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IR singularities with squared amplitudes produced by MadLoop

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06-15-2020 9:38 AM

205

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Hello,
I’m trying to use MadLoop to generate a standalone library for some processes with EW loop corrections, and have a few questions:
1. I’m confused by the IR poles in the results. For instance, for the simple QED process e- e+ -> mu- mu+, I did
define exc = g0 g+ g- h gha ghz ghwp ghwm z u c d e- b s mu- ta+ ta- w+
generate e- e+ > mu- mu+ QCD=0 [virt=QED] /exc
which produced a loop graph with a top quark loop on the photon propagator. When I did
launch -f
I got the result
|| Total(*) Born contribution (GeV^0):
| Born = 1.1012911820047121e-02
|| Total(*) virtual contribution normalized with born*alpha_S/(2*pi):
| Finite = -2.1842211753457780e-01
| Single pole = -1.1685343497245081e-01
| Double pole = 0.0000000000000000e+00
As far as I know, this loop graph should not have an IR singularity. Do you know the reason for the non-zero single pole?
2. Can I use the subtraction method in MadFKS to compute photon radiative corrections? It seems that only QCD NLO radiative corrections are available.
3. When I run MadLoop in standalone mode, how can I set a dynamical renormalization scale? The process I’m considering is again EW loop graphs. I don’t find the setscal.f file in my output.
Thanks,
```

Xiang Lv

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