# cross section and pt distribution for fixed-target

Asked by Y Zhong on 2018-07-23

We found some discrepancies between MG5 (2.6.3.2) and CalcHEP in the simulation of mu- N > mu- N s0, where N is nucleon and s0 is a new real scalar that couples to muons. The model files for MG5 and CalcHEP are both generated via FeynRules with the same fr input.

To be more specific, when we set the ms0 = 1 MeV and muon beam energy = 10 GeV, we found the cross section (with the same scalar-muon coupling and remove all the collider cuts) from MG5 is about half of that from CalcHEP. We also found there is a discontinuity in the pt distribution of the scalar momentum around 50 MeV from MG5 simulation. Such discontinuity does not happen in the CalcHEP distribution.

If we change ms0 = 100 MeV and muon beam energy = 10 GeV, we found the cross sections from the two generators agree pretty well.

We wonder what could be the reasons for the discrepancies and the discontinuity?

Thanks a lot!

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2018-07-26
2018-08-03
 Andrea (darkcele) said on 2018-07-23: #1

Hi,

* N is a Fe nucleus, mass was set to 52 GeV
* When imposing a cut on the missing momentum to be > 60 MeV, MG5 and CalcHep results agree exactly.
* Without the Pt cut, the Pt distribution looks equivalent for Pt > 55 MeV, while at lower Pt MG5 shows a discontinuity.
* The same discontinuity is visible for scalar mass = 10 MeV, muon beam=10 GeV and scalar mass = 10 MeV, muon beam=5 GeV

 Olivier Mattelaer (olivier-mattelaer) said on 2018-07-24: #2

Hi,

Can you attach your scripts? Such that I can understand how you did your simulation, how you setup your card,...
And then be able to provide usefull comment. This will be more efficient than asking you the 20 different question that I have in mind (like how did you setup the scale, how did you define the PDF,...)

Cheers,

Olivier

 Y Zhong (ymzhong) said on 2018-07-24: #3

Hi Olivier,

We simulated events through the interface. Is it good enough to just send you the banner information? (see attached) Also we can send you the model file and the scalar pt distribution we got.

Thanks!

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#* run as ./bin/mg5_aMC filename *
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set default_unset_couplings 99
set group_subprocesses Auto
set ignore_six_quark_processes False
set loop_optimized_output True
set low_mem_multicore_nlo_generation False
set loop_color_flows False
set gauge unitary
set complex_mass_scheme False
set max_npoint_for_channel 0
import model sm
define p = g u c d s u~ c~ d~ s~
define j = g u c d s u~ c~ d~ s~
define l+ = e+ mu+
define l- = e- mu-
define vl = ve vm vt
define vl~ = ve~ vm~ vt~
import model /projectnb/leptoq/mg5_simulation/mg1/MG5_aMC_v2_6_3_2/mod\
els/DarkLepton_UFO
generate mu- n > mu- n s0 HIW==0
output ft_test_1
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 Olivier Mattelaer (olivier-mattelaer) said on 2018-07-26: #4

Hi,

The issue is likely related to those setup:

False = fixed_ren_scale ! if .true. use fixed ren scale
False = fixed_fac_scale ! if .true. use fixed fac scale
91.188 = scale ! fixed ren scale
91.188 = dsqrt_q2fact1 ! fixed fact scale for pdf1
91.188 = dsqrt_q2fact2 ! fixed fact scale for pdf2
-1 = dynamical_scale_choice ! Choose one of the preselected dynamical choices
1.0 = scalefact ! scale factor for event-by-event scales

In the default dynamical scale choice, a hardcoded cut (at 4Gev^2) is present to avoid contribution from the non perturbative region.
I would be that this is what creates your cut. I would advise to either use the fixed scale, or to use another type of dynamical scale.
Assuming that such part of the phase-space is actually physical

Cheers,

Olivier

 Y Zhong (ymzhong) said on 2018-07-26: #5

Hi Olivier,

I have tried the fixed mass scale (fixed to the scalar mass) and dynamical scale choices 1, 2, 3, 4. They all give me similar results as the default scale, i.e., a cross section around 0.76 pb, which is half of the CalcHEP value. And the 50 MeV discontinuity in the scalar pt distribution is still there.

To be honest, I don't quite see why the scale factor should matter. There is no PDF. The nucleon is a newly defined particle in our model. And for the process, mu- n > mu- n s0, has QCD order equals zero.

 Olivier Mattelaer (olivier-mattelaer) said on 2018-07-26: #6

Hi,

Ok wrong guess then. Could you send me the model then?
If you have a command file with the commands to edit the param_card/run_card , it will save me some time as well.

Cheers,

Olivier

 Y Zhong (ymzhong) said on 2018-07-27: #7

Thanks Olivier! I have send the model and other necessary files to your email.

 Olivier Mattelaer (olivier-mattelaer) said on 2018-07-30: #8

Hi,

I have run a lot of computation with your model and one can observe that you have huge cancelation between your two diagrams.
the s-channel is at 2.8e+06 while the full cross-section (after the negative interference) is at 0.7665.

As the tail is probed correctly (which is highly non trivial in this case), this should not be an issue of the adaptative method of the phase-space probing. (it is actually amazing how much the code adapt itself to do the tail correctly).

I have also add a cut preventing that such cancelation is too large (comparing the interference term to the amp square) and the same peak structure is present.

I have also put the cuts.f file in debug to check that no cut was wrongly cutting part of the phase-space.

So two solutions are remaining:
1) MG5 is correct (and that such dip is related to some of the scale of the process)
2) some of the invariant are too soft to be generated.

To check the second would you be able to plot the four invariant mass (3 t -channel and 1 s-channel) -- 2 invariant mass by Feynman diagram-- and compare those between MG5AMC and calchep. It can be very instructive to see how the difference translate on those 4 key variable.

Cheers,

Olivier

 Andrea (darkcele) said on 2018-07-30: #9

Dear Oliver,
thanks for your reply. I didn't get exactly which invariant masses you suggest to plot. Can you please specify in terms of the particles 4-momenta mu N -> mu' N' S

like: invariant mass of scalar and final state muon -> (S+mu')
momentum transfer between final and initial state nucleon -> (N'-N)

THanks,
Andrea

 Olivier Mattelaer (olivier-mattelaer) said on 2018-07-31: #10

Hi,

Do not have that much time to make the math.
Just look at the diagram, and plot the mass of each propagator.

(it might be that one of the propagator is the same between both diagram in that case, you will have only three diagram to plot)

Cheers,

Olivier

> On 31 Jul 2018, at 00:02, Andrea <email address hidden> wrote:
>
> Question #671061 on MadGraph5_aMC@NLO changed:
>
> Dear Oliver,
> thanks for your reply. I didn't get exactly which invariant masses you suggest to plot. Can you please specify in terms of the particles 4-momenta mu N -> mu' N' S
>
> like: invariant mass of scalar and final state muon -> (S+mu')
> momentum transfer between final and initial state nucleon -> (N'-N)
>
> THanks,
> Andrea
>
> --

 Olivier Mattelaer (olivier-mattelaer) said on 2018-08-03: #11

Hi,

First thanks to have send me the plot.
I can now confirm that the problem is on our side and that is a pure bug.

I have modified the phase-space integrator and can clearly observe that I have a continuous contribution if I do not allow madgraph to integrate according to the double T-channel diagram.
I have made other hack to the code who convinces me that this is not related to the multi-channelling procedure (which assumes that the interference is small --which is not the case here) but really to the t-channel diagram itself.

I still have to investigate to understand the reason of this issue with the t-channel. I will keep you updated when I would have more news on this. This seems a really complicated bug so I would need you to be patient.

If you want, I can obviously show you how to hack your MG5aMC process in order to have it work.

Cheers,

Olivier

 Y Zhong (ymzhong) said on 2018-08-03: #12

Hi Olivier,

Thanks a lot for your investigation!

Yes. It will be great if you can show us how to hack so we can put MG5 in use.

Best,
Yiming

 Olivier Mattelaer (olivier-mattelaer) said on 2018-08-03: #13

Sure, the file to modify is
SubProcesses/P1_mumn_mumns0

and here is the diff:

[P1_mumn_mumns0]\$ diff matrix1.f matrix1.bk --context=10
*** matrix1.f 2018-08-03 23:36:35.000000000 +0200
--- matrix1.bk 2018-08-03 23:31:21.000000000 +0200
***************
*** 221,241 ****
WRITE(HEL_BUFF,'(20i5)')(NHEL(II,I),II=1,NEXTERNAL)
C Set right sign for ANS, based on sign of chosen helicity
ANS=DSIGN(ANS,TS(I))
GOTO 10
ENDIF
ENDDO
10 CONTINUE
ENDIF
IF (MULTI_CHANNEL) THEN
XTOT=0D0
- AMP2(1) = 0d0
DO I=1,NDIAGS
XTOT=XTOT+AMP2(I)
ENDDO
IF (XTOT.NE.0D0) THEN
ANS=ANS*AMP2(SUBDIAG(1))/XTOT
ELSE IF(ANS.NE.0D0) THEN
WRITE(*,*) 'Problem in the multi-channeling. All amp2 are'
\$ //' zero but not the total matrix-element'
STOP 1
ENDIF

Cheers,

Olivier

> On 3 Aug 2018, at 23:22, Y Zhong <email address hidden> wrote:
>
> Question #671061 on MadGraph5_aMC@NLO changed:
>
> Y Zhong posted a new comment:
> Hi Olivier,
>
> Thanks a lot for your investigation!
>
> Yes. It will be great if you can show us how to hack so we can put MG5
> in use.
>
> Best,
> Yiming
>
> --