Determination of the relative slow pion efficiency using 72/fb of Belle II experimental data collected in the years 2019/2020

Abstract

The main goal of this thesis is the investigation of the tracking efficiency of slow pions, that are produced at D∗− → D ̄0πs− decays, at Belle II. This analysis is motivated by the efforts of increasing the precision of the experimental measurement of B0 → D∗−l+νl decays, with l ∈ {e,μ}, since they provide the opportunity of extracting the Cabibbo-Kobayashi-Maskawa matrix element |Vcb |.Due to its profound linkage to numerous elementary particle physics processes, the Cabibbo-Kobayashi-Maskawa matrix is of utmost importance for the quest of gaining a deeper understanding of the Standard Model of particle physics and possibly pushing the boundaries of physics into new, yet to be explored, realms beyond the Standard Model. Therefore, seeking high precision measurements of the matrix elements is necessary.The numerical extraction of |Vcb| from experimental measurements of B0 → D∗−l+νl decays is described in detail elsewhere. Moreover, the dependence of the final |Vcb| result on the uncertainty of slow pions, using the proposed method, is shown in this thesis. Thus, any increase of the slow pion tracking precision will be directly beneficial for the calculation of |Vcb|.Currently, the uncertainty of the slow pion tracking efficiency at Belle II is considered to be about 10%, and therefore significantly higher than at its predecessor Belle with about 1.3 %. Hence, in order to perform competitive |Vcb| calculations based on data collected by the new Belle II experiment, a tremendous reduction of this uncertainty has to be achieved or otherwise, the |Vcb| uncertainty is dominated by the slow pion uncertainty.Over the course of this thesis, four different decay modes of D0 mesons originating from B0 → D∗−π+ → (D ̄ 0πs−) π+ decays have been analyzed to examine the slow pion reconstruction efficiency. This analysis is based on 72 fb−1 real data from the Belle II experiment and 200 fb−1 Monte Carlo simulated data. Using the Belle II analysis software framework basf2, the B0 mesons were recon- structed. Fitting the Monte Carlo data to the experimental data was performed with TFractionFitter. From the fit results, the yield-ratios were calculated, which are used as a benchmark for the reconstruction efficiency. Since no sig- nificant deviations of the yield-ratios from unity were observed, there was no necessity to introduce correction factors for the slow pions. Finally, over the course of this thesis, the uncertainty of the slow pion tracking efficiency could be reduced to 4.06 %.Comparing the outcome of this thesis with recent independent findings, a good compatibility of the results can be seen.