Impact of Rare Decays $t \to \ell' \nu b \ell\ell$ and $t \to q q' b \ell\ell$ on Searches for Top-Associated Physics

Searches for top quark-associated physics such as \( t\overline{t}W \) or \( t\overline{t}H \) in final states with multiple leptons require a careful accounting of expected backgrounds due to the lack of reconstructible resonances. We demonstrate that the rare top quark decays t → ℓ′νbℓℓ and t → qq′bℓℓ, when a soft lepton is not detected, can contribute a non-negligible background to such searches. Simulations in the LHC experiments typically do not account for such decays and as such backgrounds to such searches may be underestimated.