MadGraph5_aMC@NLO interface

Added in version 1.2.0.

A CepGen interface to the MadGraph5_aMC@NLO (here abbreviated as MG5_aMC) matrix element framework described in [AFF+14] was developed to generalise the two-photon (and central exclusive) processes production to advanced final states and interactions.

It relies on the standalone_cpp output mode introduced in version 2.9.10 to be used as an extension to Pythia 8.

The CepGen addon module, CepGenMadGraph requires a valid installation of MG5_aMC in the search path (MADGRAPH_DIR environment variable) at configuration/build time. Once loaded in the runtime environment manager, the mg5_aMC process type is handled with its standard parameters (see this summary page for a detailed description of these parameters).

For instance, the generation of a \(\ggll\) process can be steered as:

process = cepgen.Module('mg5_aMC',
	processParameters = cepgen.Parameters(
		model = 'sm-full',
		process = 'a a > mu+ mu-',
		mode = cepgen.ProcessMode.ElasticElastic
	),
	inKinematics = cepgen.Parameters(
		pdgIds = (PDG.proton, PDG.proton),
		sqrtS = 13.6e3  # in GeV
	),
	outKinematics = cepgen.Parameters(
		qt = (0., 10.),
		pt = (25.,)
	)
)

As seen above, the process.processParameters.process and process.processParameters.model strings are handling the MG5_aMC process to be generated, and the physics model considered in the operation.

The addon will directly call the MG5_aMC executable to produce a CepGen-compatible runtime library through the specialisation of a cepgen::MadGraphProcess interface object. Therefore, a MadGraph-generated process shared library can be shipped by processes developers and used with any compatible version of CepGen to compute a cross section or generate events according to kinematic constraints.

As for any factorised process (as pptoff and pptoww), both \(\kt\)-dependent, or collinear fluxes can be convoluted to the central matrix element to obtain the total beam-level cross section and event definition.