JUNO Conceptual Design Report (1508.07166v2)

Abstract: The Jiangmen Underground Neutrino Observatory (JUNO) is proposed to determine the neutrino mass hierarchy using an underground liquid scintillator detector. It is located 53 km away from both Yangjiang and Taishan Nuclear Power Plants in Guangdong, China. The experimental hall, spanning more than 50 meters, is under a granite mountain of over 700 m overburden. Within six years of running, the detection of reactor antineutrinos can resolve the neutrino mass hierarchy at a confidence level of 3-4$\sigma$, and determine neutrino oscillation parameters $\sin^2\theta_{12}$, $\Delta m^2_{21}$, and $|\Delta m^2_{ee}|$ to an accuracy of better than 1%. The JUNO detector can be also used to study terrestrial and extra-terrestrial neutrinos and new physics beyond the Standard Model. The central detector contains 20,000 tons liquid scintillator with an acrylic sphere of 35 m in diameter. $\sim$17,000 508-mm diameter PMTs with high quantum efficiency provide $\sim$75% optical coverage. The current choice of the liquid scintillator is: linear alkyl benzene (LAB) as the solvent, plus PPO as the scintillation fluor and a wavelength-shifter (Bis-MSB). The number of detected photoelectrons per MeV is larger than 1,100 and the energy resolution is expected to be 3% at 1 MeV. The calibration system is designed to deploy multiple sources to cover the entire energy range of reactor antineutrinos, and to achieve a full-volume position coverage inside the detector. The veto system is used for muon detection, muon induced background study and reduction. It consists of a Water Cherenkov detector and a Top Tracker system. The readout system, the detector control system and the offline system insure efficient and stable data acquisition and processing.

Jiangmen Underground Neutrino Observatory (JUNO): A Conceptual Design Report

The Jiangmen Underground Neutrino Observatory (JUNO) has been conceived as a multipurpose neutrino detector aimed at making significant advances in our understanding of neutrino physics. Located in Guangdong, China, JUNO is strategically positioned to detect reactor antineutrinos from the Yangjiang and Taishan Nuclear Power Plants. A key objective of JUNO is the determination of the neutrino mass hierarchy with a statistical significance of 3–4 σ over six years of operation. This essay provides an in-depth analysis of JUNO's design, anticipated performance, implications, and future potential.

Design and Specifications

JUNO's core consists of a 20,000-ton liquid scintillator enclosed by an acrylic sphere with a diameter of 35 meters. This large volume acts as a target for neutrino interactions, making JUNO one of the largest liquid scintillator detectors in the world. The surrounding support structure is composed of a stainless-steel truss, providing structural integrity and serving as a mounting platform for approximately 17,000 photomultiplier tubes (PMTs). Importantly, these PMTs possess a high quantum efficiency, contributing to the detector's exceptional energy resolution of 3% at 1 MeV, allowing for precise measurement of energy signatures from neutrino events.

To mitigate backgrounds, JUNO is situated underground with about 700 meters of overburden rock. This drastically reduces the cosmic-ray muon flux, minimizing noise in the antineutrino signal detection. Additionally, a 20--30,000-ton water Cherenkov veto detector effectively tags remaining cosmic-ray muons, further enhancing the background suppression. This comprehensive shielding, coupled with the precise detector calibration, will allow JUNO to achieve its primary scientific goals.

Scientific Goals and Precision Measurements

Beyond resolving the neutrino mass hierarchy, JUNO is equipped to perform precise measurements of neutrino oscillation parameters such as $\sin^2\theta_{12}$, $\Delta m^2_{21}$, and $|\Delta m^2_{ee}|$ with sub-percent accuracy. The detector's high precision will test the existing neutrino oscillation models and may shed light on new physics scenarios that lie outside the standard three-flavor framework.

JUNO's design and location also position it well for observing neutrinos from a variety of sources. These include neutrinos from supernovae, which can provide insights into the dynamics of such explosive astrophysical events, and geo-neutrinos, which hold clues about Earth's interior and mantle composition. Moreover, JUNO will attempt to detect the diffuse supernova neutrino background and solar neutrinos, contributing to our understanding of both stellar processes and solar dynamics.

Technological Innovations and Challenges

The realization of JUNO involves challenges associated with constructing and operating such a massive and complex detector. Ensuring the longevity and radiopurity of the large acrylic sphere, maintaining the uniformity and optical quality of the scintillator, and achieving high quantum efficiency from the PMTs are critical considerations. Significant R&D efforts have been invested in developing PMTs with superior performance characteristics. These tubes must withstand high pressures at the detector depth and operate reliably over JUNO’s planned multi-decade lifespan.

JUNO’s data acquisition and processing systems are also designed to handle the demands of high-throughput and precise timing necessary for processing the expected 2 GB/s of data flow. Furthermore, JUNO explores future potential upgrades, possibly incorporating options such as exploiting sterile neutrinos and other exotic physics signatures.

Broader Implications and Future Developments

The successful operation of JUNO will have broad implications for both neutrino physics and astrophysics. Understanding the neutrino mass hierarchy is critical for answering fundamental questions related to the absolute mass scale and the nature of neutrinos, whether they are Dirac or Majorana particles. Precision measurements from JUNO could also help refine models of neutrino mixing and aid in the understanding of CP violation in the lepton sector, further impacting the field of cosmology by providing insight into the matter-antimatter asymmetry of the universe.

In conclusion, the JUNO detector is poised to be an essential tool in advancing our comprehension of neutrino properties and their role in the universe. As the construction progresses and technical challenges are overcome, JUNO promises to deliver groundbreaking results, enhancing our collective pursuit of the fundamental tenets of particle physics. Beyond its primary objectives, JUNO opens avenues for probing theories beyond the Standard Model, potentially marking a new era of discovery in neutrino research.