Atmospheric neutrinos revisited

December 20, 2023• physics 16,212

A comprehensive analysis of current and upcoming atmospheric neutrino experiments may resolve outstanding puzzles in neutrino physics.

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Neutrinos are fickle. Neutrinos have a certain leptonic flavor (electron, muon or tau) that can change their flavor as they travel through space. In 1998, researchers discovered this phenomenon of neutrino oscillations beyond the Standard Model using neutrinos from natural sources such as the Earth’s atmosphere and the sun. Increasingly precise experiments also involve artificial neutrino sources, such as accelerators and nuclear reactors. These experiments have greatly improved our understanding of neutrino oscillations, but have yet to resolve two important related questions, namely the ordering of neutrino masses and the possible violation of a fundamental symmetry called charge parity by neutrinos (CP)symmetry.New research from Carlos Alberto Argüelles-Delgado of Harvard University and colleagues shows that atmospheric neutrino experiments, once key to discovering neutrino oscillations, could still play a key role in answering these questions. [1].Their analysis shows that by 2030, a comprehensive analysis of four major atmospheric neutrino experiments could resolve the neutrino mass ranking problem and provide CP Violations and other neutrino oscillation parameters.

Atmospheric neutrinos are created when cosmic rays from outer space collide with Earth’s atmosphere, creating a cascade of particles, most of which rapidly decay into neutrinos and other products. Atmospheric neutrinos come in a wide range of energies and come from all directions, so neutrino energies can be measured on many orders of magnitude and can range from 15 kilometers (the altitude at which neutrinos are produced in the atmosphere) to more than 12,000 kilometers (neutrino height) of the propagation “baseline”. diameter of the earth). These characteristics make atmospheric neutrinos unique detectors of neutrino oscillations.

Atmospheric neutrinos are indeed the key to the discovery of neutrino oscillations through the observation of “atmospheric neutrino anomalies”: Japan’s Super-Kamiokande detector observed that muon neutrinos passing through the Earth were smaller than those in muons coming down from above. Fewer neutrinos (see Focus: Nobel Prize – Neutrino Oscillations). Neutrino flavor oscillations are a quantum mechanical phenomenon whose probability depends on the energy of the neutrino and the distance the particle travels. As muon-flavored atmospheric neutrinos travel thousands of miles across the Earth, half of them oscillate into tau neutrinos. Muon neutrinos, on the other hand, travel a much shorter distance through the atmosphere and still retain most of their muon flavor.

The oscillations occur because each neutrino is a linear superposition of three eigenstates with different masses: ${𝜈}_1$, ${𝜈}_2$and ${𝜈}_3$. The absolute values ​​of the mass difference and the superposition coefficient or “mixing angle” are well constrained by various measurements using neutrinos from accelerators, reactors and the sun.However, the command of the masses – whether it is ${𝜈}_3$ heavier or lighter than ${𝜈}_2$ and ${𝜈}_1$– remains an open question (Figure 1).Another unknown parameter in neutrino oscillations is the so-called CP– Destruction phase, describing the difference in the oscillatory behavior of neutrinos and their antiparticles (antineutrinos).Characterizing neutrinos CP The violation could hold the key to solving one of physics’ biggest puzzles – the dominance of matter over antimatter in the universe.

To address these issues, researchers are establishing accelerator-based neutrino experiments with baselines hundreds of kilometers long, such as Japan’s Super-Kamiokande detector, the successor to Super-Kamiokande, and the United States’ Deep Underground Neutrino Experiment ( DUNE). These complex experiments provide high-purity, intense and directed beams of neutrinos (or antineutrinos) that can be monitored at both near and far detectors, providing controlled settings for oscillation measurements. However, atmospheric neutrinos continue to provide free neutrinos in large numbers over a wider range of energies.Through analysis of 24 years of data, Super-Kamiokande recently showed that atmospheric neutrino measurements are sensitive to mass ordering [2]. This sensitivity is attributed to the fact that the oscillation probabilities of neutrinos traveling through the Earth are affected by so-called matter effects, which lead to different behaviors for different mass orderings.

To assess what further information can be gleaned from atmospheric neutrino experiments, Argüelles and colleagues examined Super-Kamiokande and three recent atmospheric neutrino experiments: Hyper-Kamiokande, KM3Net-ORCA, and the Antarctic Neutrino Observatory. Upgrades to IceCube. As mentioned, Hyper-Kamiokande is a long-baseline experiment, but its 258-ton water detector will also be sensitive to atmospheric neutrinos. KM3Net-ORCA will detect neutrinos in 7 megatons of water in the Mediterranean Sea neutrinos interact, while IceCube Upgrade will use a cubic kilometer of ice. All these experiments are based on the discovery of neutrinos – photons emitted by charged particles produced by collisions with atoms – through Cherenkov radiation. The propagation speed is greater than the phase speed of light in the medium.

Arguelles and colleagues pointed to synergies between these experiments, proposing a combined analysis of experiments to constrain oscillation parameters and reduce associated system uncertainties. Each detector will provide a unique contribution to this global analysis. Due to their larger detector volumes and photon detection schemes, IceCube Upgrade and KM3Net-ORCA are particularly sensitive to high-energy atmospheric neutrinos.The expected statistics of its detection should be able to accurately measure mixing angles and mass differences, as demonstrated by recent results from IceCube DeepCore, which achieved comparable accuracy to accelerator experiments [3]. The Super-Kamiokande detector, on the other hand, will be more sensitive to low-energy neutrinos due to its higher density of photon sensors, which should increase sensitivity to low-energy neutrinos. CP violation. Argüelles and colleagues found CP-The violation phase is the parameter that will benefit most from combinatorial analysis. They also concluded that this analysis could determine neutrino mass ordering with 5σ statistical confidence by 2030 (this conclusion assumes that KM3Net-ORCA and IceCube Upgrade start collecting data in 2025 and 2027, respectively).

Results from atmospheric neutrino analyses, such as those presented by Arguelles and colleagues, will provide valuable input to the next generation of accelerator-based experiments, complementing them in terms of energy ranges and oscillation baselines. It is worth noting that other upcoming neutrino experiments (not considered in this work) will have an atmospheric neutrino component. Such experiments include DUNE, whose long-range detector will begin collecting data in 2029.Due to liquid argon time projection chamber technology, DUNE will be more suitable than Hyper-Kamiokande for detecting low-energy atmosphere-neutrino interactions, which will be beneficial for learning CP violation [4]. Together, these developments promise to expand our understanding of neutrino oscillations beyond the current paradigm.

About the author

Linyan Wan is a Wilson Fellow at Fermi National Accelerator Laboratory in Illinois. She received her PhD from Tsinghua University in China in 2019, then went to Boston University as a postdoctoral researcher and joined Fermilab in 2023. Her research interests lie in experimental neutrino physics, currently focusing on atmospheric neutrino oscillations.

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