Experimental Flavor Physics

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Master Thesis Topics

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Virtual Reality Visualization of Belle II Events

Modern particle physics experiments collect complex data samples. To get a better understanding of the relation between the trajectory of particles in the detector and the recorded data, visualizations can be very useful. This holds even more if the visualization is 3-dimensional and not just a 2D projection. The task would be to port and adjust code for the display of Belle II events to Unreal Engine so that it can be used with head-mounted devices and installations like the CAVE at the LRZ. Very good programming skills, some experience with graphics programming, and the ability to work independently are required.

Search for New Physics in B -> Xs l+ l- Decays

Flavor changing neutral current b -> sll transitions are sensitive to contributions from physics beyond te standard model. The LHCb experiment has seen anomalies in angular distributions and the ratio of electron to muon mode in B -> K(*) l+ l- decays. To better understand the source of these anomalies it is important to complement them with measurements of inclusive B -> Xs l+ l- decays. Such decays can be reconstructed at Belle II exploiting the B meson pair production in e+e- -> Y(4S) -> B Bbar events.

Test of Lepton Universality with B -> D(*) tau nu Decays (topic not available any more)

The decays B -> D(*) tau nu are one of the few cases where a significant deviation from the standard model was observed in the experiment. Therefore it is essential to study these decays in as much detail as possible. The expected huge dataset and the sphisticated analysis tools at Belle II will open new analysis techniques to investigate possible violations of lepton universality.

Generic New Physics Search with Machine Learning

New physics can be established by measuring deviations from standard model predictions. Usually this approach is implemented for selected observables. New physics effects may be overlooked if they show up in observables that were not considered or only in correlations of observables. A detailed and more generic comparison of measurements and theoretical predictions may be achieved by machine learning algorithms. Besides the design of the machine learning model and challenge will be to decide if observed deviations should be attributed to new physics or imperfections in theory calculations or detector simulation. This topic is explorative and requires the combination of knowledge from multiple areas.

Tagging of B Mesons

A key feature of B factories is that they produce clean events of B meson pairs in reactions e+e- -> Y(4S) -> B anti-B. This feature is exploited by many analyses, e.g. for the reconstruction of B meson decays to invisible particles. In these analyses the second B meson is reconstructed in a hadronic or semileptonic decay, called tagging. The aim of this topic is the implementation of the tagging algorithm used by the BaBar experiment in the Belle II software and the comparison with the current algorithm. The BaBar algorithm uses a semi-inclusive approach where a seed meson, e.g. a D*, is reconstructed and then longlived particles are added until a suitable B candidate is formed.

Assignment of Particles to B Mesons with Machine Learning (topic not available any more)

As described in the previous topic, many analyses rely on an efficient reconstruction of B mesons in e+e- -> Y(4S) -> B anti-B events. The essential information that the tagging algorithms yield is which particles belong to the tag side B meson. So it boils down to a classification task that assigns particles to one of the B mesons. Modern machine learning methods should be explored to solve this task. This is a rather open topic and requires developping and trying new ideas.

Semileptonic B Meson Decays to D** Mesons (topic not available any more)

Semileptonic B meson decays are use to measure standard model parameters and to search for new physics. One of the least understood parts are decays to orbitally excited D mesons, called D**. The situation could be improved with the data of the Belle II experiment. The task of this topic would be to understand the composition of D** mesons in
semileptonic B meson decays and to measure the branching fractions.

Belle II Software as a Service

Research at modern particle physics experiments usually requires an analysis of huge datasets. Technologies to address these challenges are being developed. One approach is to move data analysis tasks from local resources to centrally maintained systems, called analysis facilities. Instead of a local installation, software is then used as a service. The
task of this topic is to make the Belle II software available as a service. It requires knowledge of C++ and python programming.

Detection of decays in flight

Charged kaons and pions are usually considered stable particles in high-energy physics experiments, but they can decay within the tracking system of the detector, mainly to a muon and a neutrino. In this case the hits of the original particle and the muon are often assigned to the same track candidate which leads to a bias in the measurement of the particle properties. The task would be to develop an algorithm that can detect the kink in the track that is caused by the slight change of momentum direction from kaon or pion to muon.

Further topics not advertised here may be available. Feel free to contact us.