The most basic particles that make up the material world consist of 12 types, including 6 quarks, 3 charged leptons, and 3 neutrinos. Their fundamental properties and interactions are described by the "Standard Model" of particle physics. Since the 1950s, work related to the Standard Model has been awarded the Nobel Prize 17 times. However, the Standard Model still faces two prominent issues: the search for and confirmation of the Higgs boson, and numerous mysteries related to neutrinos.
The phenomenon of neutrino oscillation indirectly proves that neutrinos have tiny masses, which is the only experimental phenomenon that exceeds the Standard Model to date. The laws governing neutrino oscillation can be represented by six parameters: the two mass squared differences Δm212 and Δm322 between neutrinos, three mixing angles θ12, θ13, θ23, and a CP phase angle δCP. The Daya Bay experiment first measured θ13, and the sign of Δm322 (also known as the mass hierarchy) and the CP phase angle δCP are still unknown.
The discovery by the Daya Bay experiment that θ13 is much larger than expected makes it possible to measure the neutrino mass order and CP phase angle in the near future. International efforts are being made to develop research plans to determine the mass order and to consider measuring the CP phase angle.
This experiment, building on the Daya Bay experiment and leveraging our advantageous liquid scintillator detector technology and expertise in reactor neutrino physics, aims to measure the neutrino mass hierarchy. The mass hierarchy is a necessary condition for measuring the CP phase angle and is one of the intrinsic properties of neutrinos, determining their flavor structure and posing a challenge that all particle physics models must address. It directly affects the interactions of neutrinos and antineutrinos with matter, and thus has significant implications for cosmic evolution, the production and propagation of neutrinos in the sun and supernovae, various long baseline neutrino oscillation experiments, and more.
This experiment also aims to precisely measure four of the six oscillation parameters of neutrinos, surpassing the international best level of 1%, making it possible to test the unitarity of the neutrino mixing matrix and discover new physics. It can also study neutrinos from supernovae, the Earth, and sterile neutrinos, making significant contributions not only to understanding the laws of particle physics at the microscopic level but also to cosmology, astrophysics, and even geophysics.
Institute of High Energy Physics proposed the second phase of the reactor neutrino experiment (named as JUNO in 2013) in 2008. In February 2013, JUNO was approved by the Chinese Academy of Sciences and supported through the Strategic Priority Research Programme.
