91̽»¨

Abstract:

Abstract The abundant production of beauty and charm hadrons in the $$5 \times 10^{12}$$ 5 × 10 12 $$Z^0$$ Z 0 decays expected at FCC-ee offers outstanding opportunities in flavour physics that in general exceed those available at Belle II and are complementary to the heavy-flavour programme of the LHC. A wide range of measurements will be possible in heavy-flavour spectroscopy, rare decays of heavy-flavoured particles and $$C\!P$$ C P -violation studies, which will benefit from the low-background experimental environment, the high Lorentz boost and the availability of the full spectrum of hadron species. This essay first surveys the important questions in heavy-flavour physics and assesses the likely theoretical and experimental landscape at the turn-on of FCC-ee. From this certain, measurements are identified where the impact of FCC-ee will be particularly important. A full exploitation of the heavy-flavour potential of FCC-ee places specific constraints and challenges on detector design, which in some cases are in tension with those imposed by the other physics goals of the facility. These requirements and conflicts are discussed.

Abstract:

The prospects are presented for precise measurements of the branching ratios of the purely leptonic $B^+_c → τ^+ν_τ$ and $B^+ → τ^+ν_τ$ decays at the Future Circular Collider (FCC). This work is focused on the hadronic $τ^+ → π^+π^+π^− \bar{ν}_τ$ decay in both $B^+_c → τ^+ν_τ$ and $B^+ →τ^+ν_τ$ 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 $B^+ → τ^+ν_τ$ decay can be well separated from both $B^+_c → τ^+ν_τ$ and generic $Z → b \bar{b}$ processes in the FCC-ee collision environment and proposes the corresponding branching ratio measurement as a novel way to determine the CKM matrix element $|V_{ub}|$. The theoretical impacts of both $B^+ → τ^+ν_τ$ and $B^+_c → τ^+ν_τ$ measurements on New Physics cases are discussed for interpretations in the generic Two-Higgs-doublet model and leptoquark models

Abstract:

Equipping an experiment at FCC-ee with particle identification (PID) capabilities, in particular the ability to distinguish between hadron species, would bring great benefits to the physics programme. Good PID is essential for precise studies in quark flavour physics and is also a great asset for many measurements in tau, top, and Higgs physics. The requirements placed by flavour physics and these other applications are surveyed, with an emphasis on the momentum range over which PID is necessary. Possible solutions are discussed, including classical RICH counters, time-of-flight systems, and dE/dx and cluster counting. Attention is paid to the impact on the global detector design that including PID capabilities would imply.