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→ j_q W t→jqW , where j_q jq is a light quark jet. The improved strategy uses orthogonal b b - and q q -taggers, and adds a new observable, the number of light-quark-tagged jets, to the already commonly used observable, the fraction of b 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 |Vtb|=1 |Vtb|=1 at 95% 95% C.L. at the HL-LHC, and accessing directly the standard model value of |Vtd|^2+|Vₜₛ|^2 |Vtd|2+|Vts|2 .