An electron neutrino interaction in the NOvA distant detector is seen in this image. Credit: NOvA
Neutrinos are the most enigmatic of all the known elementary particles in existence. Neutrinos, first proposed by Wolfgang Pauli in 1930, were not observed experimentally until 1956 because they seldom interact with other particles. Physicists are still learning more about these mysterious particles.
One of the most fascinating aspects of neutrinos is their capacity to switch between different types of neutrinos as they move through space and matter. Physicists investigate these transitions, known as neutrino oscillations, to understand more about neutrinos’ characteristics and the role they may have had in the universe’s creation.
The NOvA experiment at the Department of Energy’s Fermilab has observed neutrino oscillations, bringing us one step closer to understanding the characteristics and behavior of these identity-changing particles.
The electron neutrino, muon neutrino, and tau neutrino are the three kinds of neutrinos identified so far by physicists. Neutrino oscillations are caused by the fact that each neutrino flavor is made up of a different combination of three neutrino masses. A neutrino’s three masses propagate as waves with various wavelengths, resulting in a combination of flavors that varies continually throughout the neutrino’s journey. This property allows a muon neutrino generated in a particle decay to interact with an electron neutrino afterwards. Antineutrinos, or antiparticles of neutrinos, are made up of the same three neutrino masses as neutrinos but have a different flavor.
Physicists seek to solve unanswered issues regarding neutrinos by detecting neutrino oscillations, such as the ordering of neutrino masses and whether neutrinos break a symmetry known as charge parity. The neutrino mass ordering — which neutrino mass is the lowest, and which is the heaviest? — provides a crucial hint to neutrino mass sources. Charge parity symmetry, or CP, is a natural symmetry in which particles and antiparticles act in the same way. Do neutrinos follow this symmetry or defy it? The finding of neutrinos’ CP violation would be revolutionary, because it might help explain the universe’s imbalance of matter and antimatter.
By correlating the oscillations of neutrinos in a muon neutrino beam with the oscillations of antineutrinos in a muon antineutrino beam, NOvA physicists learn about mass ordering and CP violation.
Asymmetry in neutrino and antineutrino oscillation rates can arise from two sources: matter effects, where the presence of rock, dirt, and other material that the beams encounter affects the oscillation rates of neutrinos and antineutrinos differently depending on the mass ordering; or CP violation, which affects how neutrinos and antineutrinos interact.
The NOvA experiment uses the Fermilab particle accelerator complex to investigate oscillations. For the experiment, the NuMI beamline produces a straight, high-intensity, high-purity beam of muon neutrinos or muon antineutrinos. Oscillations cause muon neutrinos (or muon antineutrinos) to disappear from the beam and electron neutrinos (or electron antineutrinos) to emerge in the beam, which NOvA scientists detect. NOvA employs two detectors placed along the beam’s path to make these measurements. The close detector, which is positioned near the beam source at Fermilab, observes neutrino interactions before they have a chance to oscillate. The distant detector, which is located in northern Minnesota, monitors neutrino interactions after they have gone 810 kilometers around the planet and have enough time to oscillate.
The NOvA collaboration has officially revealed the results of its most recent neutrino oscillations measurement. It was derived from data gathered between February 6, 2014, and March 20, 2020, representing a 50% increase in muon neutrino beam data over NOvA’s prior finding. The revised finding also incorporates various enhancements to the data analysis.
NOvA found no substantial imbalance in the rates of electron neutrino and electron antineutrino appearance based on this new data. The finding disfavors mass ordering and CP violation combinations that result in a significant asymmetry, but is consistent with combinations that result in a tiny asymmetry that is consistent with zero. The team is still collecting data and striving to enhance its neutrino oscillation measurements. So yet, the partnership has only acquired around half of the data collection that was originally intended.
More than 260 scientists and engineers from 49 universities in eight countries make up the NOvA cooperation. NOvA will bring scientists closer to understanding the identity-changing behavior of neutrinos with more data and improved analysis. Antineutrinos and neutrino masses mingle together. In NOvA, matter effects and CP violation may combine to produce a significant asymmetry, or they may cancel out to provide no asymmetry at all.
Fermilab is funded by the US Department of Energy’s Office of Science. The Office of Science is the single largest funder of fundamental research in the physical sciences in the United States, and it is striving to solve some of the world’s most serious problems. Visit science.energy.gov for additional details.