Bhabha scattering with photon radiation

Hi,

> I have generated e+ e- > e+ e- events in Madgraph for a center of mass energy of 500 GeV setting 'set no_parton_cut.'

Without cut this cross-section should be infinite at leading order.
So clearly you can have a lot of issue for such type of computation.

Cheers,

Olivier

> On 27 Jun 2021, at 07:41, Basabendu <email address hidden> wrote:
>
> New question #697741 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697741
>
> Dear Experts,
>
> I have generated e+ e- > e+ e- events in Madgraph for a center of mass energy of 500 GeV setting 'set no_parton_cut.' I am then passing those events through Pythia-6 with initial and final state radiation switched on. I am finally counting mono-photon events where I am selecting photons with pT > 1 GeV. Now, I am seeing one discrepancy here. When I am counting more than 2 photons, I am observing the cross-section for e+ e- > e+ e- na (where n>2) is larger than that for e+ e- > e+ e- a or e+ e- > e+ e- 2a final states. I would expect, with increase in the number of photons in the final state, the cross-section should diminish. However, I am getting expected results where the final state has missing energy (e.g., neutrino or dark matter) instead of a charged particle. I am unable to understand where I am going wrong. Could you please let me know the possible source of error?
>
>
> Thank you for your kind help.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

Thanks a lot for the quick reply. I understand, earlier the process generation was wrong itself. I have again generated the process with minimum pT cut on leptons: pTl = 1 GeV and minimum energy of photons: ea=0.1 GeV. But I see the same problem persists. Is there something more I am missing? Thank you.

Hi,

I doubt that they are any issue here.
This should be the same effect as for QCD.
when if you allow your LO computation to go to close to the singularity you have large log that destroy your precision.
In those area the LO computation typically overshoot the resummed computation as done by the parton-shower.

now do remember that
1) we do not have the running of alpha_ew
2) the computation within mg5aMC is inclusive

Cheers,

Olivier

> On 27 Jun 2021, at 09:35, Basabendu <email address hidden> wrote:
>
> Question #697741 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/697741
>
> Basabendu posted a new comment:
> Hi Olivier,
>
> Thanks a lot for the quick reply. I understand, earlier the process
> generation was wrong itself. I have again generated the process with
> minimum pT cut on leptons: pTl = 1 GeV and minimum energy of photons:
> ea=0.1 GeV. But I see the same problem persists. Is there something more
> I am missing? Thank you.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.