**Preface to "Symmetry in Particle Physics"**

This Special Issue of *Symmetry* contains seven articles dedicated to solving urgent problems of modern particle physics. In one way or another, they consider the problems associated with different symmetries and their violations in high energy physics.

Two of them are topical reviews and give a general idea of the experimental and theoretical status in the study of exotic quark states (Stanislav Dubnicka et al.) and top quark physics (Roman Lysak). ˇ

The standard bound states predicted by QCD as colorless are composed of two (mesons) and three quarks (baryons). However, modern theoretical concepts do not exclude the possibility of the existence of more complex multiquark states. The authors of the review focused on the theoretical description of the decays of five exotic XYZ states within the framework of the closed covariate quark model.

The heaviest top quark, the last of the six quarks described by the Standard Model and acting in the three-generation scheme, was experimentally discovered relatively recently by the CDF and D0 collaborations (1995). One of the co-discoverers of the top quark is the editor of this Special Issue (Dr. Jaroslav Antos). The author of the review, Roman Lysak, joined CDF after the discovery of the top ˇ quark and was very active in the experimental study of top quark properties. In the review, the main focus is on the current state of the experimental and theoretical studies of charge asymmetry in the formation of top anti-top pairs, since the explanation of such asymmetry may lead beyond the limits of the Standard Model.

A special branch in the physics of elementary particles is the study of the properties of neutrinos, primarily associated with the possibility of their mixing due to their nonzero masses. Many ongoing and planned experiments are aimed at measuring their masses and mixing angles (for example, Juno, Baikal). The authors of two articles on neutrinos (Fedor Simkovic et al. and Dimitrii Naumov et al.) are leading experts in neutrino physics and are members of the aforementioned collaborations on the experimental study of neutrinos. It is also known that the study of the fundamental properties of atmospheric, solar, and cosmic neutrinos can help to find the direction along which the expansion of the Standard Model should go.

To study the physical properties of neutrinos, Fedor Simkovic et al. proposed a scenario in which the Dirac–Majorana mass term in the corresponding Lagrangian is dominated by Dirac masses. By assuming a single small Majorana component of neutrino masses, oscillation probabilities and quantities measured in single and double beta decay experiments and in cosmology have been determined.

The article by Dimitrii Naumov et al. investigates a very important experimental aspect: propagation of high-energy neutrinos in a dense environment with accounting for neutrino masses, mixing, CP violation, refraction, and absorption.

Andrey Abruzov et al. presented results for plain QED, weak, and complete electroweak radiative corrections to various asymmetries in processes of electron–positron annihilation to be measured in future colliders. Asymmetries in electron–positron annihilation processes provide a powerful tool to verify the lepton universality hypothesis at a new level of precision. The results reveal an interplay between the weak and QED contributions to asymmetries, indicating the necessity of always considering these contributions in a combined way.

Vladimir Vechernin and Svetlana Belokurova investigated how fixing the number of quark– gluon strings affects long-range correlations between observables under the assumption of the existence of translational invariance in the rapidity space. Knowledge of this relationship is important when processing experimental data in modern collider experiments, such as those of the Relativistic Heavy Ion Collider (RHIC) or the Large Hadron Collider (LHC).

Oleg Teryaev studied the momentum energy tensor for hadrons described by QCD in the presence of classical gravity and inertia. He found the relation between the general space-time symmetry responsible for interactions of hadrons with gravity and the specific QCD dynamics.

We express our deep gratitude to all authors for their articles, which made up the content of the Special Issue. We are confident that the high professional level of their scientific research and the published results will raise the scientific standard of the *Symmetry* journal.

#### **Michal Hnatiˇc, Jaroslav Antoˇs, Juha Honkonen**

*Editors*
