These processes represent the production of a W boson that decays leptonically in association with a top quark. The lowest order diagram involves a gluon and a bottom quark from the PDF, with the b-quark radiating a W boson and becoming a top quark. The calculation can be performed up to NLO. For more details on this calculation, please see Ref. [1].
Processes 180 and 185 produce a top quark that does not decay, whilst in processes 181 and 186 the top quark decays leptonically. Consistency with the simpler processes (180,185) can be demonstrated by running process 181,186 with removebr set to true.
At next-to-leading order, the calculation includes contributions from diagrams with two gluons in the initial state, gg → Wtb. The p_{T} of the additional b quark is vetoed according to the value of the parameter ptmin_bjet which is specified in the input file. The contribution from these diagrams when the p_{T} of the b quark is above ptmin_bjet is zero. The values of this parameter and the factorization scale (facscale) set in the input file should be chosen carefully. Appropriate values for both (in the range 30-100 GeV) are discussed in the associated paper [1].
When one wishes to calculate observables related to the decay of the top quark, removebr should be false. The LO calculation proceeds as normal. At NLO, there are two options:
part=virt, real or tota : final state radiation is included in the production stage only
part = todk : radiation is included in the decay of the top quark also and the final result corresponds to the sum of real and virtual diagrams. This process can only be performed at NLO with zerowidth = .true. This should be set automatically. Note that these runs automatically perform an extra integration, so will take a little longer.
The contribution from radiation in the decay may be calculated separately using processes 182,187. These process numbers can be used with part=virt,real only. To ensure consistency, it is far simpler to use 181,186 and this is the recommended approach.
nplotter_auto.f is the default plotting routine.
[1] J.M. Campbell and F. Tramontano, Next-to-leading order corrections to Wt production and decay, Nucl. Phys. B 726 (2005) 109 [hep-ph/0506289].