Challenging the Standard Model with suppressed b to d l+l- decays

ongoing

The Standard Model (SM) of particle physics is one of the most complete theories in science with a hugely successful predicting power. However, it is unable to explain critical observed phenomena, such as the dominance of matter over antimatter in the universe, and thus needs to be extended. Rare decays of b quarks to an s quark and two leptons (b to sll) are very sensitive to the existence of New Physics (NP). Recent measurements of their properties show intriguing deviations with respect to SM predictions that could be the first clear hint of NP in decades. In this project, I will explore the related and even more suppressed b-quark decays to a d quark and two leptons (b to dll), which are so far poorly known and will shed light on the type on NP that could explain the observed discrepancies. For this purpose, my team will develop innovative analysis tools and exploit the uniquely large sample of b hadrons from the LHCb experiment. The CLIMB project will address two specific questions: are the deviations observed in b to sll decays also present in b to dll transitions? Are there new sources of matter-antimatter asymmetry beyond the SM in b to dll processes? The first will be addressed by measuring differential decay probabilities and lepton universality ratios in b to dll decays for the first time. In the SM, these transitions are related by the quark-mixing matrix, the hierarchy of which is not fully understood. NP models aim to provide an explanation for the structure observed in nature. Knowing the properties of b to dll decays precisely is a critical input in this endeavour. The second question will be answered by measuring matter-antimatter asymmetries in b to dll decays with unprecedented precision, providing very strong constraints to NP models predicting an enhanced quantity. The main challenge of this programme lies in the study of very suppressed decays. Innovative reconstruction and selection techniques will be developed to access them.