Experimental Flavor Physics
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Bachelor Thesis Topics

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Search for so far unmeasured decays at Belle II (topic not available any more)

Semileptonic B meson decays are decays of a B meson into a lepton, a neutrino, and most likely a charmed meson. Although these decays are known for decades there are still open questions concerning these decays.

One of those questions is the so-called "gap-problem". The problem is a discrepancy between the inclusive measurement, where the charmed meson is not reconstructed, and exclusive measurements, where the charmed meson is fully reconstructed. The measured exclusive branching fractions do not sum up to the measured inclusive branching fraction. A possible solution to this problem is the existence of so far unmeasured decay channels of either the B meson or the involved charmed mesons. One candidate for those is the decay of a heavy charmed meson (here called D**) into an eta and either a D or D* meson (D** → D* eta. Those decays have not been measured so far.

We offer different measurements with the Belle II experiment that study possible solutions to the gap-problem as Bachelor thesis. Three different approaches are pursued which are briefly outlined below. Each approach is topic of one bachelor thesis:

  • Direct measurement of the decay B → D* eta l nu: For this study one reconstructs the D* eta l system and identifies the signal by the signature of the not reconstructed neutrino in the decay.
  • Measurement of the decay B → D** pi followed by D** → D* eta: As the B meson is fully reconstructed, stringent selection requirements allow an efficient reduction of background. On one hand a low signal yield is expected, but on the other hand this channel is presumably quite low on background.
  • Direct measurement of D** → D* eta in the fragmentation of e+e- → c anti-c events: The analysis idea is to search for bumps in the invariant mass of the reconstructed D* eta system. This channel should offer high candidate yields but may suffer from a large amount of background.

On-the-fly intelligent background simulation (topic not available any more)

Searches for rare decays require a good understanding of backgrounds. Often huge amounts of costly produced simulated data are used for this purpose. One aspect that makes this very inefficient is that events which are not selected as signal candidates still run through the full simulation chain because the selection quantities are only known afterwards.One approach to improve this at the Belle II experiment involves predicting with a machine learning model which events fulfill the selection criteria before the expensive detector simulation and event reconstruction are performed. However, to train such a model, a large enough labeled dataset has to be produced with the inefficient process beforehand which is especially problematic for cases with low selection efficiencies. Therefore, the possibility to train the model on-the-fly while producing the data with increasing efficiency should be investigated.

Belle II Detector with Augmented Reality

A 3D printed model of the Belle II detector was created to explain to the general public how it works. To further improve the user experience and help understanding the detector concept an augmented reality application for mobile devices should be developed that shows simulated events and provides additional explanations. Web programming skills are required, experience with WebAR would be beneficial.

Program

A structured program is offered for the bachelor theses in the summer semester.