\midheading{$W+2$~jets production, processes 22,27}
\label{subsec:w2jets}

This process represents the production of a $W$ boson and $2$ jets,
where the $W$ boson decays leptonically. The calculation may be
performed up to NLO, as detailed below. Virtual amplitudes are
taken from ref.~\cite{Bern:1997sc}.
For more details on this calculation, please see
Refs.~\cite{Campbell:2002tg,Campbell:2003hd}.

For these processes (and also for $Z+2$~jet production, {\tt nproc=44,46})
the next-to-leading order matrix elements are
particularly complex and may be divided into two groups.
The division is according to the lowest order diagrams from which they
originate:
\begin{enumerate}
\item Diagrams involving two external quark lines and two external gluons,
the ``{\tt Gflag}'' contribution. The real diagrams in this case thus
involve three external gluons.

\item Diagrams where all four external lines are quarks,
the ``{\tt Qflag}'' contribution. The real diagrams in this case
involve only one gluon.
\end{enumerate}

By specifying {\tt Gflag} and {\tt Qflag} in the file {\tt input.ini} one may
select one of these options at a time. The full result may be obtained
by straightforward addition of the two individual pieces, with no
meaning attached to either piece separately.
The default is that both of these are set to {\tt .true.} simultaneously.

Virtual amplitudes are taken from ref.~\cite{Bern:1997sc}.

When {\tt removebr} is true, the $W$ boson does not decay.
