g g > b b~ angular distribution

Dear Ezequiel,

> I do so, and I plot the eta distribution of the outgoing b's. Since as energy increases the t-channel dominates more and more, I expect to have all the b's at large etas, and more and more as energy is increased. However, the eta distribution of the b’s for different colliding energies is the following: http://users.df.uba.ar/sequi/bajador/eta_distribution.eps .

This is actually what I see on your plot, larger the energy is more jet you have in the forward region.

> I wonder if Madgraph is including some other cut to avoid the collinear divergence and this is producing a bias? Or something else?

We can not integrate singularity (obviously) so I hope that you have a regulator of the divergence making it finite (at least the bottom mass).
We also have an hardcoded cut for the factorisation scale which should be larger than 2 GeV (but since our dynamical scale is the transverse mass of the b, it should never affect the generation)
You can also bypass this cut by choosing a fix scale.

Speaking of scale, all those analytical computation are done at fix scale, so do you use fix-scale in MG as well?

Cheers,

Olivier

On 23 Apr 2015, at 21:26, Ezequiel Alvarez <email address hidden> wrote:

> New question #265778 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/265778
>
> Dear experts,
>
> I am generating g g > b b~ with no-pdf in the incard.dat, so I can set by hand the energy of the incoming gluons. I am also removing any constraint on the b's: no limit on ptb, etab, mbb, etc.
>
> I do so, and I plot the eta distribution of the outgoing b's. Since as energy increases the t-channel dominates more and more, I expect to have all the b's at large etas, and more and more as energy is increased. However, the eta distribution of the b's for different colliding energies is the following: http://users.df.uba.ar/sequi/bajador/eta_distribution.eps .
>
> This is weird to me. (I am working at parton level and reading the unweighted_events.lhe file.)
>
> I have checked the analytic expression of the process (QCD and collider physics, Ellis, Stirling and Webber, page=348) and the angular distribution of gg>bb~ it is what I expected, but not the outcome of MadGraph.
>
> I wonder if Madgraph is including some other cut to avoid the collinear divergence and this is producing a bias? Or something else?
>
> Thanks so much!
>
> Ezequiel.
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Dear Olivier,

thanks for your prompt reply.

>> I do so, and I plot the eta distribution of the outgoing b's. Since as energy increases the t-channel dominates more and more, I expect to have all the b's at large etas, and more and more as energy is increased. However, the eta distribution of the b’s for different colliding energies is the following: http://users.df.uba.ar/sequi/bajador/eta_distribution.eps .

>This is actually what I see on your plot, larger the energy is more jet you have in the forward region.

Yes I agree: one can see that the larger the energy, then more jets in the forward region. But from the analytic formulae one expects considerable more jets in the forward region, see

http://users.df.uba.ar/sequi/bajador/eta_distribution_madgraph_and_analytic.eps

for a comparison of MadGraph outcome versus analytic prediction. (Histograms are for MadGraph output and continuous for analytic formulae.)

Regarding your comments: 1) yes I'm keeping mb=4.7GeV in param_card.dat to avoid divergences; 2) I've tried fixed ren and fact scales at 91 and 10 GeV and there are no substantial changes.

All the bests, Ezequiel.

Dear Ezechiel,

I have taken a look at the Webber paper and I do not see the formula that you need to compare with that plot.
Can you share your computation starting from Webber formula to the one that you plot?

On the other hand, it should be easy to compare the matrix element formula (Eq 10.55 and 10.56) with the standalone_output
of MadGraph to check that a single phase-space point return the same value for the matrix-element. (the standalone output is the average over spin/color so I expect to have a
normalisation difference)

Final question, how do you normalise your plot? (i.e. What is the measure that you use?)
Are you using the same measure for the theoretical curve and the one from the lhe file?
It is not clear to me how the integral of the blue curve can be equal to one.

Cheers,

Olivier

On 26 Apr 2015, at 10:36, Ezequiel Alvarez <email address hidden> wrote:

> Question #265778 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/265778
>
> Status: Answered => Open
>
> Ezequiel Alvarez is still having a problem:
> Dear Olivier,
>
> thanks for your prompt reply.
>
>>> I do so, and I plot the eta distribution of the outgoing b's. Since
> as energy increases the t-channel dominates more and more, I expect to
> have all the b's at large etas, and more and more as energy is
> increased. However, the eta distribution of the b’s for different
> colliding energies is the following:
> http://users.df.uba.ar/sequi/bajador/eta_distribution.eps .
>
>> This is actually what I see on your plot, larger the energy is more jet
> you have in the forward region.
>
> Yes I agree: one can see that the larger the energy, then more jets in
> the forward region. But from the analytic formulae one expects
> considerable more jets in the forward region, see
>
> http://users.df.uba.ar/sequi/bajador/eta_distribution_madgraph_and_analytic.eps
>
> for a comparison of MadGraph outcome versus analytic prediction.
> (Histograms are for MadGraph output and continuous for analytic
> formulae.)
>
> Regarding your comments: 1) yes I'm keeping mb=4.7GeV in param_card.dat
> to avoid divergences; 2) I've tried fixed ren and fact scales at 91 and
> 10 GeV and there are no substantial changes.
>
> All the bests, Ezequiel.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi Olivier,

I have verified the calculation and we were note including the phase space factors. In particular the sin(theta)^2 coming from d^3p_3 d^3p_4 dumps the distribution as theta -> 0 (or eta -> infinity ) and makes the 2 calculation agree.

Take a look what a nice agreement we found now :-) !

http://users.df.uba.ar/sequi/bajador/eta_distribution_madgraph_and_analytic_including_phase_space.eps

By the way, here are the formulae we use from Webber's book: http://users.df.uba.ar/sequi/bajador/Webber-books.2pages.ps .

All th best and thanks for your prompt answers..... Ezequiel.