Measurement of the |V_(cb)| element of the CKM matrix in tbar{t} decays with the ATLAS detector

The first measurement of the magnitude of the CKM quark mixing matrix element $|V_{cb}|$ using on-shell $W$-boson decays is presented. The measurement is made with $t\bar{t}$ events from $pp$ collision data corresponding to an integrated luminosity of $140$ fb$^{-1}$ at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. A value of: $$|V_{cb}| = (50^{+11}_{-14})\times10^{-3}$$ is measured, where the uncertainties have roughly equal contributions from the limited size of the data sample and from systematic effects. This is consistent with existing measurements made at much lower energy scales in $B$ hadron decays. While the sensitivity does not yet rival that of previous determinations, this novel approach probes a very different physical situation, namely the rare hadronic $t\rightarrow b\bar{b} c$ decay at high momentum-transfer, rather than semileptonic $B$ decays at low momentum-transfer.