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 pT’s of the individual photons (hardest, next-to hardest and softest) can be controlled independently. The remaining cut on Rγj, ηγ 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 pT, 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 pT’s of the individual photons (hardest, next-to hardest and softest) can be controlled independently. The remaining cut on Rγj, ηγ 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 pT, 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].