The Cosmic-Ray Extremely Distributed Observatory (CREDO) is a newly formed, global collaboration dedicated to observing and studying cosmic rays (CR) and cosmic-ray ensembles (CRE): groups of at least two CR with a common primary interaction vertex or the same parent particle. The CREDO program embraces testing known CR and CRE scenarios, and preparing to observe unexpected physics, it is also suitable for multi-messenger and multi-mission applications. Perfectly matched to CREDO capabilities, CRE could be formed both within classical models (e.g., as products of photon–photon interactions), and exotic scenarios (e.g., as results of decay of Super-Heavy Dark Matter particles). Their fronts might be significantly extended in space and time, and they might include cosmic rays of energies spanning the whole cosmic-ray energy spectrum, with a footprint composed of at least two extensive air showers with correlated arrival directions and arrival times. As the CRE are predominantly expected to be spread over large areas and, due to the expected wide energy range of the contributing particles, such a CRE detection might only be feasible when using all available cosmic-ray infrastructure collectively, i.e., as a globally extended network of detectors. Thus, with this review article, the CREDO Collaboration invites the astroparticle physics community to actively join or to contribute to the research dedicated to CRE and, in particular, to pool together cosmic-ray data to support specific CRE detection strategies.

For the data on the observation times and directions of the motion of extensive air showers, which are observed at two stations of the GELATICA network, for the first time we apply the method we have developed previously for identifying pairs of mutually remote extensive air showers, the ancestor particles of which arose, possibly, in a single process. A brief description of the GELATICA network, a review of the properties of used samples of data on shower observations at two stations of the network during the 2019–2021 session, and the result of applying the above method to them are given. Some properties of a single peculiar pair of remote showers are discussed. A side question arose about the cause of the observed temporal asymmetry in the locations of the regions of mutual approach of independent primary cosmic ray particles.

The GELATICA project (GEorgian Large-area Angle and TIme Coincidence Array) consists of several cosmic ray detector arrays located in two cities in Georgia. Designed to detect Extensive Air Showers (EAS) with high-precision timing, these observatories aim to reconstruct the arrival directions of cosmic ray showers. GELATICA is the part of the international CREDO collaboration (Cosmic Ray Extremely Distributed Observatory), with a primary goal of searching for Cosmic Ray Ensembles (CRE)—correlated cosmic ray events potentially originating from a common astrophysical or exotic source. This paper presents an overview of GELATICA's instrumentation and methodology, along with an analysis of multi-year data. We evaluate detector performance, characterize the distribution of recorded signals, and assess the potential for detecting CREs via coincident signals across spatially separated arrays. We report the detection of two pairs of events coinciding in time with 0.5 millisecond accuracy. However, the arrival directions of these coincident events do not suggest a common origin. Nevertheless, further data collection and an increased number of observatories could enhance the likelihood of identifying true CRE candidates.

The distribution of the angle between the arrival directions of the fronts of consecutive Extensive Air Showers(EAS) with a wide range of a total number of charged particles is studied using the experimental data taken by the EAS 4-detector array TEL in Telavi. The station is a part of the GELATICA net in Georgia (GEorgian Large-area Angle and TIme Coincidence Array), which is devoted to the study of possible correlations in the arrival times and directions of separate EAS events over large distances. It is shown that the aforementioned directions really are in the essentially independent and can be used for investigation of simultaneous correlations of the EAS arrival times and directions.

Primary Cosmic Ray Chemical Composition is investigated in energy region close to 10**16 eV. Studies are based on comparisons of gamma-hadron families observed by Pamir and Pamir-Chacaltaya Collaboration, with families generated by means of quasi-scaling model MC0. It is shown, that all characteristics of observed families, including their intensity, are in a very good agreement with simulated event properties at the normal chemical composition and are in disagreement at heavy dominant compositions. Code CORSICA with VENUS and DPM models also contradicts with experimental data of families. One- and multi-dimensional methods of recognition of Fe-like families is worked up and approved. They are based on family characteristics sensitive to atomic number of induced nuclei and are not correlated between each others. It is shown that the fraction of Fe-like families is consistent with the normal chemical composition and strongly contradicts to heavy dominant ones. The success of MC0 model, in description of families properties, is due to large inelasticity coefficient of soft interactions at superhigh energies.

We compared two methods for determining the detector performance and the cosmic ray small-surface array aperture. The comparison was performed using the GELATICA network station at Telavi Iakob Gogebashvili State University (hereafter TEL) as an example. The first method is a standard analytical method. It is based on mean values of variables and averaged distributions. This analytical approach to data analysis was the focus of the research carried out within the GELATICA project. The project is a member of the international CREDO (Cosmic-Ray Extremely Distributed Observatory) Collaboration, whose main goal is the detection and global analysis of cosmic ray ensembles. In contrast to the traditional approach, which focuses on the detection of individual cosmic ray extensive air showers, CREDO aims to connect existing and yet-to-be built cosmic ray arrays into a worldwide network, thus creating a scientific tool for the global analysis of ultra-high energy cosmic rays. The present work was the first to compare the determination of the parameters of the extensive air showers recorded by the TEL array with simulation results using the CORSIKA code. The interpretation of the results of the analytical method for the evaluation of detector registration thresholds is generally found to be inconclusive. In order to obtain definitive results, we propose additional measurements and a new method of array detector performance. We show that the energy spectrum obtained analytically is nearly in agreement with that obtained from simulations. Differences are apparent for the primary particle energy threshold. The difference in the overall counting rate for the TEL array is of the order of 4%.