MadGraph5_aMC@NLO

why is the cross-section with ISR in MGISR plugin very larger than without ISR for e- e+ > ta- ta+ ?

Asked by Reza Jafari

Dear Experts,

I calculated the cross-section of e- e+ > ta- ta+ with swiching the ISR effect on via MGISR plugin at 157.5 GeV.
I found the cross-section in this process with ISR is very much larger than the cross-section without ISR, and it seems weird (it is similar for other center-of-mass energies).

Do you think this is correct or maybe there is bug in MGISR plugin?

xsec = 11076 fb (with ISR in MGISR plugin)
xsec = 4869 fb (without ISR)

Best,
Reza

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

MGISR is not develloped/maintained by us. So for any issue with that plugin, you need to contact the author of such PLUGIN: see https://github.com/qliphy/MGISR/issues

Now we are working to provide more native support for such kind of physics and currently have two implementation.
But this is not yet ready since they do not agree so far (7.684 pb for one implementation and 10.49 pb for the second).
But since the second implementation has been under more validation than the first, and the fact that this is relatively to the one from MGISR. I would bet that this is the correct one which would then indicate that the result from MGISR to be reasonable.

Cheers,

Olivier

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Olivier Mattelaer (olivier-mattelaer) said : 		#2

Just to confirm that the 7.684 pb is certainly bugged and that I can different value if I tweak the phase-space integration to avoid the Z-resonance. In that case I get 11.01 pb.

Cheers,

Olivier

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Olivier Mattelaer (olivier-mattelaer) said : 		#3

And the physical reason for such effect is related to the Z-peak.
Having a ISR allows to reduce the center of mass energy, emitting such hard photon has a quite small probability but this is compensated by the Breit-wigner shape of the Z which boost such contribution.

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Reza Jafari (reza13) said : 		#4

Dear Olivier,

Thanks a lot for your checking the issue, and complete explanation.
I hope the second implementation will be completed as well as possible.

For more reassurance, would you please let me know that the cuts you used in run_card.dat are in default values, or you changed them?
I think this small difference may be because of different cut values and I want to check it.

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Olivier Mattelaer (olivier-mattelaer) said : 		#5

Those are default cut, but i used various model so I can not claim that I'm always using the same value for the a_ew or similar.
So I would not worry too much about such small diffference between your number and mine, the point was not to make a precise cross-check but just to understand if the number you quote is reasonable or not.

Cheers,

Olivier

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Reza Jafari (reza13) said : 		#6

Dear Olivier,

Yes, you are right.
Thanks a lot for your answer and consideration.

Best,
Reza