Process 289 represents the production of four real photons. The cross section diverges when one of the photons is very soft or in the direction of the beam. Thus in order to produce sensible results, the input file must supply values for both ptmin_photon and etamax_photon. This will ensure that the cross section is well-defined.

The calculation of process 289 may be performed at NLO using either the
Frixione algorithm [1] or standard cone isolation. The production of a photon
via parton fragmentation is included at NLO and can be run separately by
using the frag option in part. This option includes the contributions from
the integrated photon dipole subtraction terms and the LO QCD matrix
element multiplied by the fragmentation function. The phase space cuts
for the final state photons are defined in input.ini, for multiple photon
processes such as 285 - 289 the p_{T}’s of the individual photons (hardest,
next-to hardest and softest) can be controlled independently. The remaining
cut on R_{γj}, η_{γ} etc. are applied universally to all photons. Users wishing
to alter this feature should edit the file photon_cuts.f in the directory
src/User.

Note that for this process the second softest and softest photons are forced to
have equal minimum p_{T}, defined by the [ptmin_photon(3rd)] variable in the input
file.

The calculation is described in Ref. [2].

Process 289 represents the production of four real photons. The cross section diverges when one of the photons is very soft or in the direction of the beam. Thus in order to produce sensible results, the input file must supply values for both gammptmin and gammrapmax. This will ensure that the cross section is well-defined.

The calculation of process 289 may be performed at NLO using either the
Frixione algorithm [1] or standard cone isolation. The production of a photon
via parton fragmentation is included at NLO and can be run separately by
using the frag option in part. This option includes the contributions from
the integrated photon dipole subtraction terms and the LO QCD matrix
element multiplied by the fragmentation function. The phase space cuts
for the final state photons are defined in input.ini, for multiple photon
processes such as 285 - 289 the p_{T}’s of the individual photons (hardest,
next-to hardest and softest) can be controlled independently. The remaining
cut on R_{γj}, η_{γ} etc. are applied universally to all photons. Users wishing
to alter this feature should edit the file photon_cuts.f in the directory
src/User.

Note that for this process the second softest and softest photons are forced to
have equal minimum p_{T}, defined by the [gammptmin(3rd)] variable in the input
file.

nplotter_auto.f is the default plotting routine.

[1] S. Frixione, Isolated photons in perturbative QCD, Phys. Lett. B429 (1998) 369 [hep-ph/9801442].

[2] T. Dennen and C. Williams, γγγγ production at the LHC: An application of 2 → 4 analytic unitarity, Phys. Rev. D 91 (2015) 054012 [1411.3237].