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


The Standard Model of particles is currently the best theory describing the very basic building blocks of the universe. Except for gravity, it describes all fundamental interactions between the elementary particles. In the last decades, it has been improved, probed and many of its predictions have been confirmed. Despite the tremendous success of the Standard Model, there are several unexplained phenomena: the non-zero mass of neutrinos, the excess of matter over antimatter in the universe or the presence of dark matter in the universe. 

Today, in the high-energy experiment era, the focus is not on confirming the Standard Model, but on finding inconsistencies and processes where the theory breaks down.  The Large Hadron Collider at CERN, the most powerful particle accelerator up-to-date, is designed to test the Standard Model parameters and its boundaries. 

There are four large experiments at the Large Hadron Collider, \alice , \atlas, \cms and the \lhcb experiment. \alice main design goal is to investigate the origins of the universe, \atlas and \cms are focused on measuring the Standard Model properties, especially the ones of the Higgs and electroweak bosons, and \lhcb focuses on precise measurements of the predicted Standard Model parameters, searching directly for deviations from the predictions. Possible extensions of the Standard Model, New Physics, can contribute to the quantum loops of the Standard Model. As the particles contributing to the quantum loops are not limited by the available collision energy, the energy scale probed is beyond the energy scale probed by direct searches. This approach requires Standard Model predictions or constraints with similar or better accuracy than experimental measurements.

One of the smoking guns of New Physics contribution to the Standard Model are the decays of the \bquark quark to an \squark quark and a pair of leptons. These decays can be measured through the decays of a \B meson into \Kstar\mumu. Many previous measurements of this decay show tensions with the Standard Model predictions: either the branching fraction measurements~\cite{SM-LHCb_BR, ANA-BR-BPHI-LHCb} or the angular analyses~\cite{ANA-LHCb-angular3, ANA-LHCb-angular4, ANA-LHCb-angular1, ANA-LHCb-angular2, ANA-Belle_P5, ANA-CMS_P5, ANA-ATLAS_P5}. This work represents a significant step towards the angular analysis of the \BuToKstmm decay, where the the \Kstarp decays into \Kp\piz using the \lhcb dataset. The aim is to validate the observed anomalies, adding another jigsaw puzzle piece into the physics beyond the Standard Model picture.


In this thesis, the Standard Model is introduced with  an emphasis on $\decay{B}{\Kstar\mumu}$ decays. Then, the \lhcb experimental setup is explained. In the third section, the \BuToKstmm decay topology and observables are described. Then, the analysis method is briefly explained with an emphasis on the difficulties of this analysis. Section five describes the methods used for the signal candidates selection. 
The angular acceptance resolution and correction to the signal candidates is discussed in section six. The seventh section introduces the angular modeling that is validated in section eight by fitting the simulation sample and the \BuToKstJpsi channel in data. The angular model is further tested by pseudoexperiments as explained in section nine. The expected statistical uncertainty of the fit to the rare \BuToKstKppizmm decay is estimated. Lastly, the tenth section is dedicated to the authors work on the tracking efficiency measurement at the \lhcb experiment. Throughout the thesis, natural units  are assumed.


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