MadGraph5_aMC@NLO

Difference Madgraph vs Madspin

Asked by Till Arndt

Dear Madgraph Developers,

I have got a question concerning the decay of particles with Madspin compared to the decay with
just Madgraph.
I am generating the process p p > t t~ gamma (t >w+ b, w+ > all all, t~ > w- b~ , w-> all all ).
I generated the decay process both with just Madgraph and with the Madspin module.
There are significant differences in the kinematic distributions of the generated particles.
Especially in the pt of the photon and the pt of the decay products of the W boson (for example a quark)
there are differences in the lower pt ranges.
Is this behavior to be expected ?
If yes, which of the two versions should be the more accurate simulation ?
This is quite important for my analysis, as it strongly depends on the photon pt.

Thanks in advance for your answer.

Till Arndt

Question information

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MadGraph5_aMC@NLO Edit question
Assignee:
Pierre Artoisenet Edit question
Solved by:
Pierre Artoisenet
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Revision history for this message
Pierre Artoisenet (partois) said : 		#1

Dear Till Arndt,

I think differences are expected if you look at the pT distribution of the photon in the region close to the cut.
The reason is that the reshuffling procedure in MadSpin will smear at the edge of the distribution, such that some
events will be shifted to bins below the value of the cut.

If you generate the events at leading order, there is basically no reason to use MadSpin in your case
-> just use MadGraph with the decay chain syntax.

Cheers,
Pierre

Revision history for this message
Till Arndt (n-arndt) said : 		#2

Dear Pierre,

Thank you for your quick answer, but I have a follow up question.
So if I understand you correctly Madgraph does not handle the Spin correlations in
decays correctly, but Madspin does ?

Cheers
Till
________________________________________
Von: <email address hidden> [<email address hidden>]&quot; im Auftrag von &quot;Pierre Artoisenet [<email address hidden>]
Gesendet: Dienstag, 21. Januar 2014 15:36
An: Arndt, Till
Betreff: Re: [Question #242455]: Difference Madgraph vs Madspin

Your question #242455 on MadGraph5_aMC@NLO changed:
https://answers.launchpad.net/madgraph5/+question/242455

    Status: Open => Answered

Pierre Artoisenet proposed the following answer:
Dear Till Arndt,

I think differences are expected if you look at the pT distribution of the photon in the region close to the cut.
The reason is that the reshuffling procedure in MadSpin will smear at the edge of the distribution, such that some
events will be shifted to bins below the value of the cut.

If you generate the events at leading order, there is basically no reason to use MadSpin in your case
-> just use MadGraph with the decay chain syntax.

Cheers,
Pierre

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Revision history for this message
Best Pierre Artoisenet (partois) said : 		#3

Hi Till,

 Actually madgraph includes all spin correlations if you use the decay chain syntax. It is more accurate than
MadSpin as it also includes all finite width effects. The main virtue of MadSpin is that is it may be the only
viable option to generate events at NLO while keeping spin correlation effects.

Cheers,
Pierre

Revision history for this message
Till Arndt (n-arndt) said : 		#4

Thanks Pierre Artoisenet, that solved my question.