After short review of the Standard Model elementary particles, we present interactions as gauge theories. We take a closer look at the model of electroweak interactions with included Higgs mechanism, which generates masses of weak bosons $W$ and $Z$. We determine mass terms for fermions, where we diagonalise mass matrices with biunitary transformation. After we explain mixing of fermions and GIM mechanism.
When considering New Physics we first define branching fraction, which we use to compare theoretical prediction with experimental value. We describe how can we explore New Physics contributions by using Effective Theory approach. Later we present model containing leptoquarks as one of possible extensions of the Standard Model.
Afterwards we analyse New Physics contributions in rare decays $D^0\rightarrow \text{“invisible”}$ using Effective Theory approach, where we focus on neutrinos in final state. We consider especially short distance contributions, which are highly suppressed in the Standard Model due to the GIM mechanism. In the Standard Model we have only left-handed neutrinos, while in New Physics models also right-handed neutrinos can emerge. We constrain couplings of leptoquark $\bar{S}_1$ with fermions from processes $D^0\rightarrow \bar{\nu}_R\nu_R$ and $D^0-\bar{D}^0$. At the end we calculate branching fraction for $D^0\rightarrow \pi^0\bar{\nu}_R\nu_R$.
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