lhe file lists particles not present in the event

Hi,

> I use a very large value of bwcutoff as advised by you on an earlier
> occasion (https://answers.launchpad.net/madgraph5/+question/236617).

The only reason to use large value of bwcutoff is for the comparison of production and decay cross-section.
But a value of 15 is the recommended value for this parameter.

> If I use the default value of bwcutoff (bwcutoff=15), I get some events
> that have momentum information for 4 particles but that I get even for the SM
> case which is due to the fact that "when a particle is consider to be off-shell by MG,
> the particle is not written in the lhe file" (also clarified earlier by you in
> https://answers.launchpad.net/madgraph5/+question/236617 [#8]).
>
> I shall be grateful if you could help me to understand what is happening.

The problem is that we don't have a fast way to deal with 4 point interactions. So by default in presence of 3 and 4 point interactions,
we only integrate the phase-space according to the 3 point interactions. (This is JUST a technical way to perform the integration, which
is 100% accurate). So indeed 100% of the events are generated with a W and are associate to a W. We therefore rely on the bwcutoff
in order to populate the 4 point interactions diagram in the lhe file. Since you put a large value for this parameter, you forbid this effect.

If you want something smarter than the bwcutoff efect, the only solution that I can propose is the following.
1) use the reweighing module to compute the weight of the events for the two following matrix element:
- 4 point only
- W only
2) Then for each event, you can launch a random number and choose accordingly if you want to write the W or not.
This methods obviously forgets about interference effect so this is not perfect either (No method is perfect for such kind of unphysical game).

Cheers,

Olivier

On Dec 19, 2013, at 11:11 PM, Pratishruti Saha <email address hidden> wrote:

> New question #241048 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/madgraph5/+question/241048
>
> Dear MadGraph Team,
>
> I am using MadGraph 5 to study the process t > b i j, where i & j are
> a pair of Standard Model (SM) (anti-)fermions that the W+ can decay into.
>
> In trying to incorporate new physics (NP) contributions in this, we have
> introduced some 4-fermion operators coupling t, b, i & j (the model files for this
> were obtained using FeynRules).
>
> Now the top can decay into this final state via two modes - one involving an
> intermediate W+ (the SM mode) and the other involving the 4-fermion coupling.
> In the event (.lhe) file, therefore, I expect to find two kinds of events -
> a) those with momentum information for 5 particles (the SM mode)
> and
> b) those with momentum information for 4 particles (the NP mode).
> However, I find that all the events listed, in fact, contain information about
> 5 particles with the parent of the particles i & j always being W+.
>
> The Feynman diagrams generated by MadGraph are fine, the total width comes
> out to be different for the SM & NP cases, the distributions of various observables
> are different for the two cases, but the event listing shows this strange behaviour.
>
> The run_card & param_card that I use may be described as follows :
>
> Run Card :
> The default run card with the following changes :
> =================================================================
> T = fixed_ren_scale ! if .true. use fixed ren scale
> T = fixed_fac_scale ! if .true. use fixed fac scale
> 173.000 = scale ! fixed ren scale
> 173.000 = dsqrt_q2fact1 ! fixed fact scale for pdf1
> 173.000 = dsqrt_q2fact2 ! fixed fact scale for pdf2
>
> 10000 = bwcutoff ! (M+/-bwcutoff*Gamma)
>
> F = cut_decays ! Cut decay products
> =================================================================
>
> Param Card :
> The default param card obtained for the model.
> =================================================================
> SM => all NP couplings = 0.0
> NP => all or some NP couplings non-zero.
> =================================================================
>
> I use a very large value of bwcutoff as advised by you on an earlier
> occassion (https://answers.launchpad.net/madgraph5/+question/236617).
>
> If I use the default value of bwcutoff (bwcutoff=15), I get some events
> that have momentum information for 4 particles but that I get even for the SM
> case which is due to the fact that "when a particle is consider to be off-shell by MG,
> the particle is not written in the lhe file" (also clarified earlier by you in
> https://answers.launchpad.net/madgraph5/+question/236617 [#8]).
>
> I shall be grateful if you could help me to understand what is happening.
>
>
> regards,
> Pratishruti
>
> --
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