How to implement new cuts

For version before 3.5.0, see https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-1

Since 3.5.0:
1: You can prepare a file somewhere in your filesystem containing the fortran definition of your cuts (see below for LO and NLO template)
2: You can pass the path to that file within the run_card.dat within the custom_fcts entry of the run_card (this entry expects a list of path), i.e. with the following line:

['/home/users/.../mycustomfile.txt'] = custom_fcts

3: If your function needs extra parameter to be controlled directly via the run_card by adding a line in the run_card like "10.0 = min_newvar". Your fortran function could then use the parameter "min_newvar" as long as you keep the "include run.inc"

Remember that madgraph works in the center of mass of the parton and that he is using symmetry to speed-up the code.
(including flavor, beam symmetry, identical particle,...) Therefore, not all cuts are possible to implement within the code.

==================
Template for LO cuts:
==================

      logical FUNCTION dummy_cuts(P)
C**************************************************************************
C INPUT:
C P(0:3,1) MOMENTUM OF INCOMING PARTON (partonic center of mass)
C P(0:3,2) MOMENTUM OF INCOMING PARTON
C P(0:3,3) MOMENTUM OF ...
C ALL MOMENTA ARE IN THE REST FRAME of the parton
C OUTPUT:
C TRUE IF EVENTS PASSES ALL CUTS LISTED
C**************************************************************************
C For the demo: implement pt cut on particle 3 and 4 via custom entry of the run_card
C my_param and my_param2
      IMPLICIT NONE
c
c Constants
c
      include 'genps.inc'
      include 'nexternal.inc'
      include 'run.inc'
C
C ARGUMENTS
C
      REAL*8 P(0:3,nexternal)
C
C PARAMETERS
C
      real*8 PI
      parameter( PI = 3.14159265358979323846d0 )
c
c particle identification
c
      LOGICAL IS_A_J(NEXTERNAL),IS_A_L(NEXTERNAL)
      LOGICAL IS_A_B(NEXTERNAL),IS_A_A(NEXTERNAL),IS_A_ONIUM(NEXTERNAL)
      LOGICAL IS_A_NU(NEXTERNAL),IS_HEAVY(NEXTERNAL)
      logical do_cuts(nexternal)
      COMMON /TO_SPECISA/IS_A_J,IS_A_A,IS_A_L,IS_A_B,IS_A_NU,IS_HEAVY,
     . IS_A_ONIUM, do_cuts

      double precision pt
      dummy_cuts=.true.
      if (pt(P(0,3)).lt.my_param)then
        dummy_cuts=.false.
        return
      endif
      if (pt(P(0,4)).lt.my_param2)then
        dummy_cuts=.false.
        return
      endif

==================
Template for NLO cuts:
==================

c NOTE THAT ONLY IRC-SAFE CUTS CAN BE APPLIED OTHERWISE THE INTEGRATION
c MIGHT NOT CONVERGE
c
      logical function dummy_cuts(p,istatus,ipdg)
      implicit none
c This includes the 'nexternal' parameter that labels the number of
c particles in the (n+1)-body process
      include 'nexternal.inc'
c This include file contains common blocks filled with the cuts defined
c in the run_card.dat (including custom set entry)
      include 'run.inc'
      include 'cuts.inc'
c
c This is an array which is '-1' for initial state and '1' for final
c state particles
      integer istatus(nexternal)
c This is an array with (simplified) PDG codes for the particles. Note
c that channels that are combined (i.e. they have the same matrix
c elements) are given only 1 set of PDG codes. This means, e.g., that
c when using a 5-flavour scheme calculation (massless b quark), no
c b-tagging can be applied.
      integer iPDG(nexternal)
c The array of the momenta and masses of the initial and final state
c particles in the lab frame. The format is "E, px, py, pz, mass", while
c the second dimension loops over the particles in the process. Note
c that these are the (n+1)-body particles; for the n-body there is one
c momenta equal to all zero's (this is not necessarily the last particle
c in the list). If one uses IR-safe obserables only, there should be no
c difficulty in using this.
      double precision p(0:4,nexternal)
c
C external functions that can be used. Some are defined in cuts.f
C others are in ./Source/kin_functions.f
      REAL*8 R2_04,invm2_04,pt_04,eta_04,pt,eta
      external R2_04,invm2_04,pt_04,eta_04,pt,eta

      dummy_cuts = .true. ! event is okay; otherwise it is changed
c$$$C EXAMPLE: cut on top quark pT
c$$$C Note that PDG specific cut are more optimised than simple user cut
c$$$ do i=1,nexternal ! loop over all external particles
c$$$ if (istatus(i).eq.1 ! final state particle
c$$$ & .and. abs(ipdg(i)).eq.6) then ! top quark
c$$$C apply the pT cut (pT should be large than 200 GeV for the event to
c$$$C pass cuts)
c$$$ if ( p(1,i)**2+p(2,i)**2 .lt. 200d0**2 ) then
c$$$C momenta do not pass cuts. Set passcuts_user to false and return
c$$$ dummy_cuts=.false.
c$$$ return
c$$$ endif
c$$$ endif
c$$$ enddo
c
      return
      end