DUNE and the Neutrino Mass Hierarchy Problem


By: Hannah Pell

Image credit: ProtoDUNE / CERN.

Why does matter exist in the universe? Can we find evidence of proton decay, supporting Einstein’s dream of unified forces? These questions, among a host of others, are very much open for debate within high-energy physics, and one particle has the potential to help answer all of them: the neutrino.

If only we could find out how much they weigh.

This is the crux of the longstanding neutrino mass hierarchy problem that the Deep Underground Neutrino Experiment (DUNE) aims to solve. Neutrinos oscillate between three different flavor eigenstates (electron, muon, and tau) and three mass eigenstates (1, 2, and 3). Each flavor state is a quantum superposition of the three mass eigenstates, so if the flavor is known, the mass isn’t (and vice versa). Although physicists can calculate the differences between the squares of masses based on experimental results (specifically Δ13 and Δ23), the discrete mass values are still currently unknown, as well as the order from lightest to heaviest. “Normal hierarchy” is if neutrino mass eigenstate 2 is lighter than 3, and the “inverted hierarchy” would be the opposite.

 Image credit: Hyper-Kamiokande.

DUNE has been in the works for nearly the past decade. Its proposal was partially in response to the 2013 European Strategy for Particle Physics, which prioritized CERN’s long-baseline neutrino program as one of four scientific objectives requiring international infrastructure. Construction began in 2017 on a prototype for DUNE (called “ProtoDUNE, pictured above) at CERN which yielded first results in December 2020 from the DUNE collaboration, made up of over 1000 scientists in 32 countries. ProtoDUNE is a liquid argon time projection chamber (abbreviated “LArTPC”), which improves cross-section measurements in neutrino scattering experiments. Recently, researchers at Lawrence Berkeley National Laboratory and the University of California, Berkeley demonstrated a new method for capturing 3D images of particle trajectories in LArTPCs called LArPix, which will be utilized by DUNE experimentalists.

How will DUNE work? Managed by the Long Baseline Neutrino Facility at Fermilab, DUNE will accelerate beams of neutrinos and antineutrinos through 1300 km of underground rock and earth (no tunnel required) to a far detector based at the Sanford Underground Research Facility in South Dakota, which is almost one mile under the Earth’s surface. The first particle detector will characterize the beam (neutrinos or antineutrinos) and the second detector is positioned to measure how the neutrinos oscillated over their 4 millisecond journey. In addition to the targeted beams of neutrinos, DUNE could also detect incoming neutrinos from cosmological events, such as the explosion of a supernova in a neighboring galaxy.

DUNE construction began in 2017 and the experiment is expected to be ready by 2026. In 2015, the Department of Energy concluded that DUNE construction would have no significant environmental impacts on neighboring communities. Other than DUNE, additional experiments focused on neutrino masses include NOvA (also at Fermilab) and T2K based in Japan. So far, findings remain inconclusive. However, Japan is also currently building a neutrino observatory, the Hyper-Kamiokande, which is expected to start taking data in 2027.

Image credit: DUNE / Fermilab.

Will DUNE help physicists finally solve the neutrino mass hierarchy problem, among others related to proton decay and matter-antimatter asymmetry? Only time will tell, but there is certainly plenty of optimism. “I feel pretty confident in saying that in the early 2030s, we should have a definitive measurement of the mass hierarchy from at least one of the experiments,” Zoya Vallari of both the DUNE and NOvA experiments recently told symmetry magazine.

“This is science and measurements that have never been done,” Gina Rameika, newly elected co-spokesperson for DUNE, said in April. “We’re building an experiment to uncover the deepest mysteries of the neutrino.”

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