JUNO Physics and Detector (2104.02565v2)

Abstract: The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton LS detector 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 6 years of data, the neutrino mass ordering can be determined at 3-4 sigma and three oscillation parameters can be measured to a precision of 0.6% or better by detecting reactor antineutrinos. With 10 years of data, DSNB could be observed at 3-sigma; a lower limit of the proton lifetime of 8.34e33 years (90% C.L.) can be set by searching for p->nu_bar K^+; detection of solar neutrinos would shed new light on the solar metallicity problem and examine the vacuum-matter transition region. A core-collapse supernova at 10 kpc would lead to ~5000 IBD and ~2000 (300) all-flavor neutrino-proton (electron) scattering events. Geo-neutrinos can be detected with a rate of ~400 events/year. We also summarize the final design of the JUNO detector and the key R&D achievements. All 20-inch PMTs have been tested. The average photon detection efficiency is 28.9% for the 15,000 MCP PMTs and 28.1% for the 5,000 dynode PMTs, higher than the JUNO requirement of 27%. Together with the >20 m attenuation length of LS, we expect a yield of 1345 p.e. per MeV and an effective energy resolution of 3.02%/\sqrt{E (MeV)}$ in simulations. The underwater electronics is designed to have a loss rate <0.5% in 6 years. With degassing membranes and a micro-bubble system, the radon concentration in the 35-kton water pool could be lowered to <10 mBq/m^3. Acrylic panels of radiopurity <0.5 ppt U/Th are produced. The 20-kton LS will be purified onsite. Singles in the fiducial volume can be controlled to ~10 Hz. The JUNO experiment also features a double calorimeter system with 25,600 3-inch PMTs, a LS testing facility OSIRIS, and a near detector TAO.

• The paper demonstrates JUNO's capacity to determine neutrino mass ordering with 3–4σ significance by analyzing reactor antineutrino signals over six years.
• It outlines a precision measurement of neutrino oscillation parameters, including sin²θ₁₂ and Δm² values, with errors as low as 0.6%.
• The study details innovative detector features, such as a 35.4m acrylic sphere and an extensive PMT array, enhancing sensitivity and background rejection.

An Overview of the JUNO Physics and Detector Paper

The Jiangmen Underground Neutrino Observatory (JUNO) is set to become a critical facility for advancing our understanding of neutrino physics and associated astroparticle phenomena. This 20-kton liquid scintillator detector, located 700 meters underground, offers significant opportunities for a detailed exploration of neutrino properties, including neutrino mass ordering, oscillation parameters, and other pivotal physics questions.

The primary focus of JUNO is the determination of the neutrino mass ordering (NMO). By capturing antineutrinos emitted from the nearby Taishan and Yangjiang nuclear power stations, JUNO aims to establish the NMO with a 3-4$\sigma$ significance over six years. The detector will enable precision measurement of neutrino oscillation parameters: $\sin^2\theta_{12}$, $\Delta m^2_{21}$, and $|\Delta m^2_{32}|$, projected to reach precision levels of 0.6% or better. These efforts are bolstered by the detector's impressive features, including a large fiducial volume and exceptional energy resolution.

The theoretical impetus for JUNO's design is grounded in the standard three-flavor neutrino oscillation model and its associated open questions, such as the CP-violating phase and the precise nature of $\theta_{23}$. To this end, the experiment is uniquely positioned to employ reactor antineutrinos for distinguishing between the normal ordering (NO) and inverted ordering (IO) scenarios. Unlike accelerator or atmospheric experiments reliant on matter effects, JUNO utilizes the vacuum oscillations allowing an analysis independent of CP-phase uncertainties.

Beyond its primary goal, JUNO will contribute to other significant physics areas, such as supernova neutrino studies and geoneutrino detection. The observatory is expected to register approximately 5,000 inverse beta decay events from a typical galactic supernova, providing crucial data on supernova mechanisms. Further, JUNO's potential extends to detecting geo-neutrinos, crucial for understanding Earth's radiogenic heat supply.

The construction of JUNO itself presents numerous technical challenges detailed in the paper. The detector's core comprises a 35.4-meter-diameter acrylic sphere, accommodating 20 ktons of liquid scintillator, observed by an array of 17,612 20-inch PMTs and 25,600 3-inch PMTs. The surrounding water pool, functioning as a Cherenkov detector, adds a crucial muon veto layer. An elaborate purification and calibration system ensures that JUNO's sensitivity is optimized post-construction.

Complementary to the JUNO detector, the Taishan Antineutrino Observatory (TAO), a ton-level liquid scintillator experiment nearby, aims to provide precise reactor spectrum data, reinforcing JUNO's analyses by offering a high-fidelity spectral reference.

In essence, the JUNO experiment represents a vital advancement in neutrino physics exploration. By leveraging high-precision measurement capabilities, JUNO seeks to address unresolved inquiries within the sector, contributing valuable insights to the standard model and laying down theoretical prospects for neutrino mass hierarchy resolution. Data from JUNO will likely inform and complement findings from accelerator-based and global neutrino observatories, fostering a collaborative scientific approach to understanding neutrino phenomena.