NNLO non-resonant corrections to threshold top-pair production from e+ e- collisions: Endpoint-singular terms

We analyse the subleading non-resonant contributions to the e+ e- -> W+ W- b bbar cross section at energies near the top-antitop threshold. These correspond to next-to-next-to-leading-order (NNLO) corrections with respect to the leading-order resonant result. We show that these corrections produce 1/epsilon endpoint singularities which precisely cancel the finite-width divergences arising in the resonant production of the W+ W- b bbar final state from on-shell decays of the top and antitop quarks at the same order. We also provide analytic results for the (m_t/Lambda)^2, (m_t/Lambda) and (m_t/Lambda)^0 log(Lambda) terms that dominate the expansion in powers of (Lambda/m_t) of the complete set of NNLO non-resonant corrections, where Lambda is a cut imposed on the invariant masses of the b W pairs that is neither too tight nor too loose (m_t Gamma_t << Lambda^2 << m_t^2).