Off-Shell Effects for Single Top-Quark Production Processes at Hadron Colliders

We present a calculation of s- and t-channel single top-quark production and decay at next-to-leading order in QCD for the Tevatron and for the LHC. The calculation includes the off-shell effects of the intermediate top-quark and the non-factorisable corrections arising from interferences between the production and decay subprocesses, extending the results beyond the narrow-width approximation. A general method for including such effects is outlined. The method comprises a simultaneous expansion in the coupling constants, as in a standard perturbative approach, along with an expansion in the virtuality of the intermediate heavy-particle. This expansion makes it possible to identify the contributions relevant to the calculation, up to a desired accuracy, before computation of the amplitudes, allowing significant simplifications to be made to the final calculation. Results obtained using this method are presented, along with results obtained using both the standard and improved narrow-width approximations. This enables us to investigate the impact of both off-shell and spin-correlation effects on the cross section and various kinematic distributions. In general, both effects are found to be small except close to kinematic boundaries or for specific distributions, such as the top-quark invariant-mass, where their effects can become sizeable.