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45 lines
2.3 KiB
45 lines
2.3 KiB
\midheading{$t\bar{t}$ production with 2 semi-leptonic decays, processes 141--145}
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\label{subsec:ttbar}
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These processes describe $t \bar{t}$ production including semi-leptonic
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decays for both the top and the anti-top.
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Since version 6.2 we have updated this to use the one-loop amplitudes of
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ref.~\cite{Badger:2011yu}. The code for the virtual amplitudes now runs
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about three times faster than earlier versions where the virtual
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amplitudes of ref.~\cite{Korner:2002hy} were used.
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Switching {\tt zerowidth} from {\tt .true.} to {\tt .false.} only affects
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the $W$ bosons from the top quark decay, because our method of including spin
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correlations requires the top quark to be on shell.
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Process {\tt 141} includes all corrections, i.e.\ both radiative corrections
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to the decay and to the production. This process is therefore the
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basic process for the description of top production where both quarks
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decay semi-leptonically. When {\tt removebr} is true in process {\tt 141},
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the top quarks do not decay.
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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 in process {\tt 141}.
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The LO calculation proceeds as normal.
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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.
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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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Process {\tt 142} includes only the corrections in
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the semileptonic decay of the top quark. Thus it is of primary
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interest for theoretical studies rather than for physics applications.
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Because of the method that we have used to include the radiation in the decay,
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the inclusion of the corrections in the decay does not change the
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total cross section. This feature is explained in section 6 of ref.~\cite{Campbell:2012uf}.
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In the case of process {\tt 145}, there are no spin correlations in
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the decay of the top quarks. The calculation is performed by
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multiplying the spin summed top production cross section, by the decay
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matrix element for the decay of the $t$ and the $\bar{t}$. These
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processes may be used as a diagnostic test for the importance of the
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spin correlation.
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