The Baikal deep underwater neutrino telescope (or Baikal-GVD – Gigaton Volume Detector) is an international project in the field of astroparticle physics and neutrino astronomy. The construction of the Baikal-GVD neutrino telescope is motivated by its discovery potential in astrophysics, cosmology and particle physics. Its primary goal is the detailed study the diffuse flux of high-energy cosmic neutrinos and the search for their sources. It will also search for dark matter candidates (WIMPs), for neutrinos from the decay of super heavy particles, for magnetic monopoles and other exotic particles. It will also be a platform for environmental studies in Lake Baikal.

The preparatory phase of the project was concluded in 2015 with the deployment of a demonstration cluster comprising 192 OMs. The construction of the first phase of Baikal GVD (GVD-I) was started in 2016 by deployment of the first cluster in its baseline configuration, consisting of 288 OMs. Commissioning of GVD-I (8 clusters, volume 0.4 km3) is envisaged for 2021.

The Baikal-GVD Collaboration includes 9 institutions and organizations from 4 countries. The telescope is one of the three largest neutrino detectors in the world along with IceCube at the South Pole and ANTARES in the Mediterranean sea.

Neutrinos are born in nuclear reactions. Along with particles of light, photons, neutrinos are the most common particles in the Universe. Unlike photons, the Universe is transparent for neutrinos. They are almost not absorbed by matter, nor deflected by magnetic fields as being electrically neutral. A tremendous flow of neutrinos arrive the Earth in straight lines from their sources. It can be turned into a flow of undistorted information about the phenomena, objects, and events where the neutrinos were produced, even if the events took place far away in the most distant cosmic corners. It may improve our understanding of the early stage of the Universe evolution, dark matter and dark energy, processes of chemical elements generation, evolution of stars, inner structure and composition of the Sun and the Earth, and the properties of the neutrino itself.

Neutrino telescopes are infrastructures placed deep in transparent natural media in various geographical areas of the Earth that are aimed at investigating a wide spectrum of scientific problems and primarily the natural neutrino fluxes. The deep underwater detection method provides a basis for experiments to record high-energy astrophysical neutrinos with neutrino telescopes. It was first proposed by M.A. Markov in 1960 and consists in recording Cherenkov radiation from secondary muons and/or high-energy showers produced by the interaction of neutrinos with matter in transparent natural media.

 

The Baikal-GVD will studies the most violent processes in the Universe, which accelerate charged particles to highest energies, far beyond the reach of laboratory experiments on Earth. These processes must be accompanied by the emission of neutrinos. The large detection volume, combined with high angular and energy resolution and moderate background conditions in fresh lake water allows for an efficient study of the diffuse neutrino flux and of neutrinos from individual astrophysical objects, be they steady or transient. Multi-messenger methods will be used to relate our findings with those of classical astronomers and with X-ray or gamma-ray observations. A high-energy diffuse astrophysical neutrino flux has been observed recently by IceCube, using track-like and cascade-like events. GVD-I will have a detection volume for cascades of about 0.4 km3, which is approximately the same as the fiducial volume of IceCube for this detection mode. That guarantees the detection of astrophysical neutrinos during the GVD’s first years of operation. We will scrutinize the IceCube result and study in detail the energy spectrum, the global anisotropy and the neutrino flavor composition of the diffuse neutrino flux. The high angular resolution of GVD for track-like or cascade-like events (~0.25° for muon tracks and ~2° for cascades, respectively) provides a high capability for identifying point-like cosmic-ray accelerators. The closest (with respect to a terrestrial observer) astrophysical objects that are currently assumed to be capable for emitting high-intensity neutrino fluxes are located mainly in the vicinity of the Galactic center and in the Galactic plane. Supernova remnants, pulsars, the neighborhood of the black hole Sgr A* at the Galactic center, binary systems comprising a black hole or a neutron star, and clusters of molecular clouds are the most promising Galactic sources with respect to the detection of their neutrino emission. Extragalactic objects — like Active Galactic Nuclei (AGN), Gamma-Ray Bursts (GRB), starburst galaxies and galaxy clusters — are another class of neutrino sources to be targeted by Baikal-GVD. Baikal-GVD will substantially contribute to multi-messenger astronomy studies. Multi-messenger astronomy is the combination of observations in cosmic rays, neutrinos, photons of all wavelengths and even gravitational waves. It represents a powerful tool to study the physical processes driving the non-thermal Universe. The alert system of GVD will allow for a fast, on-line reconstruction of neutrino events recorded by GVD and – if predefined conditions are satisfied – for the formation of an alert message to the other communities. Combined analyses of cosmic high-energy neutrinos with spatially or temporarily coinciding gamma-rays (or spatially coinciding ultra-high energy cosmic rays) lead to a higher significance of the combined results.

The detector utilizes the deep water of Lake Baikal instrumented with optical modules (OMs) – pressure resistant glass spheres with large photo-multiplier tubes (PMTs). The PMTs record the Cherenkov radiation from secondary particles produced in interactions of high-energy neutrinos inside or near the instrumented volume. From the arrival times of light at the PMTs and from the amount of light, direction and energy of the incoming neutrinos are derived. The Baikal-GVD consists of a network of autonomous subdetectors – so-called clusters – each of them with 288 optical modules. A cluster comprises eight vertical strings attached to the lake floor: seven side strings on a radius of 60 m around a central one. Each string carries 36 OMs, arranged at depths between 735 and 1260 meters (instrumented length: 525 m). The vertical spacing between the OMs along a string is 15 m. The OMs are functionally combined in three sections. A section comprises 12 OMs with data processing and communication electronics and forms a detection unit (DU) of the array. All analogue signals from the PMTs are digitized and processed in the sections and are sent to shore if certain trigger conditions (e.g. a minimum number of fired PMTs) are fulfilled. The clusters are connected to shore (3.5 km distance) via a network of cables for electrical power and high-bandwidth data communication. The recorded events rate for one cluster is about 100 Hz, each cluster generates near 10 Gb of the raw data per day. The shore station provides power, detector control and readout, computing resources and a high-bandwidth internet connection to the data repositories. The overall design allows for a flexible and cost-effective implementation of the research infrastructure, as well as studies of astrophysical neutrinos at early construction phase. By the end of the first stage in 2021 it is planned to deploy 8 clusters with the total volume 0.4 km3. On the second stage, 30 clusters with the total volume 1.5 km3 are expected to be deployed.

The large detection volume, combined with high angular and energy resolutions and moderate background conditions in fresh lake water allows for efficient study of cosmic neutrinos, muons from charged cosmic rays and exotic particles. It is also an attractive platform for environmental studies.

It is located at the south part of lake Baikal, near 4 km distant from the scientific shore station. The place was chosen due to depth of the lake (1366 meters) and its flat bottom, clearity of water, railway availability, and possibility to assemble all the telescope equipment right on the ice in winter period.

The shore station is situated among a wildlife sanctuary on the Baikal shore. It has
all the needed infrastructure for operating the telescope, primary processing the
data, and accommodation the staff. The station is a stop of the Baikal railway and
also available through the lake. Every winter it hosts the expedition during which
some equipment are repaired or updated and new clusters are assembled and
tested.

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