Change the decay width of a vector boson

Dear Federico,

The width is fixed at zero at the model level. Even if you define a non zero-width in the param_card, the code is going to use zero-width since
the model does have any variable related to the width (i.e. the code computing the matrix-element has for each entry of the width an hardcoded value written to zero)
The solution is to go back to the way that you have created the model and allow the width to be different of zero.

Cheers,

Olivier

> On Nov 29, 2016, at 13:13, Federico Pobbe <email address hidden> wrote:
>
> New question #404486 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/404486
>
> I have to simulate events for a simple extension of the Standard Model. In my model I have an additional massive axial-vector boson coupled to a Majorana's spinor (which stands for a Dark Matter particle pair) and to the quarks. I generate events that can be tagged as 'mono-jet' with a process p p > x x j, where 'x' is the dark matter particle. The problem is that even if I changed the width of the vector boson in the param_card.dat (the vector is particle 80001):
>
> ######################################################################
> ## PARAM_CARD AUTOMATICALY GENERATED BY MG5 ####
> ######################################################################
> ###################################
> ## INFORMATION FOR BSM
> ###################################
> BLOCK BSM #
> 1 1.000000e+00 # gx
> 2 1.000000e+00 # gq
> 3 1.000000e+00 # sv
> 4 3.000000e+00 # r
> ###################################
> ## INFORMATION FOR CKMBLOCK
> ###################################
> BLOCK CKMBLOCK #
> 1 2.277360e-01 # cabi
> ###################################
> ## INFORMATION FOR MASS
> ###################################
> BLOCK MASS #
> 1 5.040000e-03 # md
> 2 2.550000e-03 # mu
> 3 1.010000e-01 # ms
> 4 1.270000e+00 # mc
> 5 4.700000e+00 # mb
> 6 1.720000e+02 # mt
> 11 5.110000e-04 # me
> 13 1.056600e-01 # mmu
> 15 1.777000e+00 # mta
> 23 9.118760e+01 # mz
> 25 1.250000e+02 # mh
> 80001 1.550000e+03 # mzp
> 80002 2.500000e+02 # mx
> 12 0.000000e+00 # ve : 0.0
> 14 0.000000e+00 # vm : 0.0
> 16 0.000000e+00 # vt : 0.0
> 21 0.000000e+00 # g : 0.0
> 22 0.000000e+00 # a : 0.0
> 24 7.982436e+01 # w+ : cmath.sqrt(mz__exp__2/2. + cmath.sqrt(mz__exp__4/4. - (aew*cmath.pi*mz__exp__2)/(gf*sqrt__2)))
> ###################################
> ## INFORMATION FOR SMINPUTS
> ###################################
> BLOCK SMINPUTS #
> 1 1.279000e+02 # aewm1
> 2 1.166370e-05 # gf
> 3 1.184000e-01 # as
> ###################################
> ## INFORMATION FOR YUKAWA
> ###################################
> BLOCK YUKAWA #
> 1 5.040000e-03 # ymdo
> 2 2.550000e-03 # ymup
> 3 1.010000e-01 # yms
> 4 1.270000e+00 # ymc
> 5 4.700000e+00 # ymb
> 6 1.720000e+02 # ymt
> 11 5.110000e-04 # yme
> 13 1.056600e-01 # ymm
> 15 1.777000e+00 # ymtau
> ###################################
> ## INFORMATION FOR DECAY
> ###################################
> BLOCK QNUMBERS 80001 # zp
> 1 0 # 3 times electric charge
> 2 3 # number of spin states (2s+1)
> 3 1 # colour rep (1: singlet, 3: triplet, 8: octet)
> 4 0 # particle/antiparticle distinction (0=own anti)
> ###################################
> ## INFORMATION FOR QNUMBERS 80002
> ###################################
> BLOCK QNUMBERS 80002 # x
> 1 0 # 3 times electric charge
> 2 2 # number of spin states (2s+1)
> 3 1 # colour rep (1: singlet, 3: triplet, 8: octet)
> 4 0 # particle/antiparticle distinction (0=own anti)
> #
> #*************************
> # Decay widths *
> #*************************
> #
> # PDG Width
> DECAY 1 0.000000e+00
> #
> # PDG Width
> DECAY 2 0.000000e+00
> #
> # PDG Width
> DECAY 3 0.000000e+00
> #
> # PDG Width
> DECAY 4 0.000000e+00
> #
> # PDG Width
> DECAY 5 0.000000e+00
> #
> # PDG Width
> DECAY 6 1.508336e+00
> #
> # PDG Width
> DECAY 11 0.000000e+00
> #
> # PDG Width
> DECAY 12 0.000000e+00
> #
> # PDG Width
> DECAY 13 0.000000e+00
> #
> # PDG Width
> DECAY 14 0.000000e+00
> #
> # PDG Width
> DECAY 15 0.000000e+00
> #
> # PDG Width
> DECAY 16 0.000000e+00
> #
> # PDG Width
> DECAY 21 0.000000e+00
> #
> # PDG Width
> DECAY 22 0.000000e+00
> #
> # PDG Width
> DECAY 23 2.495200e+00
> #
> # PDG Width
> DECAY 24 2.085000e+00
> #
> # PDG Width
> DECAY 25 4.070000e-03
> #
> # PDG Width
> DECAY 80001 8.007981e+02
> # BR NDA ID1 ID2 ...
> 1.540277e-01 2 2 -2 # 123.345080894
> 1.540277e-01 2 1 -1 # 123.345080888
> 1.540277e-01 2 3 -3 # 123.345077754
> 1.540271e-01 2 4 -4 # 123.344584056
> 1.540192e-01 2 5 -5 # 123.338276315
> 1.427890e-01 2 6 -6 # 114.345127569
> 8.708175e-02 2 80002 80002 # 69.7349061515
> #
> # PDG Width
> DECAY 80002 0.000000e+00
>
> The cross section diverges and the number of events is nearly zero. I also notice that when I type 'display particles 80001' what I get is:
>
> {
> 'name': 'zp',
> 'antiname': 'zp',
> 'spin': 3,
> 'color': 1,
> 'charge': 0.00,
> 'mass': 'mdl_MZp',
> 'width': 'ZERO',
> 'pdg_code': 80001,
> 'texname': 'Zp',
> 'antitexname': 'Zp',
> 'line': 'wavy',
> 'propagating': True,
> 'propagator': '',
> 'is_part': True,
> 'self_antipart': True,
> 'ghost': False,
> 'counterterm': {}
> }
>
> and the width is still set to 'ZERO'. How can I solve this problem?
>
>
>
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