Speaker
Description
The Future Circular Collider in its electron–positron mode (FCC-ee) will provide an exceptionally clean environment for heavy-flavour physics at the $Z$ pole, with an expected sample of order $10^{12}$ bottom and charm hadrons. This creates an opportunity to study rare and purely leptonic $B_c^+$ decays, including $B_c^+ \to \tau^+ \nu_\tau$, which are sensitive to the CKM element $|V_{cb}|$, the $B_c$ decay constant, and possible New Physics contributions to $b \to c\tau\nu$ transitions.
A major limitation in translating a future $B_c^+ \to \tau^+ \nu_\tau$ signal yield into a branching fraction is the limited knowledge of the $B_c$ fragmentation fraction $f_{B_c}$. As a first step toward a future $B_c$ fragmentation-fraction and $|V_{cb}|$ analysis, this work performs a truth-level validation of $B$-hadron flight observables in simulated $e^+e^- \to Z \to b\bar{b}$ events. Ground-state $B$ hadrons are identified using generator-level particle history and classified into $B_d^0$, $B_u^+$, $B_s^0$, $B_c^+$, $\Lambda_b^0$, $\Xi_b$, and $\Omega_b$ categories. Then, their flight distance, lab-frame decay time, proper decay length, and proper decay time are reconstructed from truth-level production and decay information.
The preliminary results show physically sensible lifetime hierarchies. The $B_c^+$ category has a much shorter mean flight distance and proper decay time than the dominant $B_d^0$, $B_u^+$, and $B_s^0$ categories, consistent with the known short $B_c$ lifetime. The validated observables provide the basis for future studies of reconstructed-level lifetime separation, signal–background discrimination, BDT-based classification, and ultimately the extraction of $B_c$ production information needed for $B_c^+ \to \tau^+ \nu_\tau$ and $|V_{cb}|$ studies at FCC-ee.
