Charmingly Direct CP Violation

Fundamental symmetries have proven to be essential in our evolving understanding of elementary particle interactions. Two of the most common examples are the discrete parity (P) and charge conjugation (C) symmetries. While P represents a system that remains unchanged under space reflection, C describes a property between particles and their anti-particles. Both C and P are preserved by strong and electromagnetic processes but violated in weak decays. The violation of the combined charge-parity (CP) symmetry is one of the requirements of the generation of baryon asymmetry in the Universe. CP violation was first observed in 1964 in decays of particles containing one strange quark. Further measurements allowed to establish CP violation also in the bottom quark meson system, leading to the Nobel prize win for Kobayashi and Maskawa in 2008. Despite its many successes, the amount of CP violation predicted by their theory is orders of magnitude smaller than what is required to explain the observed matter-antimatter asymmetry in the Universe. It is therefore crucial to continue experimental investigations to search for possible new sources of CP violation. In decays of particles containing charm quarks, CP violation is expected to be small and challenging to observe experimentally. So far CP violation in charm has only been observed by the LHCb experiment in 2019. Decays into two pions are of particular interest, as for a given particle content, CP violation effects are expected to be approximately zero. Any non-zero measurement of CP effects could therefore hint at new physics. The next generation of heavy- flavor experiments is hosted at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. The asymmetric energy collider SuperKEKB accelerates electrons and positrons on a total circumference of 3km at respective beam energies of 7 and 4GeV. Located at the interaction point of both beams, the Belle II experiment will be able to achieve leading precision in measurements of CP violation in charm decays. The determination of flavor, i.e. the quark content of the signal charm particle at the time of production is crucial for these measurements. This is usually inferred from the other particles in the decay chain. I propose a different approach that exploits information from the rest of the event, including every particle not related to the signal charm particle. In addition, the newly-developed hardware of the Belle II detector provides more advanced information related to the reconstructed particles, which in turn allows for a deeper insight into the underlying processes. This information will be fed into machine learning algorithms. With these two approaches, the efficiency of the reconstruction process will be improved which in turn allows for an overall higher precision of the measurement.

Fundamental symmetries have proven to be essential in our evolving understanding of elementary particle interactions. Two of the most common examples are the discrete parity (P) and charge conjugation (C) symmetries. While P represents a system that remains unchanged under space reflection, C describes a property between particles and their anti-particles. Both C and P are preserved by strong and electromagnetic processes but violated in weak decays. The violation of the combined charge-parity (CP) symmetry is one of the requirements of the generation of baryon asymmetry in the Universe. CP violation was first observed in 1964 in decays of particles containing one strange quark. Further measurements allowed to establish CP violation also in the bottom quark meson system, leading to the Nobel prize win for Kobayashi and Maskawa in 2008. Despite its many successes, the amount of CP violation predicted by their theory is orders of magnitude smaller than what is required to explain the observed matter-antimatter asymmetry in the Universe. It is therefore crucial to continue experimental investigations to search for possible new sources of CP violation. In decays of particles containing charm quarks, CP violation is expected to be small and challenging to observe experimentally. So far CP violation in charm has only been observed by the LHCb experiment in 2019. Decays into two pions are of particular interest, as for a given particle content, CP violation effects are expected to be approximately zero. The next generation of heavy-flavor experiments is hosted at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. The asymmetric energy collider SuperKEKB accelerates electrons and positrons on a total circumference of 3km at respective beam energies of 7 and 4GeV. Located at the interaction point of both beams is the Belle II experiment. With data sets collected by the latter, several measurements of CP violation in charm decays were conducted in this project, achieving leading precision thanks to improved analysis methods. In particular, a novel approach was developed for the determination of flavor, i.e. the quark content of the signal charm particle at the time of production, which is crucial for these measurements. This is usually inferred from the other particles in the decay chain. Our method exploits information from the rest of the event, including every particle not related to the signal charm particle. This information is fed into advanced machine learning algorithms which are trained with large simulated data sets. We are able to significantly improve the efficiency of the reconstruction process which in turn allows for an overall higher precision of the measurement.