Top pair threshold production at a linear collider with WHIZARD

We briefly describe how the Monte Carlo generator WHIZARD 2.2 can be employed to study large QCD effects enhancing the top-antitop production threshold at a next-generation lepton collider. While present state-of-the-art predictions at NNLL order are confined to inclusive total cross sections, our tool can be used to simulate differential distributions including NLL threshold resummation in the production, and with off-shell decaying tops. The new model will be shipped with WHIZARD from version 2.2.3 onwards, to be released along with this article.Comment: 4 pages, 1 figure. Proceedings of TOP2014, 7th International Workshop on Top Quark Physics, Cannes, France, September 29 - October 3 201

The Top-Antitop Threshold at the ILC: NNLL QCD Uncertainties

We discuss the top-antitop production cross section near threshold at a future linear collider accounting for the NNLL QCD corrections to the anomalous dimension of the leading S-wave production current computed recently within renormalization-group-improved NRQCD perturbation theory. We argue that the still unknown soft NNLL mixing corrections are negligible so that the NNLL QCD corrections to the total cross section can be considered complete for practical purposes. Based on combined variations of the renormalization and matching scales and the overall size of the perturbative corrections we estimate that the NNLL QCD total cross section has a normalization uncertainty of 5% at threshold. We present results for the total cross section and also for the experimentally more relevant case, when moderate cuts are imposed on the reconstructed top and antitop invariant masses.Comment: 23 pages, 7 figures; v2: plots and appendix added, journal versio

The Fully-Differential Quark Beam Function at NNLO

We present the first calculation of a fully-unintegrated parton distribution (beam function) at next-to-next-to-leading order (NNLO). We obtain the fully-differential beam function for quark-initiated processes by matching it onto standard parton distribution functions (PDFs) at two loops. The fully-differential beam function is a universal ingredient in resummed predictions of observables probing both the virtuality as well as the transverse momentum of the incoming quark in addition to its usual longitudinal momentum fraction. For such double-differential observables our result provides the part of the NNLO singular cross section related to collinear initial-state radiation (ISR), and is important for the resummation of large logarithms through N3LL.Comment: 17 pages, 1 figure; v2: journal versio