Karlsruhe 2024 – scientific programme

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T: Fachverband Teilchenphysik

T 123: Flavour physics 5

T 123.4: Talk

Friday, March 8, 2024, 09:45–10:00, Geb. 30.41: HS 2

Measuring R(D^*) in hadronic one-prong τ decays at Belle II — Florian Bernlochner, Jochen Dingfelder, Markus Prim, and •Ilias Tsaklidis — Physikalisches Institut der Rheinischen Friedrich-Wilhelms-Universität Bonn

Over the last decade, several experimental results hint at the existence of lepton universality violating processes. In this work we probe such a process by measuring the R(D^*) = B(B → D^* τ ν_ℓ)/ B(B → D^* ℓ ν_ℓ) ratio using hadronically decaying τ leptons as a signature. The Belle II experiment provides an excellent laboratory to study these processes within the clean experimental environment of e⁺   e− collisions. We present the current status of this measurement using the full Belle II Run 1 data set and by employing a hadronic reconstruction algorithm to fully reconstruct the accompanying B meson using a dedicated machine learning algorithm. This allows us to reconstruct the entire collision event, assigning each final state particle to a signal and tag side. We target B → D^* τ ν decays with a single charged hadron originating from the τ decay and two missing neutrinos in the event. This allows us to measure not only the rate, but also the polarization of the τ lepton, which has increased sensitivity to New Physics contributions.

Keywords: flavour physics; lepton universality; R(D*); Belle II