Xiao Luo, an assistant professor at UC Santa Barbara, is a huge fan of the small neutrino, a frequent but elusive and still-mysterious subatomic particle.

Neutrinos are the most abundant mass-carrying particle in our universe, albeit they interact with other matter only infrequently. They are difficult to identify due to their secretive nature. Neutrinos are also shapeshifters, swinging between three flavors predicted by the Standard Model of particle physics (electron, muon, and tau).

The study of these oscillations can lead to answers to fundamental physics puzzles, such as why our universe contains more matter than antimatter. “Being a neutrino experimentalist is satisfying since they provide numerous opportunities to learn about our cosmos, and whatever we uncover by researching them is likely to lead to new discoveries,” Luo added.

So it’s no wonder that she leaped at the chance to work on the MicroBooNE (Micro Booster Neutrino Experiment), a 170-ton neutrino detector the size of a school bus that has been running at Fermi National Accelerator Laboratory since 2015. What is its function? To provide light on some intriguing data obtained by the detector’s predecessors two decades ago.

“The major purpose of MicroBooNE is to clarify prior experimental oddities that have plagued particle physicists for decades,” said Luo, the UCSB neutrino group’s principal investigator.

Experiments around the turn of the century, first at the Liquid Scintillator Neutrino Detector (LSND) at the US Department of Energy’s Los Alamos National Laboratory, then at Fermilab in Chicago’s Mini Booster Neutrino Experiment (MiniBooNE), revealed more neutrino interactions than calculations predicted, prompting scientists to wonder what was causing this unexpected activity.

The idea of a fourth, “sterile” neutrino arose as a popular candidate for explaining these strange results. This new neutrino would interact entirely through gravity, making it much more elusive than the three known types. Its presence could explain the strange outcomes seen at LSND and MiniBooNE.

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If verified, sterile neutrinos would put the Standard Model, the most widely accepted hypothesis of how the universe operates, to the test.

Photons vs. Electrons

The detection of neutrinos is based on examining the particles that arise when a neutrino collides with an atom inside the detector. The MiniBooNE detector had a flaw: it couldn’t determine the difference between electrons and photons (light particles) close to the neutrino’s interaction point. This ambiguity created a hazy picture of the particles that arose after collisions.

If MiniBooNE is observing more electrons than expected, this suggests that extra electron neutrinos are generating the interactions. That would indicate that something unexpected was occurring in the oscillations that experts had overlooked: sterile neutrinos.

If the excess was caused by photons, the anomaly would be explained by either a mismodeled background process or new physics unrelated to sterile neutrinos.

MicroBooNE’s cutting-edge technology overcomes MiniBooNE’s limitations. It captures particle tracks with the use of sophisticated light sensors and over 8,000 painstakingly attached wires. It’s kept in a 40-foot-long cylindrical container that holds 170 tons of pure liquid argon.

When neutrinos collide with the dense, transparent liquid, they release more particles, which the electronics can record. The 3D images that arise show precise particle pathways and, more importantly, identify electrons from photons.

The MicroBooNE experiment — an international cooperation of over 200 scientists from 36 universities in five countries — recently unveiled preliminary results of their quest for the MiniBooNE anomaly in a conference held at Fermilab.

With higher than 99 percent certainty, our findings rule out electrons as the source of the MiniBooNE excess. “This first set of results shows that we don’t see anomalous excess events in the electron neutrino channel in MicroBooNE data, making the sterile neutrino hypothesis unlikely,” said Luo, who co-led a team of MicroBooNE scientists in establishing the overall strategy for the experiment’s signature analysis before arriving at UCSB.


However, while the results rule out an abundance of electrons, the photon side of the tale remains unsolved. The first photon finding from MicroBooNE(link is external) ruled out a specific photon background model, however there are many other pathways to study, such as the emergence of an electron and positron, or various results that incorporate photons.

Luo’s group is working on a broader and more exotic set of new physics models that are compatible with a photon surplus.

“Right now, I’m leading the effort with MicroBooNE to look for the abnormality in the inclusive photon channel,” she stated. Many fascinating theoretical novel physics scenarios have been offered in this channel, emphasizing the need for a more sophisticated photon search and identification.

Erin Yandel, a UCSB neutrino Ph.D. student, has made substantial progress in building the reconstruction and analysis chain with the goal of enhancing photon selection efficiency and total photon search performance. The better analysis should be able to definitively determine whether or not there is a photon excess, as well as identify the novel physics Luo and her team are looking for.

“On our quest for new physics, such as diverse heavy neutrino models and neutrino gateways to the dark sector, our next photon result will point us in the right path,” Luo added.

“Every time we look at neutrinos, we tend to find something new or surprising,” said Justin Evans, a MicroBooNE co-spokesperson and physicist at the University of Manchester. “The discoveries of MicroBooNE are leading us in a new direction, and our neutrino program will help us solve some of these puzzles.”

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Beyond MicroBooNE

Luo and fellow neutrino physicists have a lot to look forward to. MicroBooNE’s results demonstrate the physics capability of the advanced detector technology. The developments from MicroBooNE will be critical for the next generation of experiments coming online soon, including ICARUS (Imaging Cosmic and Rare Underground Signals) and the Short-Baseline Neutrino Detector at Fermilab.

These are in addition to the Deep Underground Neutrino Experiment (DUNE), a flagship international experiment hosted by Fermilab that already has more than 1,000 researchers from over 30 countries. DUNE will study neutrino oscillations by sending neutrinos 800 miles (1,300 km) through the earth to detectors at the mile-deep Sanford Underground Research Facility.

Luo’s group is focused on designing and testing hardware components for the light collection system — a key component of the detector — and developing simulation studies aimed at optimizing the detector design in order to enhance its physics reach.

This work is being carried out by Dante Totani, a postdoc in the UCSB neutrino group playing a key role in developing cutting-edge technology for DUNE’s photon detector system, as well as undergraduate students conducting simulation studies that provide critical input to the design of the detector.

The enhanced photon detection system will lead to more accurate timing, better particle identification and lower energy thresholds, all needed to broaden the physics capabilities of DUNE. With this, more new exciting physics measurements will be made in the years to come.

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