Currently, Marmoset can only handle 2→1 (Breit-Wigner), 2→2 (in a power series in X and ξ), and limited2→3 hard scattering matrix elements.
It is possible for the user to hard code new matrix elements as needed. The code that controls the matrix element definitions for 2→1, 2→2, and 2→3 processes is in Marmoset/src/pysgge.f
For 2→1 production, Marmoset allows for a Breit-Wigner resonance matrix elements. Note, however, that cos θ dependence between the beam and the resonance are lost. To implemente 2→1, define a particle
NR : charge=0 color=0 mass=900 width=25
If production is specified as,
u ubar > NR
Marmoset will produce NR through q qbar initial states with a Breit Wigner centered at mass=900 GeV with width=25 GeV.
There are a series of default 2→2 matrix elements available, based on a X and ξ series expansion. They are specified using matrix=i+10*j where the amplitude squared is
M2 = fi(X) ξj
and f1(X) = 1, f2(X)= (1-1/X) , f3(X)= (1-1/X)2, f4(X)= (X-1), f5(X)= (X-1)2.
For example,
g g > GL GL~ : matrix=32
means that Marmoset will pair produce GL using the gluon PDFs with a matrix element M2 = (1-1/X) ξ3. Recall that overall cross sections are determined after Monte Carlo generation, so the normalization of M2 is irrelevant. For the first pass, a constant matrix element matrix=1 is usually a good starting point.
2→3 is currently in it's testing stages…