Matrix element of mirror process

Hi,

In order to have the same matrix-element, you should not use the same phase-space point for the comparison but the mirror one.

Cheers,

Olivier

> On 7 Oct 2018, at 16:32, Laurent Vashmo <email address hidden> wrote:
>
> New question #674807 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/674807
>
> Hi,
>
> I am trying to compute the total cross section for the process pp > htj, but I am having some troubles, so I decided to study the different subprocesses in a more detailed way.
>
> My doubt comes when studying the processes u b > h t d and b u > htd, i.e. just the mirror process so to say. When I use the standalone output to get the matrix elements and compute both processes, I obtain different results. As information, I write here the code I use.
>
>>> import model loop_sm-no_b_mass
>>> generate u b > h t d
>>> output standalone ub_htd_st
>>> launch
>
> and the same for the other process.
>
> For u b > h t d, I get:
> Matrix element = 6.3550723077195101E-008 GeV^ -2
>
> whereas for the b u > h t d:
> Matrix element = 2.9000254780896340E-008 GeV^ -2
>
> when, given that it is exactly the same process, it should yield exactly the same result. I already checked that the set of momenta used in both of them is the same, so the problem is not here. So I am not so sure where the problem can be. Am I computing something wrong? Or missing something that I did not take into account?
>
> Also, when then - instead of using the standalone output, but the normal one - I get different values for the cross section, slightly different, but enough to be not in agreement between them.
>
> Thank you in advanced!
>
>
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Hi Olivier,

thanks, that solved my problem.

But still there is something that I don't understand. Now I get the same matrix elements, but when I integrate over the whole phase space in order to get the total cross section (using in this case the normal output, modifying the run_card.dat, and generating the events) I get different values of the cross section for both values.

With nevents = 10^6 and a scale of (m_higgs + m_top)/4, using LHAPDF set 21000 and fixed scaled, I get:

cross-section u b > h t d = 16.670 +- 0.005 fb
cross-section b u > h t d = 16.256 +- 0.005 fb

which is not in agreement. But I am applying no cuts, so they should give the same... I also get the same problem when changing the scale, for example, I tried also with scale = (m_higgs + m_top)/2 and the results are (also with nevents = 10^6):

cross-section u b > h t d = 17.566 +- 0.006 fb
cross-section b u > h t d = 17.116 +- 0.006 fb

I also created a code that integrates over the phase space and over the pdfs using VEGAS, and for both processes I get the same result using the matrix elements from Madgraph, so the problem must be somewhere in my run from Madgraph, but I have no idea where, and I checked the run card and in both processes I have the same.

Do you have an idea of where the problem could be? Thank you so much!

Best,
L.

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

Hi,

I have been working on each case separately, and I would like to add some information I found, maybe it could be helpful:

- the difference between cross-sections for mirror processes comes into play only for t-channel diagrams (for this process I mentioned before), so for the s-channel diagrams works well and yields the same result for the sub-process and the mirror one.

- if I compute both processes at the same time, e.g.
    >> generate u b > h t d
    >> add process b u > h t d
  and I run the process, then I obtain twice the result for the u b > h t d, which is in fact the correct result

So the problem comes only for the mirror processes. But I still don't know why there is this difference between results.

Thanks again!
L.

This question was expired because it remained in the 'Open' state without activity for the last 15 days.