MadGraph5_aMC@NLO

propagation of beam polarization setting to matrix.f

Asked by Bernd Stelzer

Hello,

A bit of a more technical question about the beam polarization setting and how it is propagated into the matrix.f ?

=> To explain: If I follow the MadGraph code for this setting, the configuration is done in the run_card.dat

#*********************************************************************
# Beam polarization from -100 (left-handed) to 100 (right-handed) *
#*********************************************************************
      0 = polbeam1 ! beam polarization for beam 1
      0 = polbeam2 ! beam polarization for beam 2

=> But then In setrun.f (first POL is set to 1)

C BEAM POLARIZATION
    REAL*8 POL(2)
    common/to_polarization/ POL
    data POL/1d0,1d0/

=> Then some logic for non-zero pol (does the common block for POL get updated?)
c************************************************************************
c Beam polarization
c************************************************************************
    call get_real (npara,param,value," polbeam1 " ,pb1,0d0)
    call get_real (npara,param,value," polbeam2 " ,pb2,0d0)

    if(pb1.ne.0d0.and.lp1.eq.0) pol(1)=sign(1+abs(pb1)/100d0,pb1)
    if(pb2.ne.0d0.and.lp2.eq.0) pol(2)=sign(1+abs(pb2)/100d0,pb2)

    if(pb1.ne.0.or.pb2.ne.0) write(*,*) 'Setting beam polarization ',
   $ sign((abs(pol(1))-1)*100,pol(1)),
   $ sign((abs(pol(2))-1)*100,pol(2))

=> I have now verified that polbeam1=polbeam2=0 in run_card.dat corresponds to POL(1)=POL(2)=1 in matrix.f
=> Is that intended?

If I use check_sa.f then POL(1)=POL(2)=0. I guess that is due to (as Johan explained) that check_sa.f
doesn't care about the run_card.dat. So I guess I am supposed to set it to POL(1)=POL(2)=1 for unpolarized beam?
Just seems a bit inconsistent in the definition? Thanks for your insight?
- Bernd

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Johan Alwall (johan-alwall) said : 		#1

Hello Berndt,

check_sa.f is intended to use with the standalone output mode, which doesn't use polarisation. In order to use it with madevent output, you will probably need to make quite a few additions. To answer your particular question, as you can see from the line you quote
pol(1)=sign(1+abs(pb1)/100d0,pb1)
etc,
the POL variable in madevent matrix.f goes from 0 to 2 (left-handed to right-handed). You can see in matrix.f how it is used.

All the best,
Johan

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