First measurement of the charged lepton forward-backward asymmetry in B → X l v decays at Belle II

• Applicant: Dr. Raynette van Tonder
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: since 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 545582477

The disagreement between inclusive and exclusive measurements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element |Vcb| has posed a longstanding puzzle that still remains to be solved. Inclusive determinations of |Vcb| are generally obtained by performing global fits to spectral moments of kinematic distributions in B → Xc l v decays, which allow for the simultaneous extraction of inclusive |Vcb| and the non-perturbative hadronic matrix elements in the Heavy Quark Expansion (HQE). However, these spectral moments are highly correlated and exhibit larger experimental uncertainties for higher moments. The inclusion of additional observables would improve the precision of established approaches using global fits as well as complementary data-driven methods. The goal of the proposed project is to provide the first measurement of the charged lepton forward-backward asymmetry Afb for inclusive B → X l v decays. Focusing on B → X l v decays enables the subtraction of the B → Xu l v component in the heavy quark expansion (HQE) with smaller theoretical uncertainties than the traditional background subtraction method. Furthermore, this novel measurement, corrected for detector resolution and acceptance effects, would be directly included in current global fitting frameworks, which would provide new sensitivity to previously unconstrained non-perturbative parameters of the HQE. Not only would this measurement lead to a deeper understanding of the poorly known non-perturbative parameters, but also produce a more precise determination of inclusive |Vcb| ---ultimately shedding new light on the longstanding puzzle between inclusive and exclusive measurements and the continuous effort to overconstrain the CKM unitarity condition in the search for physics beyond the Standard Model.

• DFG Programme: WBP Position