Oskar Tittel ; Christian Kiesling

2021
Max-Planck-Institut für Physik Munich

Abstract: In 2001, the BaBar and Belle experiments have convincingly confirmed the CP violation within the B meson system as predicted by the Standard Model (SM). However, ”tensions” have been observed between particular measurements and the corresponding SM prediction. To investigate these tensions and potentially find physics beyond the SM, the Belle experiment has been upgraded to Belle II. Here, a massive luminosity upgrade has been realized for the KEK B factory (KEKB) collider in Tsukuba, Japan. The new Super KEK B factory (SuperKEKB) accelerator aims for an instantaneous luminosity of L = 6⇥ 1035cm−2 s−1, which is 30 times the record of its predecessor. Together with the improved Belle II detector, this experiment will enable high precision tests on the SMin the near future. As a first look into the potential of the new detector, measurements on B physics are now performed. Interesting channels are B ! K(⇤)⇡, where tree diagrams are suppressed, enhancing the sensitivity to loop diagrams, which are sensitive to New Physics (NP). In anticipation of these more precise results, the current status of the theoretical implications from the B ! K⇡ are evaluated. It is shown, that a fit which only includes the leading order diagrams fails to explain the experimental results. This is called the K⇡-puzzle. With the introduction of the color suppressed tree diagram with a naive color suppression factor compared to the color allowed tree process, the tensions of the K⇡-puzzle can be resolved within the SM and the data can be fit with a quality of !2 d.o.f. = 5.6/4. However, including a constraint from the measurement on the branching ratio (BR) of B ! ⇡+⇡0 creates further tensions. These tensions can be partly resolved by letting the color suppressed tree amplitude float freely, which results in a fit quality of !2 d.o.f. = 7.7/4. B0 ! K0⇡0 has been identified as a key measurement in Belle II to either resolve these tensions or increase their significance. In the B ! K⇤⇡ system, neither the leading amplitudes approximation nor the introduction of a free color suppressed diagram could fit the diagrams with reasonable quality. The best fit only includes the leading orders and achieved a result of !2 d.o.f. = 12.2/3. The large measurement errors hinder a more sophisticated analysis. Thus, more precise measurements are needed to constrain the parameter space for these decays. Here, ACP (B ! K⇤0⇡0) has been identified a key measurement for Belle II. In the last part of the thesis, the current analysis on the decay B+ ! K⇤(892)+[K0S ⇡+]⇡0 is presented. The event reconstruction and optimization cut for the background suppression are discussed. Then, a working 2D fit on the decay in !E and Mbc is presented. The fit successfully models the signal, but still fails to distinguish continuum and B ¯B background. In addition, a working ACP fit is in place. In the time frame of this thesis, the analysis could not be completed. Next steps are a candidate selection and the investigation of systematic uncertainties.

Note: Presented on 15 03 2021
Note: MSc