% !TEX root = MC_snippets.tex

\chapter{Multiboson processes}
%\label{sec:MB}

In the following paragraphs, the set-ups of the current ATLAS multiboson baseline samples are described.
Details of the full process configuration are given in the PUB note~\cite{ATL-PHYS-PUB-2017-005}.


\section[Inclusive Powheg]{Inclusive \POWHEG}
%\label{sec:MB-powheg}

The descriptions below correspond to the samples in \cref{tab:MB-powheg}.

\begin{table}[htbp]
  \caption{Inclusive \(VV\) samples with \POWHEG.}%
  \label{tab:MB-powheg}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    361600--361605 & inclusive \(WW(2\ell 2\nu), WZ(3\ell\nu), ZZ(4\ell), ZZ(4\nu), ZZ(2\ell 2\nu)\) (all lepton flavours)\\
    361606--361611 & inclusive \(WW/WZ/ZZ\) semileptonic decays (\(\ell\ell qq\), \(\ell\nu qq\), \(\nu\nu qq\), all lepton flavours)\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

The \POWHEGBOX[v2]~\cite{Nason:2004rx,Frixione:2007vw,Alioli:2010xd} generator
was used to simulate the \(WW\), \(WZ\) and \(ZZ\)~\cite{Nason:2013ydw} production
processes at NLO accuracy in QCD. The effect of singly resonant
amplitudes and interference effects due to \(Z/\gamma^*\) and
same-flavour lepton combinations in the final state were included, where appropriate.
Interference effects between \(WW\) and \(ZZ\) for same-flavour charged leptons and
neutrinos were ignored.
Events were interfaced to \PYTHIA[8.210]~\cite{Sjostrand:2014zea}
for the modelling of the parton shower, hadronisation, and underlying
event, with parameters set according to the \AZNLO
tune~\cite{STDM-2012-23}. The \CT[10] PDF set~\cite{Lai:2010vv} was used
for the hard-scattering processes, whereas the \CTEQ[6L1] PDF
set~\cite{Pumplin:2002vw} was used for the parton shower. The \EVTGEN[1.2.0]
program~\cite{Lange:2001uf} was used to decay bottom and charm hadrons.

The factorisation and renormalisation scales were set to the invariant mass of
the boson pair. An invariant mass of \(m_{\ell\ell} > \qty{4}{\GeV}\) was required
at matrix-element level for any pair of same-flavour charged leptons.


\section[Sherpa]{\SHERPA}
%\label{sec:MB-sherpa}

\subsection{Fully leptonic, semileptonic and loop-induced \texorpdfstring{\(VV\)}{VV}}
%\label{sec:MB-sherpa-fully}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa}. They describe the almost
identical set-ups of fully leptonic (including loop-induced \(VV\) production)
and semileptonic \(VV\) decays. For loop-induced processes \(gg\to VV\),
the description below assumes that the `nominal' samples with
Higgs contributions are used. If you are using specialised set-ups that
exclude the Higgs component, the description should be modified appropriately.
(Get in touch with the PMG Weak Boson Processes subgroup if you are unsure.)

\begin{table}[htbp]
  \caption{\(VV\) samples with \SHERPA: fully leptonic, semileptonic,
    loop-induced fully leptonic, loop-induced semileptonic. SFOS stands for
    \enquote{same flavour opposite-charge sign}.}%
  \label{tab:MB-sherpa}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    364250--364255, 363494   & \(4\ell, 3\ell\nu, 2\ell 2\nu, 4\nu\) with \(m_{\ell\ell}\mathrm{(SFOS)} > \qty{4}{\GeV}\), \(\pTX[\ell](1,2) > \qty{5}{\GeV}\)\\
    364288--364290 & fully leptonic low \(m_{\ell\ell}\) and \(\pTX[\ell]\) complement\\
    \midrule
    345705--345727   & loop-induced leptonic\\
    \midrule
    363355-363360, 363489   & semileptonic, on-shell diboson production with factorised decays\\
    \midrule
    364302-364305   & loop-induced semileptonic, using factorised on-shell decays)\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

Samples of diboson final states (\(VV\)) were simulated with the
\SHERPA[2.2.1] or 2.2.2~\cite{Bothmann:2019yzt} generator depending on the process,
%~\footnote{This is an admixture of 2.2.1 and above versions, so the version should be kept generic to avoid confusions. As an
%alternative, the sentence can be modified indicating that samples are simulated with the \SHERPA[2.2.1] or 2.2.2 depending on the process.}
including off-shell effects and Higgs boson contributions, where appropriate.
Fully leptonic final states and semileptonic final states, where one boson
decays leptonically and the other hadronically, were generated using
matrix elements at NLO accuracy in QCD for up to one additional parton
and at LO accuracy for up to three additional parton
emissions. Samples for the loop-induced processes \(gg \to VV\) were
generated using LO-accurate matrix elements for up to one
additional parton emission for both the cases of fully leptonic and
semileptonic final states. The matrix element calculations were matched
and merged with the \SHERPA parton shower based on Catani--Seymour
dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg} using the MEPS@NLO
prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
The virtual QCD corrections were provided by the
\OPENLOOPS library~\cite{Buccioni:2019sur,Cascioli:2011va,Denner:2016kdg}. The
\NNPDF[3.0nnlo] set of PDFs was used~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters developed by the \SHERPA authors.


\paragraph{Additional description:}

The ME+PS matching~\cite{Hoeche:2011fd} was employed for different jet
multiplicities which were then merged into an inclusive sample using an
improved CKKW matching procedure~\cite{Catani:2001cc,Hoeche:2009rj}
which is extended to NLO accuracy using the MEPS@NLO
prescription~\cite{Hoeche:2012yf}. The virtual QCD corrections for
matrix elements at NLO accuracy were provided by the
\OPENLOOPS library~\cite{Buccioni:2019sur,Cascioli:2011va,Denner:2016kdg}. The
calculation was performed in the \(G_\mu\) scheme, ensuring an
optimal description of pure electroweak interactions at the
electroweak scale.

\input{sherpa_scale_pdf.tex}


\subsection[Electroweak VVjj]{Electroweak \(VVjj\)}
%\label{sec:MB-sherpa-vvjj}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa-vvjj}.

\begin{table}[htbp]
  \caption{Electroweak \(VVjj\) samples with \SHERPA.}%
  \label{tab:MB-sherpa-vvjj}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    364283--364284   & \(\ell\ell\ell\ell jj\), \(\ell\ell\ell\nu jj\)\\
    364285   &  \(\ell\ell\nu\nu jj\) opposite-sign\\
    364287   &  \(\ell\ell\nu\nu jj\) same-sign\\
    366086--366089   &  \(\ell\ell\ell\ell jj\), \(\ell\ell\ell\nu jj\), \(\ell\ell\nu\nu jj\), with the triboson contributions removed\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

Electroweak production of a diboson in association with two jets
(\(VVjj\)) was simulated with the \SHERPA[2.2.2]~\cite{Bothmann:2019yzt}
generator. The LO-accurate matrix elements were matched to a parton
shower based on Catani--Seymour dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg} using the MEPS@LO
prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
Samples were generated using the \NNPDF[3.0nnlo] PDF set~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters
%developed by the \SHERPA authors.

\paragraph{Additional description:} The calculation was performed in the
\(G_\mu\) scheme, ensuring an optimal description of pure
electroweak interactions at the electroweak scale.

\input{sherpa_scale_pdf.tex}


\subsection[Vgamma (NLO, biased)]{\(V\gamma\) (NLO, biased)}
%\label{sec:MB-sherpa-vgamma-nlo-biased}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa-vynlo-ew}.

\begin{table}[htbp]
  \caption{NLO \(V\gamma\) samples with \SHERPA.}%
  \label{tab:MB-sherpa-vynlo-ew}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    700011--700017 & biased in \(\log_{10}(\max[\pT(V), \pT(\gamma)])\)\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

The production of \(V\gamma\) final states was simulated with the
\SHERPA[2.2.8]~\cite{Bothmann:2019yzt} generator.  Matrix elements at
NLO QCD accuracy for up to one additional parton and LO accuracy for
up to three additional parton emissions were matched and merged with
the \SHERPA parton shower based on Catani--Seymour
dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg} using the MEPS@NLO
prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
The virtual QCD corrections for matrix elements at NLO accuracy were provided by
the \OPENLOOPS[2] library~\cite{Buccioni:2019sur,Cascioli:2011va,Buccioni:2017yxi,Denner:2016kdg}.
Samples were generated using the \NNPDF[3.0nnlo] PDF set~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters developed by the \SHERPA authors.


\paragraph{Additional description:}

The ME+PS matching~\cite{Hoeche:2011fd} was employed for different jet
multiplicities which were then merged into an inclusive sample
using an improved CKKW matching
procedure~\cite{Catani:2001cc,Hoeche:2009rj} which was extended to NLO
accuracy using the MEPS@NLO prescription~\cite{Hoeche:2012yf}.
The virtual QCD corrections for matrix elements at NLO accuracy were provided by
the \OPENLOOPS[2] library~\cite{Buccioni:2019sur,Cascioli:2011va,Buccioni:2017yxi,Denner:2016kdg}.
Multijet merging at NLO accuracy in the electroweak coupling was based on
the \NLOEWvirt approach~\cite{Kallweit:2014xda,Kallweit:2015dum}.
The calculation was performed in the \(G_\mu\) scheme, ensuring an optimal
description of pure electroweak interactions at the electroweak scale.

\input{sherpa_scale_pdf.tex}


\subsection[Vgamma (NLO, sliced)]{\(V\gamma\) (NLO, sliced)}
%\label{sec:MB-sherpa-vgamma-nlo}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa-vynlo}.

\begin{table}[htbp]
  \caption{NLO \(V\gamma\) samples with \SHERPA.}%
  \label{tab:MB-sherpa-vynlo}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    364500--364535   & sliced in \pTX[\gamma]\\
    345887--345900   & sliced in \(m_{\ell\ell}\)\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

The production of \(V\gamma\) final states was simulated with the
\SHERPA[2.2.2]~\cite{Bothmann:2019yzt} generator.  Matrix elements at
NLO QCD accuracy for up to one additional parton and LO accuracy for
up to three additional parton emissions were matched and merged with
the \SHERPA parton shower based on Catani--Seymour
dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg} using the \MEPSatNLO
prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
The virtual QCD corrections for matrix elements at NLO accuracy were
provided by the
\OPENLOOPS library~\cite{Buccioni:2019sur,Cascioli:2011va,Denner:2016kdg}. Samples
were generated using the \NNPDF[3.0nnlo] PDFset~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters developed by the
%\SHERPA authors.


\paragraph{Additional description:}

The ME+PS matching~\cite{Hoeche:2011fd} was employed for different jet
multiplicities which were then merged into an inclusive sample
using an improved CKKW matching
procedure~\cite{Catani:2001cc,Hoeche:2009rj} which was extended to NLO
accuracy using the \MEPSatNLO prescription~\cite{Hoeche:2012yf}.
The virtual QCD corrections for matrix elements at NLO accuracy were
provided by the \OPENLOOPS library~\cite{Buccioni:2019sur,Cascioli:2011va,Denner:2016kdg}.
The calculation was performed in the \(G_\mu\) scheme, ensuring an optimal
description of pure electroweak interactions at the electroweak scale.

\input{sherpa_scale_pdf.tex}


\subsection[Vgamma (LO)]{\(V\gamma\) (LO)}
%\label{sec:MB-sherpa-vgamma-lo}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa-vylo}.

\begin{table}[htbp]
  \caption{LO \(V\gamma\) samples with \SHERPA.}%
  \label{tab:MB-sherpa-vylo}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    366140--366154   & \SHERPA[2.2.4], \(Z(\to ee/\mu\mu/\tau\tau)\gamma\), sliced in \pTX[\gamma]\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

The production of \(V\gamma\) final states was simulated with the
\SHERPA[2.2.4]~\cite{Bothmann:2019yzt} generator. Matrix elements at LO
accuracy in QCD for up to three additional parton emissions were
matched and merged with the \SHERPA parton shower based on
Catani--Seymour dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg}
using the \MEPSatLO
prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
Samples were generated using the \NNPDF[3.0nnlo] PDF set~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters
%developed by the \SHERPA authors.


\paragraph{Additional description:}

The ME+PS matching~\cite{Hoeche:2011fd} was employed for different jet
multiplicities which were then merged into an inclusive sample using an improved CKKW matching
procedure~\cite{Catani:2001cc,Hoeche:2009rj}.
The calculation was performed in the \(G_\mu\) scheme, ensuring an optimal
description of pure electroweak interactions at the electroweak scale.

\input{sherpa_scale_pdf.tex}


\subsection{Tribosons (NLO)}
%\label{sec:MB-sherpa-vvv-nlo}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa-vvvnlo}.

\begin{table}[htbp]
  \caption{NLO \(VVV\) samples (factorised decays) with \SHERPA.}%
  \label{tab:MB-sherpa-vvvnlo}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    363507--363509   & \(3\ell1\nu2j\), \(4\ell2j\)\\
    364242--364249   & fully leptonic decays\\
    364336--364339   & \(WWW\to 2\ell2\nu jj\)\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

The production of triboson (\(VVV\)) events was simulated with the
\SHERPA[2.2.2]~\cite{Bothmann:2019yzt} generator using factorised gauge-boson decays.
Matrix elements, accurate to NLO for the inclusive process and to LO for up to
two additional parton emissions, were matched and merged with the \SHERPA parton
shower based on Catani--Seymour dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg}
using the \MEPSatNLO prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
The virtual QCD corrections for matrix elements at NLO accuracy were
provided by the \OPENLOOPS library~\cite{Buccioni:2019sur,Cascioli:2011va,Denner:2016kdg}.
Samples were generated using the \NNPDF[3.0nnlo] PEF set~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters developed by the \SHERPA authors.


\paragraph{Additional description:}

The ME+PS matching~\cite{Hoeche:2011fd} was employed for different jet
multiplicities which were then merged into an inclusive sample
using an improved CKKW matching procedure~\cite{Catani:2001cc,Hoeche:2009rj} which is extended to NLO
accuracy using the \MEPSatNLO prescription~\cite{Hoeche:2012yf}.  The
calculation was performed in the \(G_\mu\) scheme, ensuring an optimal description of pure electroweak interactions at the
electroweak scale.

\input{sherpa_scale_pdf.tex}


\subsection{Tribosons (LO)}
%\label{sec:MB-sherpa-vvv-lo}

The descriptions below correspond to the samples in
\cref{tab:MB-sherpa-vvvlo}.

\begin{table}[htbp]
  \caption{LO \(VVV\) samples with \SHERPA.}%
  \label{tab:MB-sherpa-vvvlo}
  \centering
  \begin{tabular}{l l}
    \toprule
    DSID range & Description \\
    \midrule
    407311--407315   & on- and off-shell contributions to 6-lepton production\\
    \bottomrule
  \end{tabular}
\end{table}

\paragraph{Description:}

The production of triboson (\(VVV\)) events was simulated with the
\SHERPA[2.2.1]~\cite{Bothmann:2019yzt} generator. Matrix elements accurate to LO in QCD
for up to one additional parton emission were matched and merged with the \SHERPA parton shower based on Catani--Seymour
dipole factorisation~\cite{Gleisberg:2008fv,Schumann:2007mg} using the \MEPSatLO
prescription~\cite{Hoeche:2011fd,Hoeche:2012yf,Catani:2001cc,Hoeche:2009rj}.
Samples were generated using the \NNPDF[3.0nnlo] PDF set~\cite{Ball:2014uwa}.
%along with the dedicated set of tuned parton-shower parameters
%developed by the \SHERPA authors.

\paragraph{Additional description:}

The ME+PS matching~\cite{Hoeche:2011fd} was employed for different jet
multiplicities which were then merged into an inclusive sample
using an improved CKKW matching
procedure~\cite{Catani:2001cc,Hoeche:2009rj}.  The calculation was
performed in the \(G_\mu\) scheme, ensuring an optimal description
of pure electroweak interactions at the electroweak scale.

\input{sherpa_scale_pdf.tex}
