Prospects for and at FCC-ee

Xunwu Zuo; Marco Fedele; Clément Helsens; Donal Hill; Syuhei Iguro; Markus Klute

doi:10.1140/epjc/s10052-024-12418-0

2024 Impact factor 4.8

Particles and Fields

Open Access

Eur. Phys. J. C (2024) 84: 87
https://doi.org/10.1140/epjc/s10052-024-12418-0

Regular Article - Experimental Physics

Prospects for $B_{c}^{+}$ and $B^{+} \to τ^{+} ν_{τ}$ $B^+\rightarrow \tau ^+ \nu _\tau$ at FCC-ee

Xunwu Zuo¹^a, Marco Fedele², Clément Helsens¹^,3^,6, Donal Hill⁴, Syuhei Iguro²^,5 and Markus Klute¹

¹ Institut für Experimentelle Teilchenphysik (ETP), Karlsruhe Institute of Technology, 76131, Karlsruhe, Germany
² Institut für Theoretische Teilchenphysik (TTP), Karlsruhe Institute of Technology, 76131, Karlsruhe, Germany
³ European Organization for Nuclear Research (CERN), Geneva, Switzerland
⁴ Institute of Physics, École Polytechnique Fédérale de Lausanne (EPFL), Lausanne, Switzerland
⁵ Institut für Astroteilchenphysik (IAP), Karlsruhe Institute of Technology, 76344, Eggenstein-Leopoldshafen, Germany
⁶ Institute of Bioengineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne, Switzerland

^a xunwu.zuo@cern.ch

Received: 8 October 2023
Accepted: 8 January 2024
Published online: 26 January 2024

Abstract

The prospects are presented for precise measurements of the branching ratios of the purely leptonic $B_{c}^{+} \to τ^{+} ν_{τ}$ $B_c^+ \rightarrow \tau ^+ \nu _\tau$ and $B^{+} \to τ^{+} ν_{τ}$ $B^+ \rightarrow \tau ^+ \nu _\tau$ decays at the Future Circular Collider (FCC). This work is focused on the hadronic $τ^{+} \to π^{+} π^{+} π^{-} {\bar{ν}}_{τ}$ $\tau ^{+} \rightarrow \pi ^+ \pi ^+ \pi ^- {\bar{\nu }}_\tau$ decay in both $B_{c}^{+} \to τ^{+} ν_{τ}$ $B_c^+ \rightarrow \tau ^+ \nu _\tau$ and $B^{+} \to τ^{+} ν_{τ}$ $B^+ \rightarrow \tau ^+ \nu _\tau$ processes. Events are selected with two Boosted Decision Tree algorithms to optimise the separation between the two signal processes as well as the generic hadronic Z decay backgrounds. The range of the expected precision for both signals are evaluated in different scenarios of non-ideal background modelling. This paper demonstrates, for the first time, that the $B^{+} \to τ^{+} ν_{τ}$ $B^+ \rightarrow \tau ^+ \nu _\tau$ decay can be well separated from both $B_{c}^{+} \to τ^{+} ν_{τ}$ $B_c^+ \rightarrow \tau ^+ \nu _\tau$ and generic $Z \to b \bar{b}$ $Z\rightarrow b{\bar{b}}$ processes in the FCC-ee collision environment and proposes the corresponding branching ratio measurement as a novel way to determine the CKM matrix element $| V_{ub} |$ $|V_{ub}|$ . The theoretical impacts of both $B^{+} \to τ^{+} ν_{τ}$ $B^+ \rightarrow \tau ^+ \nu _\tau$ and $B_{c}^{+} \to τ^{+} ν_{τ}$ $B_c^+ \rightarrow \tau ^+ \nu _\tau$ measurements on New Physics cases are discussed for interpretations in the generic Two-Higgs-doublet model and leptoquark models.

© The Author(s) 2024

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