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42 lines
2.3 KiB
42 lines
2.3 KiB
\midheading{$Wt$ production, processes 180--187}
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\label{subsec:wt}
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These processes represent the production of a $W$ boson that decays leptonically
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in association with a top quark. The lowest order diagram involves a gluon and
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a bottom quark from the PDF, with the $b$-quark radiating a $W$ boson and
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becoming a top quark. The calculation can be performed up to NLO.
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For more details on this calculation, please see Ref.~\cite{Campbell:2005bb}.
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Processes {\tt 180} and {\tt 185} produce a top quark that does not decay,
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whilst in processes {\tt 181} and {\tt 186} the top quark decays leptonically.
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Consistency with
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the simpler processes ({\tt 180,185}) can be demonstrated by running process
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{\tt 181,186} with {\tt removebr} set to true.
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At next-to-leading order, the calculation includes contributions from diagrams
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with two gluons in the initial state, $gg \rightarrow Wtb$. The $p_T$ of the
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additional $b$ quark is vetoed according to the value of the parameter
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{\tt ptmin\_bjet} which is specified in the input file. The contribution from
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these diagrams when the $p_T$ of the $b$ quark is above {\tt ptmin\_bjet}
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is zero. The values of this parameter and the factorization scale ({\tt facscale})
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set in the input file should be chosen carefully. Appropriate values for both
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(in the range $30$-$100$~GeV) are discussed in the associated paper~\cite{Campbell:2005bb}.
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When one wishes to calculate observables related to the decay of the top
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quark, {\tt removebr} should be false.
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The LO calculation proceeds as normal. At NLO, there are two options:
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\begin{itemize}
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\item {\tt part=virt, real} or {\tt tota} : final state radiation is included
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in the production stage only
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\item {\tt part = todk} : radiation is included in the decay of the top
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quark also and the final result corresponds to the sum of real and virtual
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diagrams. This process can only be performed at NLO with
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{\tt zerowidth = .true}. This should be set automatically.
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Note that these runs automatically perform an extra integration, so
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will take a little longer.
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\end{itemize}
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The contribution from radiation in the decay may be calculated separately using
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processes {\tt 182,187}. These process numbers can be used with {\tt part=virt,real}
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only. To ensure consistency, it is far simpler to use {\tt 181,186}
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and this is the recommended approach.
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