From: Hannah Pell
|Credit: ProtoDUNE / CERN.|
Why does matter exist in the universe? Can we find evidence of proton decay that supports Einstein’s dream of joining forces? These questions are, among other things, very open to discussions in high-energy physics, and one particle has the potential to answer all of them: the neutrino.
If only we could figure out how much they weigh.
This is the core of the longstanding problem of the neutrino mass hierarchy that is to be solved with the Deep Underground Neutrino Experiment (DUNE). Neutrinos oscillate between three different taste eigenstates (electron, muon and tau) and three mass eigenstates (1, 2 and 3). Every taste state is a quantum superposition of the three mass eigenstates. So if the taste is known, the mass is not (and vice versa). Although physicists can calculate the differences between the mass squares based on experimental results (especially Δ13 and Δ23), the discrete mass values and the order from lightest to heaviest are currently unknown. “Normal hierarchy” is when the neutrino mass eigenstate 2 is lighter than 3 and the “inverted hierarchy” would be the opposite.
DUNE has been in the works for nearly a decade. His proposal was in part a response to the 2013 European Particle Physics Strategy, which prioritized CERN’s long-term neutrino program as one of four scientific goals that require international infrastructure. In 2017, CERN began building a prototype for DUNE (“ProtoDUNE, see figure above”), which in December 2020 yielded the first results from the DUNE collaboration with over 1000 scientists in 32 countries. ProtoDUNE is a liquid argon time projection chamber (abbreviated to “LArTPC”) that improves cross-sectional measurements in neutrino scattering experiments. Recently, researchers at Lawrence Berkeley National Laboratory and the University of California at Berkeley demonstrated a new method for capturing 3D images of particle trajectories in LArTPCs called LArPix, which is used by DUNE experimenters.
How will DUNE work? Managed by the Long Baseline Neutrino Facility at Fermilab, DUNE accelerates neutrino and antineutrino rays through 1,300 miles of underground rock and earth (no tunnel required) to a distant detector at the Sanford Underground Research Facility in South Dakota, which is almost at one trades mile below the surface of the earth. The first particle detector characterizes the beam (neutrinos or antineutrinos) and the second detector measures the degree of vibration of the neutrinos during 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.
Construction of DUNE began in 2017 and the experiment is expected to be completed in 2026. In 2015, the Department of Energy came to the conclusion that the construction of DUNE would have no significant environmental impact on neighboring communities. In addition to DUNE, additional experiments that focus on neutrino masses include NOvA (also at Fermilab) and T2K based in Japan. So far the results are inconclusive. However, Japan is also currently building a neutrino observatory, the Hyper-Kamiokande, which is expected to collect data from 2027.
|Credit: DUNE / Fermilab.|
Will DUNE help physicists finally solve the problem of the neutrino mass hierarchy, among other things in connection with proton decay and the asymmetry between matter and antimatter? Only time will tell, but there is certainly a lot of optimism. “I’m pretty confident to say that by the early 2030s we should have a final measurement of the mass hierarchy from at least one of the experiments,” Zoya Vallari of the DUNE and NOvA experiment recently told Symmetry Magazine.
“This is science and measurements that have never been done before,” said Gina Rameika, newly elected co-spokeswoman for DUNE, in April. “We are building an experiment to uncover the neutrino’s deepest secrets.”