A major new neutrino experiment is nearing completion

Physicists are putting the finishing touches on the Jiangmen Underground Neutrino Observatory, or JUNO, which will explore the secret pathways of neutrinos, a family of extremely light subatomic particles with no electrical charge, and their antiparticle partners, antineutrinos.

Scheduled to begin collecting data in the summer of 2025, JUNO aims to determine which of three types of neutrinos is the heaviest. It will be the largest detector of its type in the world.

At the heart of the detector, located 700 meters underground in China, is an acrylic sphere approximately 35 meters wide. It will be filled with 20,000 tons of liquid scintillator, which emits light in response to particles produced when a passing antineutrino interacts with a proton in the liquid. Tens of thousands of photomultiplier tubes will observe the scintillator for antineutrino-induced glow.

Around the sphere, water will fill a cylindrical pit to help filter out subatomic particles that are not antineutrinos but could mimic them. Scientists began filling this pit with water on December 18.

Once operational, the detector will examine antineutrinos released as abundant byproducts from two nuclear power plants, each located about 50 kilometers apart.