MadGraph5_aMC@NLO

BSM model with small coupling get zero cross section

Asked by Si Wang

Hi,

We are working on Lmu-Ltau model, which has a new particle Z'.
We first:
generate mu+ mu- > zp, zp > ta+ ta- weight<=4
Then using the modified Plugin file of ISR(https://github.com/qliphy/MGISR):
output EE_ISR tryISR

When the coupling is small, like g'=0.001, Z' mass=100 GeV, Z' decay width=8e-6 GeV.
It will give zero cross section.

Why do we get zero cross section?
Is there a way to get a non-zero correct cross section?

Thank you!
Si

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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Did you ask MGISR if they support such sharp resonances?
You might need a dedicated phase-space integrator to handle such case.
Especialy since your syntax is associated with an onshell cut for the Z' which can lead to the phase-space integrator to miss the peak if the phase-space integrator is not setup for a (quite complex) multi-channel mode.

Additionally, I'm not aware that MGISR is supporting muon initial state (I thought it was only photon).

My advise here would be that you check the isr_only introduced in the 3.X branch of the code (I guess in 3.3.0)
Not fully sure if it does support muon, but if you have knowledge about such ISR, it should be easy enough to update such PDF for muon and that implementation defenitly has a complex multi-channel for handling such case.

Cheers,

Olivier

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Si Wang (siw34) said : 		#2

Hi Olivier,

Thank you for detailed answer.
I guess I will try to use Whizard to do ISR.

Cheers,
Si