## 1 Introduction
On the whole, it can be concluded [29] that this phenomenon is associated with MEPs generated at the early stage of EAS development, that this process is not reproduced by simulations within the framework of QCD- and QGS-based models applied in the physics of high-energy cosmic rays, and that the cross section for the coplanar generation of MEPs is relatively large. Although coplanarity was originally associated with relatively large transverse momenta in the coplanar plane, the evaluation of transverse momenta in coplanar events has not actually been carried out. Theoretical ideas relate the coplanar particle generation with; the angular momentum of a quark-gluon string rotated by interacting hadrons [30], the semihard double inelastic diffraction and some QGS tension inside a diffraction cluster between a semihardly scattered quark and spectator quarks of an interacting hadron [31], leading systems with very high spin [32; 33], QCD jet generation [34], and a temporary transition of three-dimensional space into a two-dimensional one [35; 36; 37]. The first four hypotheses [30; 31; 32; 33] include large transverse momenta as an almost indispensable element that determines a coplanar plane. In this case, transverse momenta directed perpendicular to this plane still have standard values of traditional hadronic interaction models. Besides, the fourth hypothesis [34] cannot really explain the relatively large cross section for the generation of MEPs. The hypothesis [35; 36; 37] suggests a decrease in the dimension of space from three to two dimensions at sufficiently high energies. In this case, the transverse momenta are localized in a plane without significant changes in their mean values. The lateral scale of coplanar events observed in the XREC, which are characterized by a very high lateral resolution (\(\sim\)100 microns), is very small, mainly about 1 cm or less, which corresponds to the selection of events produced by interactions of hadrons with the nuclei of air atoms at altitudes of 1\(-\)2 km above the detector. The ionization calorimeter at an altitude of the Tien Shan station has a much coarser lateral resolution of the order of the width of the ionization chambers used by the calorimeter. Therefore, the effects corresponding to the phenomenon of coplanar generation of particles and detected by ionization calorimeters should have a much wider lateral distribution (on the order of several meters). This will lead to the selection of events created by hadron-nucleus interactions at much higher altitudes above the calorimeter, namely, at altitudes to the order of tens of kilometers. All the above-considered experimental results (the delayed absorption of ionization in the thick hadron calorimeter, the flattening of the depth distribution of hadron interaction in a thick homogeneous XREC, and the coplanarity of the most energetic EAS subcores) are observed in the EAS central cores. They are associated with the most energetic secondary hadrons generated with relatively high values of Feynman variable, \(x_{\rm F}\gtrsim 0.01\)-0.05, in the very-forward region of secondary-hadron generation. These experimental data show that we do not yet understand all the features of hadronic interactions. Unfortunately, these phenomena cannot currently be studied in experiments at the Large Hadron Collider (LHC) due to the specific design of the collider and the detectors of the ALICE, ATLAS, CMS, LHCb Collaborations, which allows one to study only particles in a relatively narrow kinematic region. In terms of particle rapidity \(y\), most of the results accumulated in these experiments have been obtained in the region \(|y|\lesssim 4\), i.e. for particles with relatively low energies \(E<10\) GeV (\(x_{F}\lesssim 0.005\)). In order to explore very-forward kinematic regions in collider experiments, new large detectors are required, located far (\(\sim\)100 m) from the point of collision of beam particles.