration consists of 38 partner institutions, including national labs and universities from five countries. SBND is one of two particle detectors in the Short-Baseline Neutrino Program that provides information on a beam of neutrinos created by Fermilab's particle accelerators.

Fermi National Accelerator Laboratory contributed to this story.

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