Exclusive dilepton production at NLO

Dear Michael,
thanks for posting your questions.
To answer them
Q1) at the moment, event generation for NLO EW computations is not possible. To compute differential cross sections (histograms), you can link an analysis. Some examples are inside FixedOrderAnalysis, those with HwU in the name support a more modern histogram format. You need to specify the name of the analysis inside FO_analyse_card.dat. The procedure should be pretty simple and self-explanatory, but in case, do not hesitate to ask back.

Q2) you can look at the run_card. Fewer kinematics cuts are supported at NLO. You can add your custom cuts either in cuts.f or in dummy_fct.f. Please be very ware about IR safety at NLO.

Speaking of IR-safety, the process you are generating does not seem to be IR-finite.
In particular, the initial-state photons will split into leptons/quarks, so you need to add the relevant initial states to a multiparticle label (like p for hadron collisions). You are running lepton collisions, right?

Let me know if i can help you more.

Best wishes,

Marco

Dear Marco,

Thank you for your suggestion. I'm able to run the FO analysis and produce histograms with ROOT.

Yet, I am having some issues with the process. I'm interested in simulating elastic photons from a proton (not a lepton), and the option "set lpp1 = 2" is not working. The only option I can run is with lpp1 = 1 and taking the photons from PDF, but then NLO corrections add quarks in the initial state (in elastic photons, the proton remains intact, and no ISR in this case). Should I explicitly switch off all quarks in the model to simulate properly elastic photons?

I also want to compare the NLO correction between different lepton flavors (e, mu, tau). Should the corrections be the same? I thought some mass dependence should be present, but in the model, all leptons are massless.

btw, how can I increase statistics in my histograms?

Regards,
Michael

Hi Michael,
I am not sure the code will work at all with EW corrections and elastic photon, as this has never been validated/tested/implemented.

For the masses, in principle, if you load the model with
import model loop_qcd_qed_sm-full
you should have lepton masses, but I can't guarantee on the stability of the corrections.

Finally, to increase the statistic, just lower the req_acc_FO parameter in the run_card.

Best,

Marco

Hi Marco,

The code isn't working. The quick approximation of elastic photons: I tried to omit all quarks.

generate a a > mu+ mu- / u u~ d d~ c c~ s s~ t t~ b b~ t t~ [QED]
It works fine, but I still see incoming quarks.

In the elastic case, the only real emissions should be with outgoing photons, but I don't manage to force it this way.

Regarding the masses:
I'm running: generate a a > mu+ mu- [QED]
The massless model have 166 real emission diagrams, 2 born diagrams and 89 virtual diagrams
The loop_qcd_qed_sm-full has 6 real emission diagrams, 2 born diagrams and 224 virtual diagrams
but it returns: Poles do not cancel, run cannot continue
Maybe some real emissions are not included?

Regards,
Michael