Erlangen 2026 – scientific programme
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T: Fachverband Teilchenphysik
T 94: Flavour Physics VI
T 94.3: Talk
Friday, March 20, 2026, 09:30–09:45, KH 01.011
Measurement of the Branching Ratio and q2-spectrum of B → D** ℓ ν decays at Belle II — •Eylül Ünlü, Thomas Kuhr, and Thomas Lück — Ludwig-Maximilians-Universität München (LMU), München, Germany
There is currently some tension between the measured value of R(D*)=B(B → D* τ ντ)/ B(B → D* ℓ νℓ) and the Standard Model prediction, hinting at lepton universality violation. Semileptonic B meson decays to D** mesons are background to the R(D*) measurement, where D** denotes the orbitally excited P-wave charm mesons: D1(2420), D2*(2460), D0*(2300), and D1′(2430). These decays are not well understood, and discrepancies have been observed between past measurements of their yields made by BaBar and Belle. Hence, improving understanding of these decays reduces an important systematic uncertainty on R(D*) measurements.
The present study aims to use data from the Belle II experiment to study these decays, particularly to determine the q2 spectrum, which is a key input for theory predictions.
We reconstruct one of the B mesons from the Υ(4S)→ B B decay in the signal channel, B→ D**(D(*)π)ℓν, and the other B meson in various hadronic modes using the Full Event Interpretation algorithm. The signal yield is determined by a maximum likelihood fit to the mass difference M(D(*) π) − M(D(*)). The resulting q2 spectrum is fitted using a differential decay rate model after correcting for detector resolution effects. The status of the analysis will be presented including results on simulation and a first assessment of systematic uncertainties.
Keywords: Semileptonic B decays; Flavor physics; Belle II