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.

Measurement of the branching fraction of B0 -> D(*)- K+ (topic not available any more)

The theoretical calculation of decays like B0 -> D(*)- K+ requires a good understanding of hadronic effects. Recent calculations have achieved a high precision and show astonishing deviations from the measured values. In this project, a measurement of the branchig fractions of B0 -> D- K+ and B0 -> D*- K+ should be performed for the first time with data of the Belle II experiment. An independent result is an important step towards a solution of the puzzle.

Broad search for matter antimatter asymmetries (topic not available any more)

How to explain the origin of this asymmetry between matter and anti-matter in our universe, which is an essential condition for forming galaxies, stars, planets, and finally life, is still an outstanding mystery. So far measurements of CP violation mainly focused on exclusive decays that are expected to show significant effects. In this explorative study we want to search for matter antimatter asymmetries in a broader and more inclusive way to be sensitive to effects that may have been overlooked so far.

Search for B decays with baryon number violation by two units (topic not available any more)

A precondition for the generation of a baryon asymmetry in our universe is baryon number violation. While the violation of baryon number by one unit (∆B = 1) involving quarks of the first or second generation is severely constrained by the non-observation of proton decay, limits on baryon number violating processes involving third generation quarks are considerably weaker. Processes violating baryon number by two units are even less studied. With Belle II data we can search for ∆B = 2 processes, such as B meson decays to a deuteron and light mesons or leptons.

Measurement of the tau electric dipole moment

Electric dipole moments (EDMs) are a sensitive probe of CP violation. Tight constraints exist in particular for processes with first generation quark level transitions. The task of this topic is to measure the EDM of the tau lepton at Belle II. With a much larger dataset and an improved understanding of the detector we expect to significantly improve current constraints.

Generic New Physics Search with Machine Learning (topic not available any more)

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.

Analysis of the exotic X(3915) state

Since the discovery of the X(3872) state by Belle in 2003 many further bound states of quarks have been observed that do not fit to the pattern of mesons or baryons. A detailed understanding of these exotic states is still missing. To make progress on the experimental side the quantum numbers of the X(3915) should be measured in γ γ -> J/ψ ω events at Belle II. It requires an angular analysis of the decay products.

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.

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.

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.

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