ИСТИНА

The search for any deviations from the Standard Model (SM) predictions is the most important task of high energy physics at the moment. There are a number of experimental indications of deviations from the SM predictions, such as violation of lepton universality in rare decays of B and Bs mesons (LHCb experiment) or deviation of the magnitude of the anomalous muon magnetic moment from the SM predictions (Muon g-2 experiment at FNAL). However, the existing experimental data don't allow to assert definitely about the discovery of physics outside the SM yet and to point to a certain scenario of "new physics". Therefore, it is necessary to look for new processes where the deviations from the SM can potentially be detected. Collection of statistics on B-meson decays by the LHCb experiment and commissioning of the B-factory Belle II allows us to hope that the registration of B and Bs-decays with partial widths less than 10 ^ {- 9} - 10 ^ { -10} become a matter of the near future. The same can be said about the study of the differential characteristics of multilepton decays, the partial widths of which do not exceed 10 ^ {- 7}. The project of the future Super-tau-charm Factory in Novosibirsk opens up prospects for the experimental study of multilepton D-meson decays. Rare four-lepton decays of B-mesons in SM can be divided into two types. Decays of the first type are fundamentally forbidden at the tree level and occur in higher orders of perturbation theory due to loop diagrams of the "penguin" and/or "box" type, the contribution of which can be described using flavor-violating neutral currents (FCNC). The study of rare decays induced by neutral currents with a change in flavor is particularly promising: in SM, they are due to loop quantum corrections, where the contribution of new virtual particles can be noticeable and measurable. An example of the first type of decay is the processes B_s - >e+e-mu+mu-and B_d - >mu+mu-mu+mu-. In decays of the second type, a large number of electromagnetic and weak processes at the tree level are used to obtain the desired final lepton state. A typical example is the decay of B -- >mu+mu-mu-nu_mu and similar processes involving charged B mesons. The first and second types of processes are being studied at the Large Hadron Collider and are planned to be studied at the Belle II facility. Currently, only upper limits are found for the partial widths of 4-lepton decays B_d and B_s, which exceed theoretical predictions. The situation is different with decays of charged B - mesons. The experimental upper limit of LHCb obtained at 95% confidence level is almost an order of magnitude lower than the theoretical prediction. The discrepancy requires further analysis, both experimental and theoretical. There are currently no theoretical predictions for the rare four-lepton decays of D-mesons, either charged or neutral. Due to the unitarity of the CKM matrix, one can expect that the SM loop contributions to decays of the D -> mu + mu-mu + mu- type will be strongly suppressed, and the contribution from “weak annihilation” will dominate. Therefore, for D-mesons there is no actual division into decays of two types, as for B-mesons. This may turn out to be potentially interesting, including for the search for sources of possible violation of lepton universality. Since, in the framework of the Standard Model, the expected partial widths of neutral four-lepton decays of pretty mesons are at the level of 10 ^ {- 9}, against the background of extremely suppressed "standard physics", these processes may be sensitive to possible deviations from the predictions of the Standard Model. Four-lepton D-meson decays can provide additional information about the sources of these deviations.