Thesis defense of Dominik Suelmann

In this thesis, new physics signatures of rare charm decays and their interpretations are discussed. In this case, rare decays mean flavour changing neutral currents, including hadronic c -> u l l, c -> u + inv. and c -> u γ transitions. With new physics potentially hidden in the up sector, charm provides a unique avenue to ensure that no stone is left unturned in searches for physics beyond the standard model. Owing to particularly strong GIM and CKM suppression, many null test opportunities open up in charm and are testable by experiment without any theoretical uncertainties. With first measurements available and compatible with the standard model, theoretical uncertainties start to play a role.
This thesis aims to interpret these measurements in terms of new physics constraints and to identify possible improvements for future measurements. A data-driven approach, motivated by resonance structures, is used to parameterise standard model contributions of long-distance effects in c -> u l l decays, where especially the four-body decay D -> π π l l is highlighted as a laboratory to test different QCD models.
The first global fit to c -> u mu mu transitions is performed with measurements of branching ratios dominating the current sensitivity. However, null-test observables are found to be on the rise and observables of Λc -> p l l are analysed and implemented into the public software EOS for future research. For c -> u + inv., few searches exist in charm with branching ratios as large as O(10^-4) found not to be excluded, opening possibilities for a zoo of new physics models with missing energy. Finally, QCD effects in c -> u γ are studied by comparing QCD factorisation methods with data and analyzing improvements via light-cone sum rules.