Karlsruhe 2024 – scientific programme

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T: Fachverband Teilchenphysik

T 123: Flavour physics 5

T 123.1: Talk

Friday, March 8, 2024, 09:00–09:15, Geb. 30.41: HS 2

Measurement of R(D^*) with Inclusive B Tagging at Belle II — •Thomas Ametsbichler, Thomas Lück, and Thomas Kuhr — Ludwig-Maximilians-Universität München

A fundamental axiom of the Standard Model (SM) of particle physics is the universality of the lepton coupling, g_l (l = e, µ, τ), to the electroweak gauge bosons. The average of measurements from the BaBar, Belle and LHCb experiments have shown a 2.3 σ discrepancy of R(D^*) = B(B → D^* τ ν_τ)/B(B → D^* ℓ ν_ℓ), a ratio of branching fractions sensitive to the lepton coupling, with respect to the SM prediction. This discrepancy may be an indication of New Physics beyond the SM.

The goal of the presented analysis is an independent measurement of R(D^*) with data from the Belle II experiment. Belle II is a so-called B Factory, which produces B meson pairs in e⁺ e⁻ → Υ(4S) → B B reactions. While one B meson is reconstructed in B → D^* τ ν or B → D^* ℓ ν to measure R(D^*), there are multiple ways to reconstruct the second B meson (referred to as the tag B).

In this analysis the tag B is reconstructed inclusively instead of using a hadronic or semileptonic tag as previous measurements, which improves the reconstruction efficiency at the cost of more background. It is anticipated that this approach will reduce the statistical uncertainties of the R(D^*) measurement.

Keywords: Belle II; lepton universality; R(D*); Inclusive Reconstruction