Prospects for $$ {B}_c^{+} $$→ τ +ντ at FCC-ee

Abstract

This paper presents the prospects for a precise measurement of the branching fraction of the leptonic $$ {B}_c^{+} $$ B c + → τ+ντ decay at the Future Circular Collider (FCC-ee) running at the Z -pole. A detailed description of the simulation and analysis framework is provided. To select signal candidates, two Boosted Decision Tree algorithms are employed and optimised. The first stage suppresses inclusive $$ b\overline{b} $$ b b ¯ , $$ c\overline{c} $$ c c ¯ , and $$ q\overline{q} $$ q q ¯ backgrounds using event-based topological information. A second stage utilises the properties of the hadronic τ+→ π+π+π−$$ \overline{\nu} $$ ν ¯ τ decay to further suppress these backgrounds, and is also found to achieve high rejection for the B+→ τ+ντ background. The number of $$ {B}_c^{+} $$ B c + → τ+ντ candidates is estimated for various Tera-Z scenarios, and the potential precision of signal yield and branching fraction measurements evaluated. The phenomenological impact of such measurements on various New Physics scenarios is also explored.

Flavour prospects at the FCC-ee

“…If the efficiency of jet flavour tagging is incorporated, the sensitivity decreases to 0.4% [8]. Another method is to use branching fraction (BF) measurements of B + →τ + ν 𝜏 and B + c →τ + ν 𝜏 decays, as considered in two recent FCC-ee feasibility studies [9,10]. These can be measured by reconstructing the τ in the "3-prong" final state, 𝜏 + → π + π − π + ν 𝜏 .…”

The Future Circular Collider: a Summary for the US 2021 Snowmass Process

“…While the B c was not observed at LEP, one expects as many as few×10 9 B c mesons from the Z-pole run of FCC-ee. Starting from 5 × 10 12 Z decays and assuming the SM branching ratio of B(B c → τ ν) = (1.95 ± 0.09)% [178] one can expect a huge SM signal yield. Using three-prong tau decays, the study [178] finds that a high purity signal sample containing ∼ 4000 B c → τ ν decays is achievable.…”

“…Starting from 5 × 10 12 Z decays and assuming the SM branching ratio of B(B c → τ ν) = (1.95 ± 0.09)% [178] one can expect a huge SM signal yield. Using three-prong tau decays, the study [178] finds that a high purity signal sample containing ∼ 4000 B c → τ ν decays is achievable. Such a sample would allow the B c → τ ν branching ratio to be determined with a few percent precision.…”

High precision in CKM unitarity tests in $b$ and $c$ decays

“…The current and foreseen programs at LHCb and Belle II, in conjunction with more accurate predictions for hadronic parameters shall hopefully shed light on this puzzle. In a further future at e + e − circular colliders, one could imagine to determine |V cb | thanks to the decay B + c → τ + ν [46,47]. That determination requires however the knowledge of the production fraction of the B c meson at the Z pole, which can be a limiting factor.…”