Accessing CKM suppressed top decays at the LHC

We propose an extension of the existing experimental strategy for measuring branching fractions of top quark decays, targeting specifically $t\to j_{q}W$, where $j_q$ is a light quark jet. The improved strategy uses orthogonal $b$- and $q$-taggers, and adds a new observable, the number of light-quark-tagged jets, to the already commonly used observable, the fraction of $b$-tagged jets in an event. Careful inclusion of the additional complementary observable significantly increases the expected statistical power of the analysis, with the possibility of excluding $\mathrm{\mid }{V}_{tb}\mathrm{\mid }=1$ at $95\mathrm{\%}$ C.L. at the HL-LHC, and accessing directly the standard model value of $\mathrm{\mid }{V}_{td}{\mathrm{\mid }}^2+\mathrm{\mid }{V}_{ts}{\mathrm{\mid }}^2$.