New era of physics uncovered by Neutrino experiment’s first results

A major new physics experiment has used four complementary analyses to show no signs of a theorised fourth kind of neutrino known as the sterile neutrino. Its existence is considered a possible explanation for anomalies seen in previous physics experiments.

New results from the MicroBooNE experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory deal a blow to a theoretical particle known as the sterile neutrino. For more than two decades, this proposed fourth neutrino has remained a promising explanation for anomalies seen in earlier physics experiments. Finding a new particle would be a major discovery and a radical shift in our understanding of the universe.

However, four complementary analyses released by the international MicroBooNE collaboration and presented during a seminar today all show the same thing: no sign of the sterile neutrino. Instead, the results align with the Standard Model of Particle Physics, scientists’ best theory of how the universe works. The data is consistent with what the Standard Model predicts: three kinds of neutrinos—no more, no less.

“MicroBooNE has made a very comprehensive exploration through multiple types of interactions, and multiple analysis and reconstruction techniques,” said Bonnie Fleming, physics professor at Yale University and co-spokesperson for MicroBooNE. “They all tell us the same thing, and that gives us very high confidence in our results that we are not seeing a hint of a sterile neutrino.”

MicroBooNE is a 170-ton neutrino detector roughly the size of a school bus that has operated since 2015. The international experiment has close to 200 collaborators from 36 institutions in five countries. They used cutting-edge technology to record spectacularly precise 3D images of neutrino events and examine particle interactions in detail—a much-needed probe into the subatomic world.

The University of Manchester is one of the leading institutes on MicroBooNE. Professor Justin Evans, co-spokesperson of the experiment, has been leading this analysis for the past two years. The Manchester group, which also includes Professor Stefan Soldner-Rembold, has been heavily involved in the operation of the MicroBooNE detector and the analysis of the data. Of particular note, the Manchester group is leading a broad programme of searches for physics beyond the Standard Model with MicroBooNE, including searches for new heavy neutral leptons, and new particle sectors that couple to the Standard Model particles through the Higgs boson.

What we have achieved here with MicroBooNE is a transformative step for the field of neutrino physics. The questions of short-baseline anomalies - unexpected appearance or disappearance of activity consistent with electron neutrinos - have been with us for two decades now, and those anomalies can be interpreted as the existence of new types of neutrino.

Professor Justin Evans

MicroBooNE is part of the Manchester group’s world leading liquid-argon neutrino-physics programme, in which we are also leading the construction of the new Short-Baseline Near Detector (SBND) that will sit in the same neutrino beam as MicroBooNE and a third detector called ICARUS, to form an exciting new facility called the Short-Baseline Neutrino programme at Fermilab. And looking further to the future, the Manchester group are playing a leading role in the construction of DUNE, the upcoming international flagship experiment that will enable us to understand the neutrino, and its role in the evolution of the universe, at an even deeper level.

Professor Justin Evans said: “What we have achieved here with MicroBooNE is a transformative step for the field of neutrino physics. The questions of short-baseline anomalies - unexpected appearance or disappearance of activity consistent with electron neutrinos - have been with us for two decades now, and those anomalies can be interpreted as the existence of new types of neutrino.

“Today, MicroBooNE has released results in which, with three independent analyses, we have studied, with exquisite precision, the interactions of neutrinos traveling over short baselines; and we have revealed a clear picture in which we see no excess of electron-neutrino-like interactions. This illustrates the power of the liquid-argon technology, and heralds the start of a new era of precision for neutrino physics, in which we will deepen our understanding of how the neutrino interacts, how it impacted the evolution of the universe, and what it can reveal to us about physics beyond our current Standard Model of how the universe behaves at the most fundamental level."

Neutrinos are one of the fundamental particles in nature. They’re neutral, incredibly tiny, and the most abundant particle with mass in our universe—though they rarely interact with other matter. They’re also particularly intriguing to physicists, with a number of unanswered questions surrounding them. These puzzles include why their masses are so vanishingly small and whether they are responsible for matter's dominance over antimatter in our universe. This makes neutrinos a unique window into exploring how the universe works at the smallest scales.

MicroBooNE’s new results are an exciting turning point in neutrino research. With sterile neutrinos further disfavored as the explanation for anomalies spotted in neutrino data, scientists are investigating other possibilities. These include things as intriguing as light created by other processes during neutrino collisions or as exotic as dark matter, unexplained physics related to the Higgs boson, or other physics beyond the Standard Model.

Neutrinos come in three known types—the electron, muon and tau neutrino—and can switch between these flavors in a particular way as they travel. This phenomenon is called “neutrino oscillation.” Scientists can use their knowledge of oscillations to predict how many neutrinos of any kind they expect to see when measuring them at various distances from their source.

Neutrinos are produced by many sources, including the sun, the atmosphere, nuclear reactors and particle accelerators. Starting around two decades ago, data from two particle beam experiments threw researchers for a loop.

MiniBooNE scientists also saw more particle events than calculations predicted. These strange neutrino beam results were followed by reports of missing electron neutrinos from radioactive sources and reactor neutrino experiments.

Sterile neutrinos emerged as a popular candidate to explain these odd results. While neutrinos are already tricky to detect, the proposed sterile neutrino would be even more elusive, responding only to the force of gravity. But because neutrinos flit between the different types, a sterile neutrino could impact the way neutrinos oscillate, leaving its signature in the data.

But studying the smallest things in nature isn’t straightforward. Scientists never see neutrinos directly; instead, they see the particles that emerge when a neutrino hits an atom inside a detector.

The MicroBooNE detector is built on state-of-the-art techniques and technology. It uses special light sensors and more than 8,000 painstakingly attached wires to capture particle tracks. It’s housed in a 40-foot-long cylindrical container filled with 170 tons of pure liquid argon. Neutrinos bump into the dense, transparent liquid, releasing additional particles that the electronics can record. The resulting pictures show detailed particle paths and, crucially, distinguish electrons from photons.

MicroBooNE’s first three years of data show no excess of electrons—but they also show no excess of photons from a background process that might indicate an error in MiniBooNE’s data.

MicroBooNE ruled out the most likely source of photons as the cause of MiniBooNE’s excess events with 95% confidence and ruled out electrons as the sole source with greater than 99% confidence, and there is more to come.

MicroBooNE still has half of its data to analyse and more ways yet to analyse it. The granularity of the detector enables researchers to look at particular kinds of particle interactions. While the team started with the most likely causes for the MiniBooNE excess, there are additional channels to investigate—such as the appearance of an electron and positron, or different outcomes that include photons.

Neutrinos are surrounded by mysteries. The anomalous data seen by the earlier MiniBooNE and LSND experiments still need an explanation. So too does the very phenomenon of neutrino oscillation and the fact that neutrinos have mass, neither of which is predicted by the Standard Model. There are also tantalising hints that neutrinos could help explain why there is so much matter in the universe, as opposed to a universe full of antimatter or nothing at all.

Liquid argon will also be used in the Deep Underground Neutrino Experiment, a flagship international experiment hosted by Fermilab that already has more than 1,000 researchers from over 30 countries. DUNE will study oscillations by sending neutrinos 800 miles (1,300 km) through the earth to detectors at the mile-deep Sanford Underground Research Facility. The combination of short- and long-distance neutrino experiments will give researchers insights into the workings of these fundamental particles.

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