The neutrino-mass hierarchy is one of the most elusive puzzles in particle physics, and solving it could rewrite our understanding of the universe. While quarks play by well-defined rules, the lepton sector, particularly neutrinos, remains a frontier of mystery, characterized by large mixing angles and significant uncertainties. To unravel this enigma, neutrino physicists are deploying a new generation of detectors, each a monumental leap in scale and precision. Leading this charge is the Jiangmen Underground Neutrino Observatory (JUNO) in Guangdong Province, China, which began its data collection on August 26. This 20-kiloton liquid-scintillator detector aims to answer a fundamental question: is the third neutrino-mass eigenstate (ν3) heavier or lighter than the second (ν2)?
But here's where it gets controversial: while the Standard Model (SM) predicts neutrinos to be massless, decades of observations from solar, atmospheric, reactor, and astrophysical neutrinos suggest they do have mass—albeit tiny and undetectable by current instruments. This discrepancy has sparked debates about the nature of neutrinos and the limitations of our current theories. Could the SM be incomplete, or are we missing something fundamental about neutrino behavior?
JUNO’s chief engineer, Ma Xiaoyan, describes the project as a journey of extraordinary challenges, requiring not just cutting-edge technology but also meticulous planning and teamwork. “Meeting the stringent requirements of purity, stability, and safety demanded the dedication of hundreds of engineers and technicians,” Ma explains. “Their collaboration transformed a bold design into a functioning detector, poised to unlock the secrets of neutrinos.”
