Unraveling the D**

• Applicant: Dr. Markus Prim, Ph.D.
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 526218088

The anomalies observed in b-hadron decays over the last decade have attracted considerable interest as they have put the Standard Model of particle physics to the test and hint at new physics. Two prominent examples in charged-current interactions are the discrepancy between the two methods of determining the CKM matrix element Vcb, and an apparent excess in the ratio R(D(*)), a test of lepton flavor universality (LFU). The largest source of potential systematic bias in these measurements will be addressed: the background produced by semileptonic B→D**lν decays with an orbitally excited charmed meson in the final state. With the proposed research program, the Emmy-Noether group will be capable to perform the most precise determinations of the LFU ratio R(D(*)) and the CKM matrix element Vcb by not only utilizing the soon to be largest recorded B-factory data sample by Belle II, but also by tackling the systematic effects that can cause significant bias in these measurements. The dominant systematic uncertainty is caused by B→D**lν processes. These are relatively unexplored, and open questions remain up to date regarding the nature of the D** spectrum and the branching ratios of the B→D**lν decays. With this proposal, the B→D**lν decay is approached from two sides to improve the situation: Spectroscopic measurements will be performed to extract the D** line shapes, complementary to the existing LHCb measurement. In addition, measurements of the differential branching ratio of the B→D**lν decay will be performed to extract its form factors describing the hadronic transition element. Special attentions is paid to the D** states with large width. The commonly used narrow-width approximation for the form factors of these states might be inappropriate and the impact of this approximation will be investigated. Equipped with a much-improved understanding of B→D** lν, three topical measurements will be performed: the LFU R(D(*)), exclusive Vcb in a combined B→Dlνl and B→D* lνl measurement, and inclusive B→Xc lν moments to extract inclusive Vcb. Emphasis is placed on determining previously neglected correlations within and between different measurements to allow consistent interpretations of the experimental data. This Emmy-Noether group bridges the gap between spectroscopic and semileptonic measurements with excited charmed mesons by building a comprehensive measurement program that links the two topics. This is needed to develop a full understanding of the nature of D** states and the B→D** lν, process. With the proposed measurements, and with the close collaboration with theorists in the respective fields, the group will eliminate the largest source of potential bias for semileptonic precision physics evolving around charged-current b→clν interactions and confirm or resolve the present day tension in LFU and Vcb.

• DFG Programme: Independent Junior Research Groups
• International Connection: Netherlands, Switzerland, USA
• Cooperation Partners: Dr. Matteo Fael; Dr. Zoltan Ligeti; Dr. Dean J. Robinson; Kimberley Keri Vos