Study of $B → D^{**} l \nu$ decays for a $R(D^{ (∗) })$ measurement at Belle

One of the assumptions the standard model of particle physics makes is the lepton flavor universality.
This assumes that the couplings of the gauge bosons do not depend on the flavor of the interacting lepton.
Taking this assumption into account, one can calculate the branching fraction ratios of the semileptonic $ B \rightarrow D^{(**)} \tau \nu $ decay to the $ B \rightarrow D^{(**)} l \nu $ decay, with $ l $ being one of the light leptons $ e $ or $ \mu $.
The combined result shows a $ 3.1\sigma $ deviation from the value expected from the SM.
To check whether the discrepancy reported by the previous $ \mathcal{R}(D^{(**)}) $ measurements is actually there or caused by a systematic error in the measurement, a new  $ \mathcal{R}(D^{(**)}) $ measurement is performed with the Belle dataset.

Two of the main systematic uncertainties, which are reported for the previous measurements, are the limited knowledge about both the shapes and the branching fractions of the $ B \rightarrow D^{**} l \nu $ decays.
To reduce the above mentioned uncertainties for the $ \mathcal{R}(D^{(**)}) $ analysis, this thesis provides studies on the form factor modeling of the four different $ D^{**} $ states.
Additionally, this thesis introduces a setup to measure the different $ B \rightarrow D^{**} l \nu $ branching fractions for each of the four different $D^{**} $ states.
Both the form factor study and the branching fraction measurement are expected to improve the understanding of the decay $ B \rightarrow D^{**} l \nu $ and consequently reduce the systematic uncertainty on the underlying $ \mathcal{R}(D^{(**)}) $ measurement.