Search for the visible ττ decay of a Z′ dark boson in µµττ final state with Belle II

The standard model (SM) of particle physics is currently the best known description of the fundamental constituents of matter and their interactions. It is a highly predictive and very successful theoretical framework, however it cannot be considered as a complete description of nature at a fundamental level as it does not explain some observations. Examples are the difference between the measured and the predicted value of the (g-2) of the muons, the tensions in b→sl+l− decays, and the phenomenology related to existence of the dark matter, specifically in the prediction of the observed relic density. Part of these observations can be explained with the introduction of new interactions mediated by non-SM neutral bosons. The work presented in this thesis concerns the first search for the τ+τ− decay of a Z' dark boson in the process e+e- → μ+μ-Z', Z' → τ+τ− with 54.7 fb-1 collected in 2020 by the Belle II detector, installed at the asymmetric energy e+e- collider SuperKEKB, at KEK Laboratory (Tsukuba, Japan). The measurement is currently performed on the 10% of the dataset, in which no excess compatible with signal events events was found, and a 90% confidence level upper limits on the cross sections of the process was set. It is also translated, in the framework of an Lμ-Lτ theory, into upper limits on the Z’ coupling constant for 3.6 ≤ MZ’ ≤ 10 GeV/c2. Although the analysis does not improve the existing constraints on the Lμ − Lτ model because of the small dataset used, the result is still interesting mainly because it provides a unique measurement that can be interpreted in other dark sector models; it demostrates the feasibility of dark sector analysis involving taus.