On October 10th, 2023, the Tsung-Dao Lee Institute (TDLI) at Shanghai Jiao Tong University officially released the conceptual design of the TRopIcal DEep-sea Neutrino Telescope (TRIDENT) project [1]. Spearheaded by the TDLI, this endeavor aims to establish China's first deep-sea, multi-cubic-kilometre neutrino telescope in the western Pacific Ocean. By detecting high-energy astrophysical neutrinos ranging from sub-TeV to PeV, the initiative intends to explore the mysteries of the extreme universe. This project will not only bridge the existing gaps in China's research in this area but will also contribute to the global multi-messenger astronomical network. Furthermore, it promises to advance cutting-edge interdisciplinary research across particle physics, astrophysics, geophysics, and marine sciences.

A New Era: Multi-Messenger Astronomy

In 1609, the invention of the telescope by Galileo transformed astronomy. For centuries, astronomical observations primarily depended on capturing photons from cosmic sources, using instruments such as the Hubble and James Webb Space Telescopes. Yet, the universe communicates through various "messengers." In 2013, IceCube detected cosmic high-energy neutrinos [2], and in 2015, LIGO identified the merger of two black holes via gravitational waves [3]. These milestones heralded the era of multi-messenger astronomy, which integrates data from photons, gravitational waves, and neutrinos.

Neutrinos rank among the most abundant subatomic particles in the universe, with hundreds of trillions emanating from the sun and passing through our bodies every second. Electrically neutral and primarily interacting via the weak force, they act as ghost-like travelers through the cosmos. They exist in three forms—electron, muon, and tau—and can oscillate among these types due to quantum effects. Although neutrinos were predicted in 1930 and first detected in 1956, their exploration has since earned four Nobel Prizes. These studies have revolutionized our understanding of fundamental physics. Yet many mysteries, such as their precise mass, remain unsolved.

Neutrinos, known for their ghost-like ability to penetrate matter, can escape from intense cosmic events such as supernova explosions and black hole eruptions. This makes them ideal messengers for studying the universe's most extreme phenomena. Detecting these neutrinos not only helps us understand the mechanisms behind these powerful events but also offers insights into fundamental physics.

In the 1960s, Soviet physicist M. Markov first proposed to "determine the direction of charged particles with the help of Cherenkov radiation", which had insipred physicists to build neutrino telescopes. Today, the IceCube experiment, located 2,500 meters deep in Antarctica, stands as the world's leading neutrino telescope. Completed in 2010, it had detected high-energy neutrinos from space by 2013 and identified a corresponding celestial source in 2017. Other projects, such as KM3NeT in the Mediterranean and Baikal-GVD in Lake Baikal, enhance these endeavors. As neutrino astronomy approaches significant breakthroughs, there's a global push to develop the next-generation telescopes for deeper exploration of the cosmos and a better understanding of fundamental physics.

The TRIDENT Collaboration Published the Conceptual Design of the Detector on "Nature Astronomy"

The Tsung-Dao Lee Institute at Shanghai Jiao Tong University leads the development of the TRIDENT project, aiming to build China's first deep-sea neutrino telescope in the western Pacific Ocean.

Thanks to the rapid development of ocean engineering technology in China in the past decade, building a deep-sea neutrino telescope in the country has become feasible. This represents an opportune moment for China to venture into the emerging field of neutrino astronomy and provide a platform for international collaboration in this nascent area.

In September 2021, the TRIDENT Pathfinder team, led by Shanghai Jiao Tong University, completed its first sea trial. The team was led by the project spokeperson Prof. Donglian Xu, ocean engineer Prof. Xinliang Tian, and comprised professionals from several institutions, including Peking and Tsinghua Universities. Within a development span of just one year, the team designed a detector suitable for 4,000-meter deep-sea environments, thanks to the collaborative efforts of multiple teams. The sea trial's success collects vital data, confirming the feasibility of constructing a neutrino telescope at the chosen site. Their findings were published on "Nature Astronomy" on October 9, 2023, with Donglian Xu as the corresponding author. Co-first authors include postdoctoral researcher Ziping Ye and Ph.D. student Tian Wei from the Tsung-Dao Lee Institute at Jiao Tong University, as well as Ph.D. student Hu Fan from Peking University's Department of Astronomy.

The main scientific findings of the paper are:

1. The selected site offers an excellent deep-sea environment for the construction of a neutrino telescope. The site is situated on a deep-sea plain in the northern part of the west pacific ocean, approximately 3.5 kilometers deep. The seabed is flat, and the current flow is very gentle within a few hundred meters of the sea floor. The deep-sea water's radioactivity measurements at the selected site align with the standard seawater data.  The neutrino telescope utilizes the Earth as a shield, capturing neutrinos that penetrate from the planet's opposite side. Located near the equator, TRIDENT can detect neutrinos from 360 degrees of the entire sky due to the Earth's rotation, complementing IceCube in Antarctica and other Northern Hemisphere neutrino telescopes.

1. The optical properties of the seawater at the selected site meet the requirements for constructing a large-scale telescope array. The Pathfinder team deployed custom-designed, highly sensitive optical modules, enabling the simultaneous use of two independent optical measurement systems: the photomultiplier tube system and the camera system. These systems were used to assess the optical properties of seawater in-situ at a depth of approximately 3420 meters at the chosen site. The findings indicate that the average absorption and scattering lengths are around 27 meters and 63 meters, respectively. For perspective, the attenuation length of typical tap water is usually just 2-3 meters.  Clear seawater can more effectively "record" a neutrino's infomation, such as its type, source direction, and the energy it carries. The successful deployment of the measurement system at the selected site also offers partial validation for the core technologies anticipated for the future TRIDENT telescope. These technologies encompass high-pressure glass vessels, photoelectric sensors, data readout systems, and deep-sea deployment techniques.

1. Based on the above results, the team performed simulation using the "Siyuan-1" scientific computing platform of Shanghai Jiao Tong University, and propose the conceptal design of TRIDENT. The TRIDENT array, anchored to the seabed like seaweeds, consists of 1,200 vertical strings, each 700 meters long and spaced 70-100 meters apart. Each string carries 20 high-resolution digital optical modules. Spanning 4-kilometer in diameter, the array covers 12 square kilometers and monitor around 8 cubic kilometers of seawater for high-energy neutrino interactions. The designed lifespan is 20-year. The team also proposed a hybrid Digital Optical Module (hDOM), which features a pixelized design --- its inner surface is densely covered by small photomultiplier tubes (PMTs), while the gaps between these PMTs are filled with ultra-fast SiPM. This hybrid design will further enhance the photon coverage and angular resolution of the telescope. The TRIDENT is expected to discover neutrinos from the active galaxy NGC 1068 within 1 year of operation, while IceCube takes 10 years to reach 4.2 sigma. For transient neutrino sources such as TXS 0506+056 blazar, TRIDENT will detect its burst with over 10 sigma significance, comparing to IceCube's 3.5 sigma in 2018 [4].

Reference

1. 3.TRIDENT Collaboration, A Multi-cubic-kilometre Neutrino Telescope in the Western Pacific Ocean, Nature Astronomy 10.1038/s41550-023-02087-6 (2023)
2. IceCube Collaboration, Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector, Science 342,1242856(2013)
3. LIGO Scientific and Virgo Collaborations, Observation of Gravitational Waves from a Binary Black Hole Merger,. Phys.Rev.Lett. 116.061102 (2016)
4. IceCube Collaboration, Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert, Science 361,147-151(2018)