Fully-differential Top-Pair Production at a Lepton Collider: From Threshold to Continuum

We present an approach to predict exclusive $W^+bW^-\bar{b}$ production at lepton colliders that correctly describes the top-anti-top threshold as well as the continuum region. We incorporate $t\bar{t}$ form factors for the NLL threshold resummation derived in NRQCD into a factorized relativistic cross section using an extended double-pole approximation, which accounts for fixed-order QCD corrections to the top decays at NLO. This is combined with the full fixed-order QCD result at NLO for $W^+bW^-\bar{b}$ production to obtain predictions that are not only valid at threshold but smoothly transition to the continuum region. Our implementation is based on the Monte Carlo event generator WHIZARD and the code TOPPIK and allows to compute fully-differential threshold-resummed cross sections including the interference with non-resonant background processes. For the first time it is now possible to systematically study general differential observables at future lepton colliders involving the decay products of the top quarks at energies close to the pair production threshold and beyond