Cross section bb+Phi(chichi) larger than bb+Phi

Asked by Tristan

Dear MadGraph experts

Using a private model of SM with an extra (pseudo) scalar mediator ('phi') that couples to dark matter ('chi'), I observe unexpected features when going to large mediator masses, comparing the following processes:

1) 'p p > b b~ phi'
2) 'p p > b b~ phi, phi > chi chi~'

- Around Mphi~2TeV, the production cross section of bb+phi(chichi), i.e. with the mediator decaying to DM particles, becomes larger than the production of bb+phi, i.e. without decaying the mediator.
- For larger mass, around Mphi~5TeV, the same effect is observed for tt+phi production compared to tt+phi(chichi).

Naively, I would conclude that the BR(phi->chichi)>100%, but this is unphysical.
So maybe the production process with the mediator in the final state (1) is not expected to give physical results due to the width?
Then, still, I am surprised that the effect seems to be more present when producing the mediator in association with b's than with t's.

Would you have an idea what could be going on?

Thanks
Tristan

PS: Results for 'p p > b b~ phi, phi > chi chi~' are the same as for 'p p > b b~ chi chi~', so this is not the reason. Just saying to avoid confusion.

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

Did you use the correct value for the width?
Did you enter that value by hand? or did you ask MG to compute that value automatically via the MadWidth tool (see arXiv:1402.1178)?

Cheers,

Olivier

On 27 May 2015, at 09:46, Tristan <email address hidden> wrote:

> New question #267437 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/267437
>
> Dear MadGraph experts
>
> Using a private model of SM with an extra (pseudo) scalar mediator ('phi') that couples to dark matter ('chi'), I observe unexpected features when going to large mediator masses, comparing the following processes:
>
> 1) 'p p > b b~ phi'
> 2) 'p p > b b~ phi, phi > chi chi~'
>
> - Around Mphi~2TeV, the production cross section of bb+phi(chichi), i.e. with the mediator decaying to DM particles, becomes larger than the production of bb+phi, i.e. without decaying the mediator.
> - For larger mass, around Mphi~5TeV, the same effect is observed for tt+phi production compared to tt+phi(chichi).
>
> Naively, I would conclude that the BR(phi->chichi)>100%, but this is unphysical.
> So maybe the production process with the mediator in the final state (1) is not expected to give physical results due to the width?
> Then, still, I am surprised that the effect seems to be more present when producing the mediator in association with b's than with t's.
>
> Would you have an idea what could be going on?
>
> Thanks
> Tristan
>
> PS: Results for 'p p > b b~ phi, phi > chi chi~' are the same as for 'p p > b b~ chi chi~', so this is not the reason. Just saying to avoid confusion.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Tristan (tristan-dupree) said :
#2

Hi Olivier,

Thanks for the fast feedback. What do you mean "the correct value for the width"? It depends on the model, right?

The width is implemented within the MG model itself, following the formulas in arXiv/1411.0535, see formula 14.

The model contains a scalar mediator, coupling to SM b's and t's (Yukawa) and to DM (coupling 1).

Thanks

Tristan

Revision history for this message
Tristan (tristan-dupree) said :
#3

Hi Olivier,

Thanks for the fast feedback.

The model contains a scalar mediator, coupling to SM b's and t's (Yukawa) and to DM (coupling 1).

The width is implemented within the MG model itself, following the formulas in arXiv/1411.0535, see formula 14.
I am quite sure the width is correctly implemented. The width has also been synchronized with other models.

So what do you mean "the correct value for the width"? It depends on the model, right?
Could you let me know if you think this could cause the effect I am observing?

Thanks
Tristan

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#4

Hi,

> The width is implemented within the MG model itself, following the formulas in arXiv/1411.0535, see formula 14.

Note that this paper keeps the width as a free parameter.

> I am quite sure the width is correctly implemented. The width has also been synchronised with other models.

ok, could you check it anyway?

> So what do you mean "the correct value for the width"? It depends on the model, right?

Yes it depends of the model and of the benchmark point

> Could you let me know if you think this could cause the effect I am observing?

Sure it can, if you have a total width which is too small then you can have arbitrary enhancement factor.
In particular if the total width is smaller than the partial width phi > chi chi, then you expect to have BR>1
I would just run phi > chi chi~ to check that the total width used by your formula is indeed larger than this contribution.

Now if you expect a BR=1 (i.e. that the partial width equals the total width), it is still possible that MG returns an higher cross-section for the process with the decay
due to the (small) off-shell effect which are not captured by the Narrow width approximation, but those effects should be of the order of width/M. So I would not worry at all if you have BR>1 but lower than BR < 1 + width/M, is this your case?

Cheers,

Olivier

On 27 May 2015, at 11:51, Tristan <email address hidden> wrote:

> Question #267437 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/267437
>
> Status: Answered => Open
>
> Tristan is still having a problem:
> Hi Olivier,
>
> Thanks for the fast feedback.
>
> The model contains a scalar mediator, coupling to SM b's and t's
> (Yukawa) and to DM (coupling 1).
>
> The width is implemented within the MG model itself, following the formulas in arXiv/1411.0535, see formula 14.
> I am quite sure the width is correctly implemented. The width has also been synchronized with other models.
>
> So what do you mean "the correct value for the width"? It depends on the model, right?
> Could you let me know if you think this could cause the effect I am observing?
>
> Thanks
> Tristan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Tristan (tristan-dupree) said :
#5

Thanks for the suggestions.

I just ran standalone 'phi > chi chi~' and 'phi > t t~', for Mphi=2000, and I find values of 80 and 111.
Together they add up to 191, which is exactly the total width we use for the mediator at that mass.
So this is consistent and does not seem the origin of the feature I observe.

FYI: At large mediator mass width/M ~ 10%, so this would also not explain why
'pp>ttchichi' > 'pp>ttphi'

Do you have any other idea why the production of pp>ttphi would give strange values wrt pp>ttchichi at large mediator masses?

Thanks
Tristan

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said :
#6

Hi Tristan,

> Do you have any other idea why the production of pp>ttphi would give
> strange values wrt pp>ttchichi at large mediator masses?

The natural candidate is that the narrow width approximation do not held at such large mass.
You are so high in energy that the PDF starts to penalise you quite a lot, and that it can be competitive to
be at lower energy (and therefore offshell) making the concept of Branching ratio irrelevant.
To check if this is the case, you can plot the invariant mass of the chi chi~ system and check if you only have one peak at the phi mass,
or if you have a significative structure at lower energy.

Cheers,

Olivier

PS: I was originally discarding this possibility because you were stated that
 ‘p p > b b~ phi, phi > chi chi~' are the same as for 'p p > b b~ chi chi~',
the low energy part should in principle be remove the on shell requirement cut in the first syntax.
But since your width is ~10%, the —default— on shell requirement —be on shell up to 15 times the width— do not
cut the low energy contribution.

On 27 May 2015, at 13:02, Tristan <email address hidden> wrote:

> Question #267437 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/267437
>
> Status: Answered => Open
>
> Tristan is still having a problem:
> Thanks for the suggestions.
>
> I just ran standalone 'phi > chi chi~' and 'phi > t t~', for Mphi=2000, and I find values of 80 and 111.
> Together they add up to 191, which is exactly the total width we use for the mediator at that mass.
> So this is consistent and does not seem the origin of the feature I observe.
>
> FYI: At large mediator mass width/M ~ 10%, so this would also not explain why
> 'pp>ttchichi' > 'pp>ttphi'
>
> Do you have any other idea why the production of pp>ttphi would give
> strange values wrt pp>ttchichi at large mediator masses?
>
> Thanks
> Tristan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Tristan (tristan-dupree) said :
#7

Thanks for the feedback, Olivier, we will check this. Also the 'PS' was very helpful :-)

Just one question: given your explanation, why would you think the effect seen for bbchichi happens for ttchichi only at larger mediator mass? For bbchichi the effect is seen for Mphi=2TeV, whereas for ttPhi a larger mediator mass is needed.

Do you have an idea?

Thanks!
Tristan

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said :
#8

Hi,

> Just one question: given your explanation, why would you think the
> effect seen for bbchichi happens for ttchichi only at larger mediator
> mass? For bbchichi the effect is seen for Mphi=2TeV, whereas for ttPhi a
> larger mediator mass is needed.

I do not know for sure, but this can an effect of the PDF.

Cheers,

Olivier

On 27 May 2015, at 15:11, Tristan <email address hidden> wrote:

> Question #267437 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/267437
>
> Status: Answered => Open
>
> Tristan is still having a problem:
> Thanks for the feedback, Olivier, we will check this. Also the 'PS' was
> very helpful :-)
>
> Just one question: given your explanation, why would you think the
> effect seen for bbchichi happens for ttchichi only at larger mediator
> mass? For bbchichi the effect is seen for Mphi=2TeV, whereas for ttPhi a
> larger mediator mass is needed.
>
> Do you have an idea?
>
> Thanks!
> Tristan
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Tristan (tristan-dupree) said :
#9

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Tristan (tristan-dupree) said :
#10

Thanks Olivier, indeed the offshell contribution was very big at Mphi=2TeV.

And indeed this contribution was much bigger at that mass for bbPhi than for ttPhi.

I have two nice plots as proof, but I don't see how to attach them ;-)

Anyway, thanks!