Scientists from NSU Proposed a Facility for Recording Widespread Atmospheric Showers Together with the Baikal Neutrino Telescope Baikal-GVD

The creation of a large megascience-class installation — the Baikal-GVD neutrino telescope (Gigaton Volume Detector), is entering its final stage. Currently, the active volume has reached 0.8 cubic kilometers, which is 80% of the planned volume. An important component of this large-scale project could be a ground station for registering extensive atmospheric showers, whose operation must be synchronized with the deep-water detector system. Scientists from the laboratory of new methods for detecting ionizing radiation of the Interdisciplinary Center for Elementary Particle Physics and Astrophysics Faculty of Physics, NSU They proposed a technical solution to this problem by equipping the facility with scintillation detectors developed by them and conducted modeling of the ground station of the Baikal neutrino telescope.

Baikal-GVD — one of the three operating neutrino telescopes in the world and, along with the IceCube telescopes at the South Pole and KM3NeT in the Mediterranean Sea. It is located in the southern part of Lake Baikal, about 3.6 km from the shore. It is a large deep-water observatory on the bottom of Lake Baikal designed to search for and study ultra-high-energy neutrino fluxes from space. It is the largest neutrino telescope in the Northern Hemisphere.

Baikal-GVD is a system of vertical “strings” with optical modules fixed under the ice in winter and submerged on steel cables into the water column. The immersion depth ranges from 750 to 1300 meters. The telescope also includes electronic modules for power supply, data collection, telescope control, and other tasks. Stations are combined into groups connected to the Shore Center. The first cluster was commissioned back in 2016, and since then the project has been continuously expanding. The modules capture weak blue light (Cherenkov radiation) that occurs when neutrinos interact with water. Currently, the working volume of Baikal-GVD is approaching one cubic kilometer.

This facility is being created through the efforts of an international collaboration with the leading role of the Institute for Nuclear Research of the Russian Academy of Sciences (Moscow) and the Joint Institute for Nuclear Research (Dubna).

IceCube — the largest neutrino observatory. It is located at the Amundsen-Scott Antarctic station within the ice sheet. The detector records Cherenkov radiation from secondary particles produced when cosmic neutrinos interact with the ice. Its volume is one cubic kilometer of ice. The installation consists of 86 “strings” with attached optical detectors (photomultiplier tubes, PMTs) — 60 photodetectors on each. The “strings” are spaced 125 meters apart. Above the installation is the IceTop detector, which allows distinguishing single cosmic neutrinos from neutrinos produced in the atmosphere as a result of interactions of high-energy cosmic protons, by recording, in the latter case, the flux of secondary particles at ground level. The IceCube installation was completed in 2010.

Neutrino is a neutral elementary particle with an extremely small mass that hardly interacts with ordinary matter. The particle has no electric charge, so it is not affected by electromagnetic forces. Neutrinos are born in the thermonuclear reactions powering stars, and also arrive from supernova explosions and distant galaxies.

– Astrophysical neutrinos arriving from the depths of space are virtually indistinguishable from neutrinos that are produced in the upper layers of our planet’s atmosphere by interactions of cosmic protons. It should be noted that the latter are much more numerous. Distinguishing neutrinos born in deep space from neutrinos formed in the Earth’s atmosphere is statistically possible — atmospheric neutrinos are detected at low energies, while astrophysical neutrinos are detected in the range of higher energies. Much here depends on the accuracy of calculations made using specially developed simulation programs, which need to be adjusted and verified for each specific experiment. In the IceCube experiment, specialized detectors are installed on the ice surface above the deep detectors located 2.5 km deep in the ice, which register extensive atmospheric showers in which atmospheric neutrinos are born. This detector system greatly helps the IceCube experiment advance in modeling and calibration, as well as in solving specific astrophysical problems.

A similar system is required for the Baikal-GVD project as well, but this is a rather complex task and its solution is associated with overcoming certain difficulties. Installing atmospheric shower detectors directly above the deep detectors in Baikal is quite challenging. The garlands with deep detectors are securely anchored and support themselves, but securing detectors at the water surface is much more difficult. While the ice at the South Pole does not melt year-round, Baikal is freed from ice every spring; moreover, the stable operation of the detectors is impeded by winds, rains, snow melting, or water freezing., — said Evgeny Anatolyevich Kravchenko, head of the laboratory of new methods for detecting ionizing radiation at the Interdisciplinary Center for Elementary Particle Physics and Astrophysics of the Physics Department of NSU.

During a scientific discussion with a well-known Russian astrophysicist, deputy director of the Institute for Nuclear Research of the Russian Academy of Sciences by Grigory Igorevich Rubtsov A proposal was born: to place a setup of detectors registering extensive air showers not directly above the Baikal Neutrino Telescope, but approximately four kilometers away on the shore, and to register extensive air showers on the earth’s surface not vertically but at a significant inclination. This would create the opportunity to register extensive air showers on the surface and obtain synchronous signal data at the Baikal Neutrino Telescope. It was proposed to name the shore station Baikal-Top.

With this problem setup, the project participants faced a number of very serious questions regarding the scale of the coastal installation. When it is placed at such a significant distance from the Baikal neutrino telescope, the so-called solid angle decreases, and the signal absorption in the water increases. It is quite possible that, in order to obtain sufficient statistics for joint event studies, a recording installation of very large size will be required, construction of which will need substantial financial investments. It is also necessary to create a specialized simulation program capable of calculating the effective area of the detectors and estimating the number of events that will be registered with this approach, as well as taking into account a number of other important parameters. This work was undertaken by Evgeny Anatolyevich Kravchenko and a laboratory engineer Daniil Sergeyevich Zhadan.

— To fulfill the task set before us under the contract for conducting research work with the Institute of Nuclear Research of the Russian Academy of Sciences, Daniil Zhadan created a certain code, which we used in the performance of our calculations. There is a widely used, standard package capable of simulating extensive atmospheric showers called Corsika. We had to develop a program using this package, together with Proposal – a package describing the passage of high-energy muons through a large layer of water (muons are heavy unstable elementary particles possessing enormous kinetic energy and capable of passing through dense layers of matter). All this described the so-called joint events. The obtained results were first presented to the scientific community last year. Initial calculations showed that the idea of such a setup is quite feasible and its operation will be effective with a detector registration area from 100 to 600 square meters. The particle registration area in modern astrophysical installations reaches several thousand square meters.Our calculations were made for a setup using scintillation detectors produced by our laboratory for the “TAIGA” experiment. A setup with an area of about 100 square meters, according to preliminary results of our calculations, would allow registering from a hundred joint events per year — detecting particles from a widespread atmospheric shower on the shore, and in the Baikal neutrino telescope in the water thickness — high-energy muons from the same shower. Using a setup with an area of 500-600 square meters, the number of registered joint events would reach tens of thousands annually. The calculations show that this dependence is nonlinear, because as the detector area increases, the particle registration threshold decreases, which directly affects the statistics of registered events. — explained Evgeny Anatolyevich Kravchenko.

The results of these calculations were published in the journal “Experimental theoretical physics” (Towards Baikal-Top: Feasibility Study of an Onshore Detector System for the Joint Registration of EAS with Baikal-GVD) in the current year.

This was followed by work on detailing the conducted modeling with a more detailed description of possible detectors and the energy distribution of atmospheric showers. Preliminary information was presented by Evgeny Anatolyevich Kravchenko at the All-Russian Conference on Cosmic Rays held at the end of June and beginning of July this year, in which he participated with a poster presentation. He emphasized that the obtained and published estimates were very conservative, and it is possible that the number of registered particles, especially for small detector areas, will actually be approximately 10 times higher than in the initial calculations, indicating that this type of experiment can expect a fairly rapid start.

Currently, scientists are busy verifying the results and plan to prepare an updated, more detailed publication.

The installation proposed by NSU scientists is of interest not only for calibrating the Baikal neutrino telescope. It can be used for other research in astrophysics and elementary particle physics, for example, it will help advance the solution to the so-called muon puzzle, which represents a discrepancy between actual data from ground detectors and theoretical calculations, where the number of muons in extensive air showers is 30–60% higher than predicted by existing physical models.

— In many experiments recording extensive air showers at high energies, an excess of particles—so-called muons—is observed, which is not confirmed within the framework of existing theoretical models embedded in the corresponding simulation programs. To study this discrepancy in detail, it is necessary to simultaneously measure the presence of muons of both ultra-high and high energies in extensive air showers. In this area, the setup we proposed is more attractive than the pair of physical installations located at the South Pole and involved in the IceCube experiment., — said Evgeny Anatolyevich Kravchenko.

Baikal-Top will register secondary muons with less background noise because it is designed to capture not vertical, but inclined atmospheric showers, which means the systematic error will be smaller. This will help verify hadronization models used in simulation programs. This, in turn, will provide additional experimental information for theoretical energy research that is not accessible at particle accelerators, including the Large Hadron Collider.

To detect extensive atmospheric showers, scientists at NSU propose using scintillation detectors developed at the Laboratory of New Methods for Detecting Ionizing Radiation of the Interdisciplinary Center for Elementary Particle Physics and Astrophysics of the Physics Faculty of NSU together with colleagues from the Budker Institute of Nuclear Physics SB RAS for the “TAIGA” experiment. It is these detectors that equip the setup for recording extensive atmospheric showers.

These scintillation detectors use components exclusively of domestic production. Inside the hermetic casing are scintillation plates based on polystyrene. When a charged particle hits them, it produces a flash; then the light passes to the wavelength shifter-light guide and is output to the photomultiplier tube (PMT), from where it is read by specialized electronics. The detector design uses a PMT with a small photocathode area—this noticeably reduces the cost of the device, as the price depends on the size of the PMT.

For NSU scientists, the advantages of using these detectors are obvious.

— When creating scintillation detectors, we initially focused exclusively on domestic materials and developments, so the problem of import substitution will not have to be solved, and there will be no problems with the production of detectors. A 100 square meter installation is already effectively operating in the “TAIGA” experiment. During this time, it has undergone testing, and all weak points have been identified and eliminated. The electronics involved in the “TAIGA” experiment are similar to those used in the Baikal-GVD project. They are quite capable of very accurately synchronizing the operation of the Baikal-GVD and Baikal-Top stations in time. It is precisely the synchronization in time that allows the reconstruction of large inclined events of this type. Essentially, there are no technical problems for creating the Baikal-Top installation. We intend to work out various physical tasks in more detail: it is necessary to determine with what accuracy this installation will allow the corresponding calibration of the direction angle based on joint events, and so on. If we talk about studying hadronic models of extensive atmospheric showers, it is necessary to identify which statistical data and critical parameters are important to consider in further work.But this task is quite solvable and, in my view, quite interesting and promising, — explained Evgeny Anatolyevich Kravchenko.