• electron positron collider
• dark photon
• dark mediators
• dark matter
• experimental particle physics
• B-factory

The Standard Model (SM) of particle physics has been proven by many experimental results to be a predictive theory and currently the best known description of the fundamental constituents of nature and their interactions. However, it cannot account for some known phenomena, such as the existence of dark matter, established by many astrophysical and cosmological observations which provide the measurement for its relic abundance. Dark matter (DM) is among the most compelling issues for new physics beyond the SM, but remains a complicated mystery to solve, since almost nothing is known about its origin and its nature. Therefore it deserves to be searched for with all available experimental tools.
The dark matter puzzle can be addressed by assuming a thermal production in the early universe. In most of the theoretical frameworks, to account for the measured relic abundance and avoid DM overproduction, a new mediator that can couple to DM and SM particles is required to enhance the DM annihilation rate. A simple solution to extend the SM and account for this additional mediator is by considering a dark gauge $U_D(1)$ invariance which is associated with a new massive boson that can connect the SM particles to the unknown constituents of a new hidden sector.
The work presented in this thesis concerns the first searches for the invisible decay of a Z’ boson in the process $e^+ e^- \to \mu^+ \mu^- Z^{\prime}, Z^{\prime} \rightarrow$invisible and of a lepton-flavor-violating Z’ in $e^+ e^- \to e^{\pm} \mu^{\mp} Z^{\prime}, Z^{\prime} \rightarrow$invisible. The exploited data set has been collected in 2018 during the first physics run of the Belle II detector, installed at the asymmetric energy electron-positron collider SuperKEKB, at KEK Laboratory (Tsukuba, Japan). We do not find any excess of events and set 90$\%$ credibility level upper limits on the cross sections of these processes. The former is also translated, in the framework of an L$_{\mu}$-L$_{\tau}$ theory, into upper limits on the Z’ coupling constant at the level of $5 \times 10^{-2}\div1$ for $M_{Z^\prime}$ ≤ 6 GeV/$c^2$.
Moreover, this thesis work assesses the capability to provide competitive constraints on the L$_{\mu}$-L$_{\tau}$ model with the upcoming Belle II 2019 data, that could solve both the known tension in the SM regarding the anomalous magnetic moment of the muon and one of the most compelling issue for modern particle physics such as dark matter.