ISR effects vary with particle mass

Asked by 王世钰

dear expert
I tried to use Madgraph version 3.2.0 to test the effect of ISR on the e+e->Twb process.
where T is the new physical particle, which means its mass can be arbitrary, and wb is the particle in SM.
In this process, the energy of the center of mass I set is 3TeV, and the range of T mass is 400-3000GeV. I found that when the mass of T particles is below 1400GeV, the ISR will raise the scattering cross section of this process, and when the mass of T particles is above 1400GeV , the ISR will depress the scattering cross section of this process.
Raising the scattering cross section I can understand: the Breit-wigner shape of the Z which boost the contribution. But as the quality changes first up and then down it confuses me.

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Olivier Mattelaer (olivier-mattelaer) said :


With that few information about the diagram involved in the process, It is difficult to comment.
Now if i do remember correctly the ISR correction at LEP for the Z peak, Those where sometimes positive and sometimes negative depending of the mass of Z . So I do guess that this make sense.

In a way in order to produce such high energy T, you need to have two electon that have NOT radiated ISR, and therefore it makes sense that the presence of ISR decrease the cross-section of high-energy processes.



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