MadGraph5_aMC@NLO

Madgraph Results vs. Analytic Formulae

Asked by Christopher W. Murphy

I computed several QED processes at various center-of-mass energies with no PDFs, e+ e- > u u~, u u~ -> e+ e-, e+ e- -> mu+ mu-, and the agreement with the analytic formulas given is Peskin and the PDG was excellent.

However when I computed several QCD processes with no PDFs, t t~ -> u u~, u u~ ->t t~, u u~ -> d d~, d d~ -> u u~, Madgraph gives answers that are larger than what I expected from the analytic formulas by factors of 1.8-2.5 depending on the process and energy. However, ratios of cross section computed using Madgraph did agree with what I expected.

In addition, I computed p p -> t t~ using every PDF available in Madgraph with 2 choices of scales. I also integrated analytic formulae using MSTW2008 PDFs (LO, NLO, & NNLO). The results were consistent enough that the differences were probably just due the choice of PDF and scale.

However, when I computed p p -> b b~ with a minimum invariant mass of the b b~ pair of 35 GeV., the answers Madgraph gives are larger than what I expected from the analytic formulas integrated with MSTW2008 PDFs by a factor of ~3.

Does anyone have any advice as the what I should do? Thank you in advance.

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Christopher W. Murphy
Solved:
Last query:
Last reply:
Revision history for this message
Johan Alwall (johan-alwall) said : 		#1

Hello Christopher,

When comparing between MadGraph and analytical formulas, please be careful to set the scales in the same way. By default, MadGraph uses a scale choice that is dependent on the event topology, e.g., in p p > b b~ you will get the m_T = sqrt(m^2+p_T^2) of the b as scale, so typically close to m_b. When comparing with analytical formulas, I would suggest to use fixed scales (i.e., set fixed_renormalization_scale and fixed_factorization_scales to T in the run_card.dat). Also note that the value for alpha_s(MZ) is given by the PDF - for cteq6l1 (which is the default pdf) this value is 0.130.

These two settings can very well explain your factor ~3 comparing to your analytical result.

All the best,
Johan

Revision history for this message
Christopher W. Murphy (v-cmurphy) said : 		#2

Hi Johan,

I'm still stuck and hoping you can help me a little more. I've tried what
you suggested and I've also run some other processes, but the results
still don't agree at the level I was expecting, a few percent. Rather
than send you an enormous email with everything I've done, is there
anything specifically that I can send you that would be useful?

Thank you very much for helping me with this!
-Chris

> Your question #212531 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/212531
>
> Status: Open => Answered
>
> Johan Alwall proposed the following answer:
> Hello Christopher,
>
> When comparing between MadGraph and analytical formulas, please be
> careful to set the scales in the same way. By default, MadGraph uses a
> scale choice that is dependent on the event topology, e.g., in p p > b
> b~ you will get the m_T = sqrt(m^2+p_T^2) of the b as scale, so
> typically close to m_b. When comparing with analytical formulas, I would
> suggest to use fixed scales (i.e., set fixed_renormalization_scale and
> fixed_factorization_scales to T in the run_card.dat). Also note that the
> value for alpha_s(MZ) is given by the PDF - for cteq6l1 (which is the
> default pdf) this value is 0.130.
>
> These two settings can very well explain your factor ~3 comparing to
> your analytical result.
>
> All the best,
> Johan
>
> --
> If this answers your question, please go to the following page to let us
> know that it is solved:
> https://answers.launchpad.net/madgraph5/+question/212531/+confirm?answer_id=0
>
> If you still need help, you can reply to this email or go to the
> following page to enter your feedback:
> https://answers.launchpad.net/madgraph5/+question/212531
>
> You received this question notification because you asked the question.
>

Revision history for this message
Johan Alwall (johan-alwall) said : 		#3

Hello Chris,

You can send me your enormous mail by email (jalwall_at_ntu.edu.tw). We'll report back here once we understand what is happening.

All the best,
Johan

Revision history for this message
Christopher W. Murphy (v-cmurphy) said : 		#4

 I found two problems in my analysis that resolved the discrepancy.

1.) In the semi-analytic calculation I forgot to run alpha_s
2.) When an intermediate particle goes on-shell it is included in the LHE file. In my analysis I always assumed there were only 4 particles per event, not sometimes 5.