MCFM-10.4/Doc/sections/gobbets/gobbet1610.tex

\midheading{Single-top-quark production and decay at NNLO, process 1610}
\label{single-top-quark-production-and-decay-at-nnlo}

This calculation is based on ref.~\cite{Campbell:2020fhf}. See also
ref.~\cite{Campbell:2021qgd} for the role of double-DIS scales and the
relevancy for PDFs.

This process can be run by using process number 1610. The resulting
histograms and cross-sections are printed for a strict fixed-order
expansion as well as for a naive addition of all contributions. The
fixed-order expansion assembles pieces according to the following
formula. Please see ref.~\cite{Campbell:2020fhf} for more details.

\[\begin{aligned}
\mathrm{d}\sigma_{\text{LO}} & = \frac{1}{\Gamma_t^{(0)}}
\mathrm{d}\sigma^{(0)}\otimes\mathrm{d}\Gamma_t^{(0)} \,,\\
\mathrm{d}\sigma_{\delta \text{NLO}} & = \frac{1}{\Gamma_t^{(0)}} \Bigg[ 
\mathrm{d}\sigma^{(1)}\otimes\mathrm{d}\Gamma_t^{(0)} + 
\mathrm{d}\sigma^{(0)}\otimes\left(\mathrm{d}\Gamma_t^{(1)} 
- \frac{\Gamma_t^{(1)}}{\Gamma_t^{(0)}} \mathrm{d}\Gamma_t^{(0)}  \right)
\Bigg] \,,\\
\mathrm{d}\sigma_{\delta \text{NNLO}} & = \frac{1}{\Gamma_t^{(0)}} \Bigg[
\mathrm{d}\sigma^{(2)}\otimes\mathrm{d}\Gamma_t^{(0)} + 
\mathrm{d}\sigma^{(1)}\otimes\left(
\mathrm{d}\Gamma_t^{(1)} - 
\right) \\
& \qquad\qquad  + \mathrm{d}\sigma^{(0)}\otimes \left(
\mathrm{d}\Gamma_t^{(2)} - 
\frac{\Gamma_t^{(2)}}{\Gamma_t^{(0)}}\mathrm{d}\Gamma_t^{(0)}
-\frac{\Gamma_t^{(1)}}{\Gamma_t^{(0)}} \left(
\mathrm{d}\Gamma_t^{(1)} - \frac{\Gamma_t^{(1)}}{\Gamma_t^{(0)}}
\mathrm{d}\Gamma_t^{(0)}
\right)
\right) \Bigg] \,.
\end{aligned}\]

At each order a corresponding top-decay width is used throughout all
parts. The NNLO width is obtained from ref.~\cite{Blokland:2005vq} and at
LO and NLO from ref.~\cite{Czarnecki:1990kv}. These widths agree with
numerical results obtained from our calculation of course.

This process can be run with a fixed scale or with dynamic DIS (DDIS)
scales by setting \texttt{dynamicscale\ =\ DDIS},
\texttt{renscale\ =\ 1.0} and \texttt{facscale\ =\ 1.0}.

At NNLO there are several different contributions from vertex
corrections on the light-quark line, heavy-quark line in production, and
heavy-quark line in the top-quark decay. Additionally there are one-loop
times one-loop interference contributions between all three
contributions. These contributions can be separately enabled in the
\texttt{singletop} block:

\begin{verbatim}
[singletop]
    nnlo_enable_light = .true.
    nnlo_enable_heavy_prod = .true.
    nnlo_enable_heavy_decay = .true.
    nnlo_enable_interf_lxh = .true.
    nnlo_enable_interf_lxd = .true.
    nnlo_enable_interf_hxd = .true.
    nnlo_fully_inclusive = .false.
\end{verbatim}

For a fully inclusive calculation without decay the last setting has to
be set to \texttt{.true.} and the decay and decay interference parts
have to be removed. Additionally jet requirements must be lifted, see
below.

When scale variation is enabled with DDIS scales then automatically also
a variation around the fixed scale \(\mu=m_t\) is calculated for
comparison.

This process uses a fixed diagonal CKM matrix with
\(V_{ud}=V_{cs}=V_{tb}=1\). The setting \texttt{removebr=.true.} removes
the \(W\to \nu e\) branching ratio.

This process involves complicated phase-space integrals and we have
pre-set the initial integration calls for precise differential
cross-sections with fiducial cuts. The number of calls can be tuned
overall with the multiplier setting
\texttt{integration\%globalcallmult}. For total fully inclusive
cross-sections the number of calls can be reduced by a factor of ten by
setting \texttt{integration\%globalcallmult\ =\ 0.1}, for example.

For scale variation uncertainties and PDF uncertainties we recommend to
start with the default number of calls and a larger number of warmup
iterations \texttt{integration\%iterbatchwarmup=10}, for example. For
the warmup grid no scale variation or PDF uncertainties are calculated
and this ensures a good Vegas integration grid that can be calculated
fast. The setting \texttt{integration\%callboost} modifies the number of
calls for subsequent integration iterations after the warmup. For
example setting it to \texttt{0.1} reduces the calls by a factor of ten.
This is typically enough to compute the correlated uncertainties for a
previously precisely determined central value.

At NNLO the default value for \(\tau_{\text{cut}}\) is \(10^{-3}\), which
is the value used for all the plots in our publication. We find that
cutoff effects are negligible at the sub-permille level for this choice.
We strongly recommend to not change this value.

\paragraph{Using the plotting routine with b-quark
tagging}\label{using-the-plotting-routine-with-b-quark-tagging}

The calculation has been set up with b-quark tagging capabilities that
can be accessed in both the \texttt{gencuts\_user.f90} routine and the
plotting routine \texttt{nplotter\_singletop\_new.f90}. The plotting
routine is prepared to generate all histograms shown in our publication
in ref.~\cite{Campbell:2020fhf}. By default the top-quark is
reconstructed using the leading b-quark jet and the exact W-boson
momentum, but any reconstruction algorithm can easily be implemented.

The version of the \texttt{gencuts\_user.f90} file
used for the plots in our paper~\cite{Campbell:2020fhf} is available as
\href{\mcfmprocs/Files1610/gencuts_user_singletop_nnlo.f90}{{\tt gencuts\_user\_singletop\_nnlo.f90}}.
It can be used as a guide on how to access the b-quark tagging in the \texttt{gencuts\_user} routine.

See also \texttt{nplotter\_ktopanom.f} (used for the NLO off-shell
calculation in ref.~\cite{Neumann:2019kvk} for a reconstruction of the
W-boson. It is based on requiring an on-shell W-boson and selecting the
solution for the neutrino $z$-component that gives the closest on-shell
top-quark mass by adding the leading b-quark jet.

\paragraph{Calculating fully inclusive
cross-sections}\label{calculating-fully-inclusive-cross-sections}

When calculating a fully inclusive cross-section without top-quark decay
please set \texttt{zerowidth\ =\ .true.},
\texttt{removebr\ =\ .true.} in the general section of the input file;
\texttt{inclusive\ =\ .true}, \texttt{ptjetmin\ =\ 0.0},
\texttt{etajetmax\ =\ 99.0} in the basicjets section;
\texttt{makecuts\ =\ .false.} in the cuts section; also set
\texttt{nnlo\_enable\_heavy\_decay\ =\ .false.} and
\texttt{nnlo\_enable\_interf\_lxd\ =\ .false.},
\texttt{nnlo\_enable\_interf\_hxd\ =\ .false.} and
\texttt{nnlo\_fully\_inclusive\ =\ .true.} in the singletop section.

These settings ensure that neither the decay nor any production times
decay interference contributions are included. The last setting makes
sure that only the right pieces in the fixed-order expansion of the
cross-section are included. It also ensures that the b-quark from the
top-quark decay is not jet-tagged and just integrated over.

\paragraph{Notes on runtimes and demo files}\label{notes-on-runtimes-and-demo-files}

Running the provided input file \\
\texttt{input\_singletop\_nnlo\_Tevatron\_total.ini} with
-integration\%globalcallmult=0.1 and without histograms takes about 4-5
CPU days. So depending on the number of cores, this can be run on a
single desktop within a few hours.

Running the input file \\
\texttt{input\_singletop\_nnlo\_LHC\_fiducial.ini} with the default set
of calls and histograms takes about 3 CPU months (about 3 wall-time
hours on our cluster with 45 nodes). For the fiducial cross-section
(without precise histograms) a setting of
\texttt{-integration\%globallcallmult=0.2} can also be used.

Note that \texttt{-extra\%nohistograms\ =\ .true.} has been set in these
demonstration files, so no further histograms from
\texttt{nplotter\_singletop\_new.f90} are generated.

The input file \href{\mcfmprocs/Files1610/input_singletop_nnlo_LHC_fiducial.ini}{{\tt input\_singletop\_nnlo\_LHC\_fiducial.ini}} 
together with the file \\
\href{\mcfmprocs/Files1610/gencuts_user_singletop_nnlo.f90}{{\tt gencuts\_user\_singletop\_nnlo.f90}}.
replacing \texttt{src/User/gencuts\_user.f90} reproduces the fiducial
cross-sections in ref.~\cite{Campbell:2020fhf} table 6.