MadGraph5_aMC@NLO

'p p > z > e+ e-' vs decay syntax

Asked by Mario Alberto Werle Barela

I'm sure that I'm missing something embarassingly simple (and possibly purely physical), but I cannot figure out what is going on.

I want to understand why the following (most simple of) tests

(i) generate p p > z > e+ e- ### 639 pb

(ii) generate p p > z, z > e+ e- ### 1420 pb

give incompatible results.

In my understanding, (i) only selects every diagram with an s-channel Z (taking advantage of the entire phase space, i.e., on and off-shell Z), while (ii) puts the Z on-shell.

The result, if different at all, should be larger for (i).

I don't think the disparity could be explained by the breaking of the Narrow Width Approximation, too.

What is happening in this simple case? Again, I apologize if I ask something trivial.

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

What is the value of your cuts? and in particular the value of cut_decays in your case?

Cheers,

Olivier

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Mario Alberto Werle Barela (mariobarela) said : 		#2

Isn't cut_decays a parameter of MadSpin only? I am not using it, but only the syntax of pure MadGraph.

In any case, all the cuts are the standard ones (I have not changed anything in the cards).

Thank you!

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Olivier Mattelaer (olivier-mattelaer) said : 		#3

No cut_decays are not part of MadSpin (madspin never applies cut, this is (one) reason to have cut_decay within the run_card to prevent (most) cut on the decay and be comparable to madspin (and/or to branching ratio computation).

Cheers,

Olivier

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Mario Alberto Werle Barela (mariobarela) said : 		#4

Oh, I'm sorry for the confusion, I had looked at the wrong run card.

Indeed, `cut_decays` is set to `False` and the bwcut is set at the standard 15.

I also tested implementing the decay through madspin to obtain a similar ~1420 pb cross section, more than twice of that from p p > z > e+ e-.

What can be causing this?

Thank you!

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Olivier Mattelaer (olivier-mattelaer) said : 		#5

That's the impact of the cut, for your first syntax, you do have cuts on the lepton (by default) while in the second syntax (and when using madspin), you do not have cuts.

Cheers,

Olivier

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Mario Alberto Werle Barela (mariobarela) said : 		#6

Ohhhhhhhhhh! Of course! I'm sorry for missing the meaning of the cut_decays parameter.

Thank you very much!

One more question, if you can help me.

Why doesn't the syntax

p p > mu+ mu- mu+ mu-

include the contributions given by t-channel fermion exchange followed by production of a pair of virtual Z/gamma with following decay to 4mu?

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Best Olivier Mattelaer (olivier-mattelaer) said : 		#7

I do see such diagram (around diagram number 30).
If you check the diagram from the web page, not all of them are included since generating diagram as jpeg is super slow. The printable format eps (not supported on mac anymore) has all of them.

Cheers,

Olivier

> On 19 Oct 2023, at 03:25, Mario Alberto Werle Barela <email address hidden> wrote:
>
> p p > mu+ mu- mu+ mu-

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Mario Alberto Werle Barela (mariobarela) said : 		#8

Great!

Thank you very much!

Best,
Mario

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Mario Alberto Werle Barela (mariobarela) said : 		#9

Thanks Olivier Mattelaer, that solved my question.