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21 lines
1.3 KiB
21 lines
1.3 KiB
\midheading{$\gamma\gamma\gamma$ production, process 287}
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\label{subsec:trigam}
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Process {\tt 287} represents the production of three real photons.
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The cross section diverges
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when one of the photons is very soft or in the direction of the beam.
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Thus in order to produce sensible results, the input file must supply values for both
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{\tt gammptmin} and {\tt gammrapmax}. This will ensure that
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the cross section is well-defined.
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The calculation of process {\tt 285} may be performed at NLO using either the
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Frixione algorithm~\cite{Frixione:1998jh} or standard cone isolation. The production of a photon via parton
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fragmentation is included at NLO and
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can be run separately by using the {\tt frag} option in {\tt part}. This option includes the contributions from the
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integrated
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photon dipole subtraction terms and the LO QCD matrix element multiplied by the fragmentation function.
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The phase space cuts for the final state photons are defined in {\tt{input.ini}}, for multiple photon processes such
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as {\tt 285 - 287} the $p_T$'s of the individual photons (hardest, next-to hardest and softest) can be controlled
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independently.
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The remaining cut on $R_{\gamma j}$, $\eta_{\gamma}$ etc. are applied universally to all photons. Users wishing to alter
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this feature should edit the file {\tt{photon\_cuts.f}} in the directory {\tt{src/User}}.
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