Göttingen 2025 – scientific programme

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T: Fachverband Teilchenphysik

T 73: Flavour Physics IV

T 73.6: Talk

Thursday, April 3, 2025, 17:30–17:45, VG 1.103

Measurement of the branching ratio and q²-spectrum of B → D^** ℓ ν decays at Belle II — •Eyluel Uenlue, Thomas Lueck, and Thomas Kuhr — Ludwig-Maximilians-Universitaet Muenchen

There is currently some tension between the measured value of R(D^*)=B(B → D^* τ ν_τ)/ B(B → D^* ℓ ν_ℓ) 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 there have been discrepancies between past measurements of their yields made by BaBar and Belle. Hence, improving understanding of these decays decreases an important systematic uncertainty on R(D^*) measurements.

The aim of the present study is to use simulation and data from the Belle II experiment to study these decays, in particular to determine the q² spectrum, which is a key input for theory.

We reconstruct one of the B mesons from the Υ(4S)→ B B decay in the signal channel, B→ D^**(D^*π)ℓν. The other B meson is reconstructed in various hadronic modes using the Full Event Interpretation algorithm, which provides a tag B sample with well known kinematics. 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 by a differential decay rate model after correcting for detector resolution effects. The current status of the analysis will be presented including results on simulation and some sources of systematic uncertainty.

Keywords: B meson decay; D mesons; Branching fraction measurement; Belle II