\section*{Conclusions}
\addcontentsline{toc}{section}{\protect\numberline{}Conclusions}


In this thesis, the first study of the rare \BuToKstmmFull is presented. This work is the first attempt to perform the angular analysis with a neutral particle in the final state with the \lhcb dataset. The full dataset of 9\invfb collected by the \lhcb experiment is utilized in this measurement. 


Due to the challenging reconstruction of this decay channel, the selection criteria is carefully chosen and tested in order to maximally suppress the background contributions while preserving high signal efficiency and an even angular acceptance.
On top of applying linear cuts in the selection, a multi-variate analysis is used to suppress the background pollution even more.
On top of applying a set of simple requirements in the selection, a multi-variate analysis is used to suppress the background pollution even more.
In total, 271$\pm$28 signal candidates are selected. 
 

The fit model and the shape of the angular background present in the selected data sample is thoroughly investigated using simulation and data samples with large number of signal candidates. The angular shape of the background is modeled to maximize the fit stability and to avoid introducing biases in the angular parameters. 
The fit model is validated using a fit to the reference resonant \BuToKstJpsi decay. The results of the fit to the \BuToKstJpsi decay agree with previous measurements at other experiments.


A detailed study of the maximum likelihood fit to the rare signal channel is done using pseudoexperiments. Multiple angular foldings are employed to maximize the stability of the fit. However, it is shown that the complicated background structure together with the low statistical power of the current data sample results in large uncertainties. The precision of the angular observables to be measured is estimated in five intervals of the dimuon invariant mass squared. 
Using the \flavio package, a likelihood scan as a function of the real part of the Wilson coefficient Re(\C9 ) is performed, assuming a New Physics scenario with Re(\C9 )=-2, as observed in the previous studies. The best possible sensitivity to the deviation of the Wilson coefficient from the Standard Model value is estimated to be $\approx2.4$ standard deviations. 



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