At NRNU MEPhI, the world’s largest coordinate detector TREK is being created using drift chambers to study cosmic ray muons. The basis of the installation is multiwire drift chambers (DC), previously used in the neutrino detector at the Institute of High Energy Physics (Protvino, Moscow oblast) at the U-70 accelerator. To study the characteristics of DCs before installation in the TREK and the projected MDM detector, a stand was created that makes it possible to determine their efficiency, coordinate and angular accuracy, and zone characteristics. The article presents the design of the stand and the results of testing 394 DCs for the TREK and MDM installations.

A trigger system of the Experimental Complex NEVOD (EC NEVOD) unique scientific facility is described. Detectors and setups that differ in area, physical principles of detection, and background counting rate are included in the EC NEVOD to detect various components of cosmic rays. Each detector is equipped with its own original data-acquisition and trigger system and is able to operate independently. In addition, the detectors are all combined by a single system of data triggering and synchronization. Features of the individual detector systems, their main characteristics and methods of their initialization and integration are presented.

The data of cosmic ray NEVOD-DECOR experiment on the investigation of inclined muon bundles for a long time period (May 2012 - March 2021) are presented. The analysis showed that the observed intensity of muon bundles at primary cosmic ray energies of about 1 EeV and higher can be compatible with the expectation in frame of widely used hadron interaction models only under the assumption of an extremely heavy mass composition. This conclusion is consistent with data of several experiments on investigations of muon content in air showers, but contradicts the available fluorescence data on X_{max} Xmax which favor a light mass composition at these energies. In order to clarify the nature of the "muon puzzle", investigations of the muon bundle energy deposit in the detector material were carried out. For the first time, experimental estimates of the average energy of muons in the bundles of inclined air showers initiated by primary particles with energies from 10 to 1000 PeV have been obtained.

Results of the analysis of multiparticle events in cosmic rays in coordinate-tracking setups on the drift chambers CTUDC and ProtoTREK are presented. The analysis of the ProtoTREK data show that it is possible to reconstruct events with a density of up to 15 particles per square meter. The estimation of the energies of primary cosmic rays responsible for the generation of muon bundles detected in the CTUDC was made.

Results are presented from analyzing multiparticle events in cosmic rays on coordinate-tracking setups on the CTUDC and ProtoTREK drift chambers. Analysis of the ProtoTREK data shows that multiwire drift chambers allow the reconstruction of events with densities of up to 15 particles per square meter. The energies of primary cosmic rays responsible for the generation of muon groups registered by the CTUDC installation are estimated.

NEVOD-DECOR is the unique experiment where systematic studies of cosmic ray muon bundles in a wide range of zenith angles and, accordingly, the energies of primary cosmic rays are carried out. Impressive experimental material (more than 100 thousand events) has been accumulated over a long time period from May 2012 to December 2022. The earlier developed method of local muon density spectra allows us to compare experimental data on muon bundles with the results of the EAS muon component simulations. The analysis showed that the observed intensity of muon bundles at primary cosmic ray energies of about 1 EeV and higher can be compatible with the expectation (in frame of widely used hadronic interaction models) only under the assumption of an extremely heavy mass composition. It is consistent with data of several other experiments on investigations of air shower muon content, but contradicts the available measurements of the depth of the shower maximum in the atmosphere by means of fluorescent technique, which favor a light mass composition at these energies. This probably leads to the need to revise the existing hadronic interaction models.

In the classification of the Ministry of Science and Higher Education in the National Project “Nauka” (2018), the concept of MegaScience is the name of a class of unique scientific installations, and in general terms, these are large expensive international scientific and research complexes for solution of actual scientific problems. It is emphasized that world-class breakthrough research is impossible within the framework of one university or organization, without attracting multi-billion budgets. However, the practice of MEPhI shows that it is possible to construct the unique scientific installations and to achieve results comparable to the largest international projects within a university in cooperation with other Russian and foreign organizations.

The new coordinate-tracking detector ProtoTREK was developed in the Experimental complex NEVOD, MEPhI for the study of near- vertical extensive air showers. The installation consists of two planes of drift chambers, seven in each, and has an effective area of 13 m(2). The registration system of the new detector gets trigger from scintillator counters and uses the TDC that has been specially designed for this detector. It is based on FPGA Altera Cyclone V and has GPS synchronization with the joint triggering system of the experimental complex. A deep learning approach is used to reconstruct multi-muon events registered by ProtoTREK. The paper presents the design of the detector and first experimental results.

The new coordinate-tracking detector TREK based on drift chambers is being developed at National Research Nuclear University MEPhI to study inclined extensive air showers. To reconstruct the events with a high multiplicity from the data of drift chambers, the histogram method, which is designed to search for parallel tracks, is currently used. However, we observe afterpulses in the experimental data obtained using a coordinate-tracking unit based on drift chambers (CTUDC). The afterpulses lead to fake track reconstructions. To solve this problem, a new method is being developed using deep learning. This paper presents the results of the development of this method and its application to simulated data.

Data of the NEVOD-DECOR experiment on investigations of inclined cosmic ray muon bundles for a long time period (May 2012 – March 2021) are presented. Their comparison with the results of calculations based on simulations of EAS muon component allows one to study the behavior of the energy spectrum and mass composition of primary cosmic rays and/or to check the validity of hadron interaction models in a wide energy range from about 10^16 to more than 10^18 eV. The analysis showed that the observed intensity of muon bundles at primary particle energies of about 10^18 eV and higher can be compatible with the expectation only under the assumption of an extremely heavy mass composition of cosmic rays. This conclusion is consistent with data of a number of other experiments investigating the muon component of air showers at ultra-high energies. On the contrary, measurements of the depth of the shower maximum in the atmosphere (Xmax) in the experiments using air fluorescence technique favor a light mass composition of primary cosmic rays at these energies. This contradiction (so-called “muon puzzle”) cannot be resolved without serious changes of the existing hadron interaction models.

The muon puzzle is an excess of muon bundles generated by primary cosmic rays (PCR) at energies above 10 $${}^{17}$$ eV compared to estimations that assume even a heavy composition of PCR. The appearance of such excess of muons can be caused both by cosmophysical (the change in the spectrum and composition of cosmic rays) and nuclear-physical (the changing features of the hadron interaction) reasons. To separate these two possibilities it is necessary to measure energy characteristics of muon bundles and their dependence on energy of primary particles. Today the complex NEVOD-DECOR is the only one for such type experiment conduction. To improve the conditions of this experiment a further development of the experimental complex is planned: construction of new coordinate-tracking detector TREK for increasing of the area and improving of spatial resolution of muon track detection, modernization of the Cherenkov water detector for optimization of its structure and improvement of accuracy of energy deposit of muon bundle measurement, inclusion of the installation NEVOD-EAS in the experiment for independent evaluation of primary particle energy.

The new coordinate-tracking detector TREK based on multiwire drift chambers is being developed in the National Research Nuclear University MEPhI to study the muon component of extensive air showers. Its prototype named the coordinate-tracking unit based on drift chambers (CTUDC) has been designed. Investigation of the multiparticle events registered by the unit has shown all the complexity of reconstruction of such events. The analytical reconstruction methods applied earlier demonstrate their inefficacy in dealing with these events. A new approach based on deep learning methods is being developed to solve this problem. The paper presents the results of application of artificial neural networks to experimental data obtained by the CTUDC.

The TREK large-scale coordinate tracking detector is being created at MEPhI on the basis of drift chambers for detecting the near-horizontal flux of muon bundles generated by ultrahigh-energy primary cosmic rays. The detector’s operation is modeled comprehensively in three stages to interpret experimental data: modeling an EAS in the CORSIKA environment, calculating interaction between EAS particles and the detector and its supporting structures in the Geant4 environment; and modeling of responses from drift chambers and on-chamber electronics in the Garfield ++ environment. Each stage of modeling is described, along with the exchange of data between them and the initial results.

The Experimental Complex (EC) NEVOD includes a number of detectors used to carry out basic research of cosmic rays (CR) and their interactions in the energy range 1011–1019 eV and applied research of the heliosphere, magnetosphere and atmosphere of the Earth by the muonography method which is based on the analysis of spatial-angular variations of the muon flux generated by primary CR particles with energies of 10^9–10^11 eV. The EC NEVOD is being constantly developed. Nowadays, it consists of three basic groups of experimental facilities unique in the world: the major facilities (Cherenkov water detector NEVOD, calibration telescope system — CTS, coordinate-tracking detector DECOR), the peripheral facilities (extensive air shower array NEVOD-EAS, distributed thermal neutron detector systems PRISMA and URAN), the muon hodoscopes for cosmophysical and geophysical investigations (TEMP and URAGAN). All detectors and installations of the first two groups are combined by a multilevel triggering system and the time synchronization system ensuring timestamping of registered multicomponent events. Today, the Experimental Complex NEVOD is the only facility capable of studying such a wide set of fundamental and applied scientific problems in the field of cosmic rays and solar-terrestrial physics.

Обсуждается калибровка и сравнение характеристик оптических модулей нейтринных телескопов в объеме черенковского водного детектора НЕВОД. Приводятся результаты измерения отклика прототипа оптического модуля с фотоумножителем Hamamatsu R877 на одиночные мюоны и события с большим энерговыделением.

The possibility of measuring the characteristics of the response of optical modules of neutrino telescopes to various classes events registered in the volume of the Cherenkov water detector NEVOD is discussed. Results are presented from measuring the response of a prototype optical module with a Hamamatsu R877 photomultiplier to single muons and high-energy events.