Measuring the W − t − b interaction at the ILC

The large top quark mass suggests that the top plays a pivotal role in electroweak symmetry-breaking dynamics and, as a result, may have modified couplings to electroweak bosons. Hadron colliders can provide measurements of these couplings at the $\ensuremath{\sim}10%$ level, and one of the early expected triumphs of the International Linear Collider (ILC) is to reduce these uncertainties to the percent level. In this article, we propose the first direct measurement of the standard model $W\mathrm{\text{\ensuremath{-}}}t\mathrm{\text{\ensuremath{-}}}b$ coupling at the ILC, from measurements of $t\overline{t}$-like signals below the $t\overline{t}$ production threshold. We estimate that the ILC with $100\text{ }\text{ }{\mathrm{fb}}^{\ensuremath{-}1}$ can measure a combination of the coupling and top width to high precision, and when combined with a direct measurement of the top width from the above-threshold scan, results in a model-independent measurement of the $W\mathrm{\text{\ensuremath{-}}}t\mathrm{\text{\ensuremath{-}}}b$ interaction of the order of $\ensuremath{\sim}3%$.