4. Analysis 14
4.4. Results
As discussed in sec. 4.3.1 not the best performing classifier was chosen. This is because the
»stronger« classifiers would cause an image where the correlation between the classifier output andMbcis stronger as well. One has to balance between a good separation and efficiency or a low correlation between these variables.
5. Conclusion
In this thesis, the Belle II Analysis Framework was used to generate and analyze large amounts of simulated data. I reconstructed the rare B meson decayB+ →K∗(892)+µ+µ− and selec-tively filtered it by applying hard constraints on different variables, choosing specific events based on a ranked feature and the use of a classifying algorithm. The algorithm was chosen to be a boosted decision tree because it performed the best with its default parameters. After I tweaked some of these to be optimized for this task, a good separation between the signal and the background class was achieved.
The reconstruction efficiency and the amounts of signal candidates versus background candi-dates in the signal region ofMbcfor different integrated luminosities were calculated. The inte-grated luminosities were chosen in steps relevant for the Belle II data taking period. Compared to the results of Belle [5] on the same decay, the figure of merit value for the reconstruction was more than doubled. While the amount of signal was improved from 26 to 55 candidates, the amount of background candidates was reduced from 122 to only 15. These numbers dis-play the significant improvement to the reconstruction of this decay which I achieved with the introduction of a machine learning algorithm, the new software framework and the upgraded detector.
In conclusion, a reconstruction was found that allows an estimate for future calculations of RK∗. The latest paper published by the LHCb collaboration [10] used roughly 640 signal candidates from theB0→K∗0µ+µ− decay to calculateRK∗0. It can be reconstructed well since it does not require a reconstruction of neutral particles in the final state. In theB0→K∗0e+e− channel however only 200 candidates could be used which accounts for a large portion of the error onRK∗. In the Belle II experiment it is expected that the electron channel reconstruction performs similarly to the muon channel reconstruction. Therefore, by the time 5 ab−1 of data are collected, Belle II’s measurement ofRK∗+should at least be comparable to LHCb’s in terms of deviation from the Standard Model. 5 ab−1should be reached in 2021, giving a prospect of New Physics in the near future.
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A. Appendix
A.1. Features
In this section every feature that is used for the classifier training is displayed. Each figure contains five differently colored sets. While the mixed and charged set correspond to the back-ground sets described in 3.5.2, the continuum backback-ground is divided into charm and "uds".
Charm contains all background that is produced via e+e− →cc¯while "uds" contains the rest of the continuum background, this being e+e− →qq,¯ q=u,d,s. This separation was made because charm behaves differently than "uds" in some variables. Everything is normed so that the shape differences become apparent.
Hiermit bestätige ich, dass die vorliegende Arbeit von mir selbständig verfasst wurde und ich keine anderen als die angegebenen Hilfsmittel - insbesondere keine im Quellenverzeichnis nicht benannten Internet-Quellen - benutzt habe und die Arbeit von mir vorher nicht einem an-deren Prüfungsverfahren eingereicht wurde. Die eingereichte schriftliche Fassung entspricht der auf dem elektronischen Speichermedium. Ich bin damit einverstanden, dass die Bachelorar-beit veröffentlicht wird.
Jasper Riebesehl 10. November 2017, Hamburg