MadGraph5_aMC@NLO

Syntax for Spin Correlations

Asked by Matthew Low

Hi,

I'm interested in generating p p > t t~ with the full spin correlations between t and t~. I would like for both t and t~ to decay leptonically and then use the angles of the leptons to find the spin correlations. Which of the following will give me the correct spin correlations?

a) p p > t t~ , t > b l+ vl , t~ > b~ l vl
b) p p > b l+ vl b~ l vl
c) p p > t t~ , then use MadSpin for decays

A colleague of mine has compared these in a different process and found the different methods give different answers.

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Sihyun Jeon (shjeon) said (last edit ): 		#1

They should give spin correlations modeled properly as far as I know if for c) you just used default spinmode.

However b) might have p p > w+ w- b b~ where b b~ is just from g decays I think so comparing with b) could be different (or even you might get p p > z z b b~).
a) and c) should be the same if there are no cuts are not applied

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Olivier Mattelaer (olivier-mattelaer) said : 		#2

Just To be more complete,

If you use dynamical_scale_choice=-1 (the default) the choice of the scale will likely be different for a and c
which can lead to quite some difference (but within theoretical uncertainty obviously).
Also even if you use a fix scale to avoid such difference, a and c will not compute the cross-section in the same way and you should expect a difference of the order of Width/Mass.

Cheers,

Olivier

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Matthew Low (matthewlow) said : 		#3

Hi Olivier and Sihyun,

Thanks, this is very helpful. If b contains the same diagrams as a, should it give the same result as a?

Did a always maintain spin correlations or is that a newer feature? Since a does keep spin correlations, what is the purpose of having MadSpin?

Thanks for the helpful responses!

Best,
- Matt

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Sihyun Jeon (shjeon) said : 		#4

1. I would say there could be some level of differences but hard to quantify. You would start getting offshell top pair productions with that syntax so kinematics could be different but not sure.

2. a) always maintained correlation. MadSpin is particularly useful for NLO cases, where you should end up writing "generate p p > t t~ [QCD]" and cannot decay top right away. For LO it depends on use cases I would say, but if you introduce jet merging "generate p p > t t~ (with additional j)", using MadSpin seemed to be faster based on my experience (but also depends on how many j you add.

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Matthew Low (matthewlow) said : 		#5

Thanks again! Which of a,b,c do you recommend I use to most accurately generate spin correlations?

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Olivier Mattelaer (olivier-mattelaer) said : 		#6

I would say that you should take the decision to use (or not to use ) b since that one differ for off-shell contribution.
So if you want to include effect far off-shell, you should use b otherwise either a or c (whatever is the simplest for you).

Cheers,

Olivier

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