\midheading{Di-boson production, processes 61--89}
\label{subsec:diboson}

These processes represent the production of a diboson pair $V_1 V_2$,
where $V_1$ and $V_2$ may be either a $W$ or $Z/\gamma^*$.
All the processes in this section may be calculated at NLO with the exception
of {\tt nproc=69}. There are various
possibilities for the subsequent decay of the bosons, as specified in the
sections below. Amplitudes for the $V_1 V_2$ process at $O(\alpha_s)$
are taken from ref.~\cite{Dixon:1998py}.
We also include singly resonant diagrams at NLO for all processes
in the case {\tt zerowidth = .false.}.
For more details on this calculation, please see
Refs.~\cite{Campbell:1999ah,Campbell:2011bn}.

For processes {\tt 62}, {\tt 63}, {\tt 64}, {\tt 65}, {\tt 74}
and {\tt 75} the default behaviour is that the hadronic decay products
of the bosons are clustered into jets using the supplied jet
algorithm parameters, but no cut is applied on the number of jets.
This behaviour can be altered by changing the value of the
variable {\tt notag} in the file \verb|src/User/setnotag.f|.

Calculations of processes {\tt 61}, {\tt 71}, {\tt 76}, {\tt 81} and {\tt 82}
can be performed at NLO by subtraction, zero-jettiness slicing and $q_T$-slicing.
They can be computed at NNLO using  zero-jettiness slicing and $q_T$-slicing,
as described in 'Non-local slicing approaches for NNLO QCD in MCFM',\cite{Campbell:2022gdq}.
For processes {\tt 61}, {\tt 81} and {\tt 86} the NNLO corrections
include glue-glue initiated box diagrams which first contribute at order $\alpha_s^2$.
Two loop results for virtual diagrams at $O(\alpha_s^2)$ are taken from \cite{Gehrmann:2015ora}.