JUNO physics and detector

The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid scintillator detector in a laboratory at 700-m underground. An excellent energy resolution and a large fiducial volume offer exciting opportunities for addressing many important topics in neutrino and astro-particle physics. With six years of data, the neutrino mass ordering can be determined at a 3–4σ significance and the neutrino oscillation parameters sin²θ₁₂, Δm₂₁ ², and |Δm₃₂ ²| can be measured to a precision of 0.6% or better, by detecting reactor antineutrinos from the Taishan and Yangjiang nuclear power plants. With ten years of data, neutrinos from all past core-collapse supernovae could be observed at a 3σ significance; a lower limit of the proton lifetime, 8.34×10³³ years (90% C.L.), can be set by searching for p→ν̄K⁺; detection of solar neutrinos would shed new light on the solar metallicity problem and examine the vacuum-matter transition region. A typical core-collapse supernova at a distance of 10 kpc would lead to ∼5000 inverse-beta-decay events and ∼2000 (300) all-flavor neutrino–proton (electron) elastic scattering events in JUNO. Geo-neutrinos can be detected with a rate of ∼400 events per year. Construction of the detector is very challenging. In this review, we summarize the final design of the JUNO detector and the key R&D achievements, following the Conceptual Design Report in 2015 (Djurcic et al., 2015). All 20-inch PMTs have been procured and tested. The average photon detection efficiency is 28.9% for the 15,000 MCP PMTs and 28.1% for the 5000 dynode PMTs, higher than the JUNO requirement of 27%. Together with the >20 m attenuation length of the liquid scintillator achieved in a 20-ton pilot purification test and the >96% transparency of the acrylic panel, we expect a yield of 1345 photoelectrons per MeV and an effective relative energy resolution of 3.02%/E(MeV ) in simulations (Abusleme et al., 2021). To maintain the high performance, the underwater electronics is designed to have a loss rate <0.5% in six years. With degassing membranes and a micro-bubble system, the radon concentration in the 35 kton water pool could be lowered to <10 mBq/m³. Acrylic panels of radiopurity <0.5 ppt U/Th for the 35.4-m diameter liquid scintillator vessel are produced with a dedicated production line. The 20 kton liquid scintillator will be purified onsite with Alumina filtration, distillation, water extraction, and gas stripping. Together with other low background handling, singles in the fiducial volume can be controlled to ∼10Hz. The JUNO experiment also features a double calorimeter system with 25,600 3-inch PMTs, a liquid scintillator testing facility OSIRIS, and a near detector TAO.