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70 lines
3.6 KiB
70 lines
3.6 KiB
\midheading{Diphoton production}
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\label{subsec:gamgam}
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Process 285 represents the production of a pair of real photons. Since
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this process includes two real photons, the cross section diverges when
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one of the photons is very soft or in the direction of the beam. Thus in
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order to produce sensible results, the input file must supply values for
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both ptmin$_{photon}$ and etamax$_{photon}$. This will ensure that the cross
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section is well-defined.
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The calculation of process 285 may be performed using either the
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Frixione algorithm or standard cone isolation. Since version 10.1 also a
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fixed cone size can be specificed as well as a simple hybrid cone
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isolation, see ref.~\cite{Neumann:2021zkb}.
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This process also includes the one-loop gluon-gluon contribution as
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given in ref.~\cite{Bern:2002jx}. The
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production of a photon via parton fragmentation is included at NLO and
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can be run separately by using the frag option in part. This option
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includes the contributions from the integrated photon dipole subtraction
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terms and the LO QCD matrix element multiplied by the fragmentation
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function.
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The phase space cuts for the final state photons are defined in
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{\tt{input.ini}}, for multiple photon processes such as {\tt 285 -
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287} the $p_T$'s of the individual photons (hardest, second hardest
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and third hardest or softer) can be controlled independently.The
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remaining cuts on $R_{\gamma j}$, $\eta_{\gamma}$ etc. are applied
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universally to all photons. Users wishing to alter this feature should
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edit the file {\tt{photon\_cuts.f}} in the directory {\tt{src/User}}.
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This process can be calculated at LO, NLO, and NNLO.
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NLO calculations can be performed by subtraction, zero-jettiness slicing and $q_T$-slicing.
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NNLO calculations can be performed by zero-jettiness slicing and $q_T$-slicing.
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Input files for these 6 possibilities are given in the link below.
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The fixed-order NNLO calculation has been implemented in ref.
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\cite{Campbell:2016yrh}. Transverse
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momentum resummation at the level of $\text{N}^3\text{LL}+\text{NNLO}$
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has been implemented in ref. \cite{Becher:2020ugp}. By including the three-loop hard
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\cite{Caola:2020dfu} and beam functions
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\cite{Luo:2020epw},\cite{Ebert:2020yqt},\cite{Luo:2019szz} it has been upgraded to $\text{N}^3\text{LL}'$ in ref. \cite{Neumann:2021zkb}.
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\subsection{Transverse momentum resummation}
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Transverse momentum resummation can be enabled for process {\tt 285} at
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highest order $\text{N}^3\text{LL}'+\text{NNLO}$ with `part=resNNLOp`.
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The setting `part=resNNLO` resums to order
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$\text{N}^3\text{LL}+\text{NNLO}$ ($\alpha_s^2$ accuracy in improved
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perturbation theory power counting) and `part=resNLO` to order
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$\text{N}^3\text{LL}+\text{NLO}$. Note that process 285 with resummation
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only includes the $q\bar{q}$ channel. The $gg$ channel enters at an
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increased relative level of $\alpha_s$, so has to be added with process
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number 2851 at order `part=resNLO` for overall
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$\text{N}^3\text{LL}'+\text{NNLO}$ precision. For an overall consistent
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precision of $\text{N}^3\text{LL}+\text{NNLO}$ the $gg$ channel can be
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added with `part=resLO`.
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Note that at fixed-order the $gg$ channel is included at NNLO
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automatically at the level of $\alpha_s^2$.
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The fixed-order NNLO calculation has been implemented in ref.
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\cite{Campbell:2016yrh}. Transverse
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momentum resummation at the level of N$^3$LL+NNLO
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has been implemented in ref. \cite{Becher:2020ugp}. By including the three-loop hard
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\cite{Caola:2020dfu} and beam functions
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\cite{Luo:2020epw},\cite{Ebert:2020yqt},\cite{Luo:2019szz} it has been upgraded to N$^3$LL' in ref. \cite{Neumann:2021zkb}.
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