Progress towards the first measurement of the Vcb element of the CKM matrix in semi-leptonic tt¯ decays

A method of measuring the |Vcb| element of the CKM matrix for the first time in tt decays using the ATLAS detector at the Large Hadron Collider is presented. This method uses a binned template profile likelihood fit to measure the fraction of tt events where one top decays as t → W b → lνb and the other as t → W b → cbb. The expected statistical and systematic uncertainties are evaluated for the Run 2 data-taking period corresponding to a centre-of-mass energy √ s = 13 TeV and an integrated luminosity of 139 fb−1 . Using an optimised flavour tagging working point not yet calibrated gave a fractional uncertainty of ∆|Vcb|/|Vcb| = 0.13(stat.)⊕0.13(syst.) = 0.19, a different flavour tagging working point likely to be calibrated on a shorter timescale gave an uncertainty of ∆|Vcb|/|Vcb| = 0.16(stat.) ⊕ 0.16(syst.) = 0.22.