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HUNT.Abstract Neutrinos are electrically neutral and weakly interact with matter, so their propagation is not affected by magnetic fields and their arrival direction is directed to the source object, making them almost the only messenger for finding the origin of extragalactic cosmic rays, but also making them difficult to be effectively detected. The arrangement of large-scale photosensor arrays under water or ice has been confirmed by several experiments (e.g. IceCube and Baikal-GVD) as an effective way to detect high-energy neutrinos. Based on a series of advanced detection techniques of the Large High Altitude Air Shower Observatory(LHAASO) and its important scientific results of observing a large number of PeV ultra-high energy gamma sources scattered in the galaxy, we propose a high-energy neutrino telescope(HUNT) with an effective volume of about 30 km3 or more underwater, aiming at the sensitive observation of individual neutrino sources above 10 TeV, which is expected to solve the century-old problem of high-energy cosmic ray origin once and for all.

Neutrinos are produced through the interaction between high-energy cosmic rays and the matter and radiation inside or near their originating celestial bodies. Since neutrinos are electrically neutral, they are not deflected by magnetic fields in space like cosmic rays but can directly point back to their source. Moreover, as particles of weak interaction, neutrinos have an extremely small cross-section for interactions with surrounding particles, allowing them to escape from the cores of dense celestial objects and propagate freely over cosmological distances without hindrance. Moreover, neutrinos can escape from regions that are opaque to electromagnetic radiation, making them unique probes near the event horizons of exploding stars and black holes, bringing us information about the most extreme physical processes in the universe. Therefore, neutrinos are key messengers in the search for the origins of high-energy cosmic rays and in the investigation of the extreme astrophysical environments. Not to mention, detection of high-energy neutrinos can help to probe the nature of weak interactions in detail at high energy scales, test the standard model of particle physics and possibility of new physics.

However, detecting them is extremely challenging due to the extremely weak interaction between neutrinos and the media used to detect them. Another issue is the ubiquitous cosmic rays colliding with atomic nuclei in the atmosphere, producing a large number of neutrinos known as the atmospheric neutrino background. On one hand, the signal is very weak, and on the other hand, the background is very strong, which are the two major challenges in neutrino detection.

In 1960, Soviet physicist Moisei Markov and others suggested that deploying three-dimensional detectors underwater or in transparent ice could measure the Cherenkov radiation produced by charged particles from extraterrestrial neutrinos. In December 1993, as the first pioneering experiment, the DUMAND project deployed its first set of detector equipment at a depth of 4,800 meters near Keahole Point on Hawaii Island. The IceCube Neutrino Observatory in Antarctica first detected astrophysical high-energy neutrino radiation above TeV energy levels in 2013. In the study of the source population generating these neutrinos, it was found that IceCube's neutrino observations show no clear correlation with the list of blazars, which contribute up to 30% of the diffuse neutrino observational flux (below 100 TeV). There is also a lack of spatial and temporal correlation between Gamma-Ray Bursts and neutrino observations, contributing up to 1% of the diffuse neutrino observational flux. Active Galactic Nuclei core directions show a 2.6σ excess of neutrino signals relative to the background, contributing 27%-100% of the diffuse neutrino observational flux at 100 TeV. Starburst Galaxies and Galaxy Clusters are also considered candidate sources of neutrinos, but these bodies are generally thought to be transparent to the gamma rays accompanying neutrino production, and extragalactic gamma-ray background observations limit the contribution of these bodies to the diffuse neutrino flux, making it difficult for these bodies to explain all of the IceCube neutrino observations. In terms of point source detection, the IceCube experiment, through twelve years (2008-2020) of data accumulation, found only two sources where neutrino events exceeded with the significance over 3σ: blazar TXS 0506+06 3.5σ, and active galaxy NGC 1068 4.2σ. Tidal Disruption Event AT2019dsg was also found to be associated with a high-energy neutrino event, but with lower significance.

Apart from IceCube, there are several high-energy neutrino telescopes currently in operation or under construction: KM3NeT-ARCA located in the Mediterranean Sea, Baikal-GVD deployed in the Lake Baikal, P-ONE on the Pacific coast of Canada and IceCube-Gen2 in Antarctica. The next-generation experiment, IceCube-Gen2, aims to complete the detector construction by 2034, intending to observe the neutrino sky from TeV to EeV energies, with at least five times the sensitivity to individual sources than IceCube. It will collect at least ten times more neutrino events annually than IceCube and will be able to study their distribution across the sky, energy spectrum, and flavor composition in detail, as well as test new physics on the cosmic baseline.

The currently existing, various international experiments are based on decades of development and are limited by detection technology or financial investment. The effective volume of their detectors remains within 1-8 km3, which may not be enough to discover the high energy neutrino sources, particularly the Galactic neutrino sources.

The first 12 ultra-high-energy gamma-ray sources detected by LHAASO are promising PeVatron candidates. To achieve significant observations of their neutrino emissions, for example, at least one event per year, a neutrino telescope with an effective area exceeding 100 m2 for muon neutrinos at 100 TeV is necessary if the gamma-rays (>100 TeV) from LHAASO J1825-1326 are totally generated through hadronic processes. IceCube's effective area for muon neutrinos is approximately 100 m2 at 100 TeV. This implies that the array volume needs to be expanded by more than ≈103/2=30 times.

From the perspective of multi-messenger astronomy, enhancing the detection capability for neutrinos above 100 TeV can provide more realtime neutrino alerts for multi-messenger observations, especially for time-domain astronomy.

IceCube neutrino telescope provide ≈10 alerts per year in “Gold” channel. These events are at least 50% likely to be induced by astrophysical neutrinos. A neutrino array with the instrumented volume around 30 km3 is expected to provide more Gold alerts which will help us to understand the PeV cosmic ray acceleration process in transient sources, such as Gamma-Ray Bursts, Gravitational Wave Bursts, Core-Collapse Supernovae, and Tidal Disruption Events.

Therefore, the LHAASO team has proposed the High-energy Underwater Neutrino Telescope (HUNT) with the instrumented volume around 30km3 to detect high-energy neutrinos above 10 TeV. The main physical objectives of this project are to significantly and effectively discover Galactic high-energy neutrino sources in a short period (e.g., 5 years), confirm the Galactic origin of PeV cosmic rays, understand the PeV cosmic-ray acceleration mechanism, and solve the century-old mystery of the origin of cosmic rays. This project also aims to perform an all-sky survey of high-energy neutrino sources above 100 TeV, to analyze the populations of extragalactic neutrino sources, and to investigate the cosmic-ray acceleration process within these sources.

Considering the experimental physics objectives and construction costs, the basic layout of HUNT is as follows (see Figure 1). Within a 6-kilometer by 6-kilometer area, 2,304 strings of Optical Modules(OM) are uniformly arranged. For instance, the horizontal distance between each string is ≈130 meters, and each string is equipped with 24 OMs, with a vertical spacing of 36 meters. Under such a layout, the total geometric volume of the detector reaches 30 km3. The ideal sites for the HUNT project are Lake Baikal or the South China Sea, which can geographically complement the IceCube-Gen2 in South Pole. Figure 2 shows the horizontal coverage of neutrino telescopes at the Lake Baikal, the South China Sea, and the South Pole. The horizontal direction is the ideal window of high-energy neutrinos above 100 TeV, where the neutrinos are not affected by the Earth absorption and the atmospheric muons are mostly obscured. The Galactic center is visible for ≈40% of time for HUNT in the Lake Baikal, while almost the entire sky is visible for at least 20% of time for HUNT in the South China Sea.

The layout of the HUNT array
Figure 1, the layout of the HUNT array.

The optical sensitive device used in OM is a 20-inch PMT, which is used to measure the intensity and time information of light signals; each OM integrates an LED calibration source, high and low voltage module power supply, readout electronics system, thermometer and other equipment. All OMs achieve clock synchronization at the level of 1 ns through the White Rabbit clock distribution system, and the readout electronics adopt waveform sampling storage of 500 MHz or 1 GHz. The detector will set up an acoustics positioning system to monitor the position of all strings in realtime with an accuracy of 20 cm. It is planned to use both LED light sources and lasers to calibrate the time difference for all probes with an accuracy better than 0.3 ns. The trigger system will plan to use a global online trigger mode, that is, first uploading the charge and time information of OM to the onshore center's DAQ system for event selection, and then reading the digital waveform information within the relevant OMs. These data are transmitted through optical fibers, and onshore facilities include a power distribution station, data acquisition system, and a computing storage center, etc.

FOV
Figure 2. Horizontal coverage for neutrino telescopes in the Lake Baikal (red region), the South China Sea (blue region) and the South Pole (crosshatched region). The darker color represents greater visibility (i.e., the fraction of visible time). The visibility of the crosshatched region is 100%.


Reference

  1. IceCube Collaboration, “Evidence for High-Energy Extraterrestrial Neutrinos at the IceCube Detector,” Science 342 (2013)
  2. LIGO Scientific, Virgo Collaboration,“Observation of Gravitational Waves from a Binary Black Hole Merger,” Phys. Rev. Lett. 116 no. 6, (2016) 061102
  3. LHAASO Collaboration, “Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources,” Nature 594 no. 7861, (2021) 33–36
  4. M. A. Markov, “On high energy neutrino physics,” in 10th International Conference on High Energy Physics, pp. 578–581. 1960
  5. Dumand Collaboration, “DUMAND-II: Status report,” AIP Conf. Proc. 220 (1991) 285–289
  6. The Baikal Collaboration, “Reconstruction of Atmospheric Neutrinos with the Baikal Neutrino Telescope NT-96,” arXiv e-prints (May, 1997) astro–ph/9705244
  7. NESTOR Collaboration, “NESTOR: A deep-sea neutrino telescope,” Nucl. Instrum. Meth. A 502 (2003) 150–154
  8. ANTARES Collaboration, “ANTARES: the first undersea neutrino telescope,” Nucl. Instrum. Meth. A 656 (2011) 11–38
  9. NEMO Collaboration, “The Neutrino Mediterranean Observatory project,” Nucl. Phys. B Proc. Suppl. 143 (2005) 359–362
  10. AMANDA Collaboration, “The AMANDA neutrino telescope,” Nucl. Phys. B Proc. Suppl. 77 (1999) 474–485
  11. IceCube Collaboration, “Icecube - the next generation neutrino telescope at the south pole,” Nucl. Phys. B Proc. Suppl. 118 (2003) 388–395
  12. IceCube Collaboration, “Analysis of the cumulative neutrino flux from Fermi-LAT blazar populations using 3 years of IceCube data,” EPJ Web Conf. 121 (2016) 05006
  13. M. G. Aartsen, et al., “Search for prompt neutrino emission from gamma-ray bursts with icecube,” The Astrophysical Journal Letters 805 no. 1, (May, 2015) L5
  14. IceCube Collaboration, “Search for neutrino emission from cores of active galactic nuclei,” Phys. Rev. D 106 no. 2, (2022) 022005
  15. K. Bechtol, M. Ahlers, M. Di Mauro, M. Ajello, and J. Vandenbroucke, “Evidence against Star-forming Galaxies as the Dominant Source of Icecube Neutrinos,” 836 no. 1, (Feb., 2017) 47
  16. E. Peretti, P. Blasi, F. Aharonian, G. Morlino, and P. Cristofari, “Contribution of starburst nuclei to the diffuse gamma-ray and neutrino flux,” 493 no. 4, (Apr., 2020) 5880–5891
  17. IceCube Collaboration, “Neutrino emission from the direction of the blazar TXS 0506+056 prior to the IceCube-170922A alert,” Science 361 (2018) 147–151.125
  18. IceCube Collaboration, “Evidence for Neutrino Emission from the Nearby Active Galaxy NGC 1068,” Science 378 (2022) 538–543
  19. R. Stein et al., “A tidal disruption event coincident with a high-energy neutrino,” Nature Astronomy 5 (Feb., 2021) 510–518
  20. KM3Net Collaboration, “Letter of intent for KM3NeT 2.0,” J. Phys. G 43 no. 8, (2016) 084001
  21. Baikal-GVD Collaboration, “Diffuse neutrino flux measurements with the Baikal-GVD neutrino telescope,” 107 no. 4, (Feb., 2023) 042005
  22. M. Agostini,et al., “The Pacific Ocean Neutrino Experiment,” Nature Astronomy 4 (Sept., 2020) 913–915
  23. Z. P. Ye, et al., “A multi-cubic-kilometre neutrino telescope in the western Pacific Ocean,” Nature Astronomy 7 (Dec., 2023) 1497–1505
  24. IceCube-Gen2 Collaboration, “IceCube-Gen2: the window to the extreme Universe,” J. Phys. G 48 no. 6, (2021) 060501
  25. IceCube Collaboration, “All-sky Search for Time-integrated Neutrino Emission from Astrophysical Sources with 7 yr of IceCube Data,” 835 no. 2, (Feb., 2017) 151
  26. IceCube Collaboration, “IceCat-1: The IceCube Event Catalog of Alert Tracks,” 269 no. 1, (Nov., 2023) 25