SMuK 2023 – wissenschaftliches Programm

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T: Fachverband Teilchenphysik

T 129: Flavor X

T 129.2: Vortrag

Donnerstag, 23. März 2023, 17:45–18:00, HSZ/0304

Measurement of R(D^*) with inclusive B meson tagging at Belle II — •Stephanie Steinmetz, Thomas Lück, and Thomas Kuhr — Ludwig-Maximilians-Universität München

The measured ratio R(D^*) = B(B → D^* τ ν) / B(B → D^* ℓ ν) of branching fractions, where ℓ = e, µ, has consistently shown an excess of B → D^* τ ν events. The deviation between Standard Model predictions and the current world average lies at 2.8 σ, made even more interesting by the fact that many systematic uncertainties cancel in the ratio. In combination with the analogous R(D), the discrepancy exceeds 3 σ and has therefore attracted much attention as a possible hint towards new physics phenomena such as leptoquarks or a charged Higgs.

In this analysis at Belle II, we investigate the decay chains B → D^* τ (ℓ ν ν) ν as the signal channel, and B → D^* ℓ ν as normalisation. As both provide the same final state (up to neutrinos), a D^* and a lepton are reconstructed in both cases. The other B meson ("tag B") in the Υ(4S) → BB event is reconstructed ("tagged") inclusively, i.e. by assuming all particles not assigned to the signal B belong to the tag B without reconstructing intermediate particles. The resulting higher event yields are especially useful when only limited data is available, but come at the cost of higher background levels compared to previous approaches where specific tag B decay trees are reconstructed. The goal of this analysis is to determine the feasibility of applying the inclusive tagging approach to early Belle II data in order to gain competitive results w.r.t. other approaches. The current status of the analysis will be presented in this talk.