The neutrino-mass hierarchy is one of the most tantalizing mysteries in particle physics, and the Jiangmen Underground Neutrino Observatory (JUNO) is poised to crack it wide open. But here's where it gets controversial: while the Standard Model predicts neutrinos should be massless, decades of observations suggest they do have mass—albeit tiny. JUNO, a groundbreaking experiment in China, aims to settle a crucial debate: does the third neutrino mass state (ν3) outweigh the second (ν2), or is it the other way around? This seemingly small detail could upend our understanding of the fundamental building blocks of the universe.
Compared to quarks, leptons—the family neutrinos belong to—are like the Wild West of particle physics, with large mixing angles and significant uncertainties. To tame this frontier, scientists are deploying a new generation of detectors, each vastly larger than its predecessor. JUNO, the first of these behemoths, began collecting data in August 2023. Its 20,000-ton liquid-scintillator detector is a marvel of engineering, designed to capture the faintest whispers of neutrino interactions.
