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!

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Last query:
2018-07-26
Last reply:
2018-08-03
Andrea (darkcele) said : #1

Hi,
I'd like to add few details about this.

* 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

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 : #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!

<LesHouchesEvents version="3.0">
<header>
<!--
#*********************************************************************
# *
# MadGraph5_aMC@NLO *
# *
# Going Beyond *
# *
# http://madgraph.hep.uiuc.edu *
# http://madgraph.phys.ucl.ac.be *
# http://amcatnlo.cern.ch *
# *
# The MadGraph5_aMC@NLO team *
# *
#....................................................................*
# *
# This file contains all the information necessary to reproduce *
# the events generated: *
# *
# 1. software version *
# 2. proc_card : code generation info including model *
# 3. param_card : model primary parameters in the LH format *
# 4. run_card : running parameters (collider and cuts) *
# 5. pythia_card : present only if pythia has been run *
# 6. pgs_card : present only if pgs has been run *
# 7. delphes_cards : present only if delphes has been run *
# *
# *
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-->
<MGVersion>
2.6.3.2
</MGVersion>
<MG5ProcCard>
<![CDATA[
#************************************************************
#* MadGraph5_aMC@NLO *
#* *
#* * * *
#* * * * * *
#* * * * * 5 * * * * *
#* * * * * *
#* * * *
#* *
#* *
#* VERSION 2.6.3.2 2018-06-22 *
#* *
#* The MadGraph5_aMC@NLO Development Team - Find us at *
#* https://server06.fynu.ucl.ac.be/projects/madgraph *
#* *
#************************************************************
#* *
#* Command File for MadGraph5_aMC@NLO *
#* *
#* run as ./bin/mg5_aMC filename *
#* *
#************************************************************
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
]]>
</MG5ProcCard>
<MGProcCard>
#*********************************************************************
# MadGraph/MadEvent *
# http://madgraph.hep.uiuc.edu *
# *
# proc_card.dat *
#*********************************************************************
# *
# This Files is generated by MADGRAPH 5 *
# *
# WARNING: This Files is generated for MADEVENT (compatibility issue)*
# This files is NOT a valid MG4 proc_card.dat *
# Running this in MG4 will NEVER reproduce the result of MG5*
# *
#*********************************************************************
#*********************************************************************
# Process(es) requested : mg2 input *
#*********************************************************************
# Begin PROCESS # This is TAG. Do not modify this line
mu- n > mu- n s0 #Process
# Be carefull the coupling are here in MG5 convention
HIW==0

end_coup # End the couplings input

done # this tells MG there are no more procs
# End PROCESS # This is TAG. Do not modify this line
#*********************************************************************
# Model information *
#*********************************************************************
# Begin MODEL # This is TAG. Do not modify this line
DarkLepton_UFO
# End MODEL # This is TAG. Do not modify this line
#*********************************************************************
# Start multiparticle definitions *
#*********************************************************************
# Begin MULTIPARTICLES # This is TAG. Do not modify this line

# End MULTIPARTICLES # This is TAG. Do not modify this line
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<MGRunCard>
<![CDATA[
#*********************************************************************
# MadGraph5_aMC@NLO *
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# run_card.dat MadEvent *
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# This file is used to set the parameters of the run. *
# *
# Some notation/conventions: *
# *
# Lines starting with a '# ' are info or comments *
# *
# mind the format: value = variable ! comment *
# *
# To display more options, you can type the command: *
# update full_run_card *
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#
#*******************
# Running parameters
#*******************
#
#*********************************************************************
# Tag name for the run (one word) *
#*********************************************************************
  tag_1 = run_tag ! name of the run
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# Number of events and rnd seed *
# Warning: Do not generate more than 1M events in a single run *
# If you want to run Pythia, avoid more than 50k events in a run. *
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  10000 = nevents ! Number of unweighted events requested
 30 = iseed ! rnd seed (0=assigned automatically=default))
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# Collider type and energy *
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# 3=photon from electron *
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     0 = lpp2 ! beam 2 type
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# PDF CHOICE: this automatically fixes also alpha_s and its evol. *
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     nn23lo1 = pdlabel ! PDF set
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#
#
#*********************************************************************
# BW cutoff (M+/-bwcutoff*Gamma) ! Define on/off-shell for "$" and decay
#*********************************************************************
  15.0 = bwcutoff ! (M+/-bwcutoff*Gamma)
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 0.0 = ptl ! minimum pt for the charged leptons
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 -1.0 = missetmax ! maximum missing Et (sum of neutrino's momenta)
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#*********************************************************************
# Minimum and maximum E's (in the center of mass frame) *
#*********************************************************************
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  0.0 = eb ! minimum E for the b
  0.0 = ea ! minimum E for the photons
  0.0 = el ! minimum E for the charged leptons
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 -1.0 = ebmax ! maximum E for the b
 -1.0 = eamax ! maximum E for the photons
 -1.0 = elmax ! maximum E for the charged leptons
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#*********************************************************************
# Maximum and minimum absolute rapidity (for max, -1 means no cut) *
#*********************************************************************
  -1.0 = etaj ! max rap for the jets
  -1.0 = etab ! max rap for the b
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 -1.0 = etal ! max rap for the charged leptons
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# Minimum and maximum DeltaR distance *
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 -1.0 = drajmax ! max distance between gamma and jet
 -1.0 = drjlmax ! max distance between jet and lepton
 -1.0 = drabmax ! max distance between gamma and b
 -1.0 = drblmax ! max distance between b and lepton
 -1.0 = dralmax ! maxdistance between gamma and lepton
#*********************************************************************
# Minimum and maximum invariant mass for pairs *
# WARNING: for four lepton final state mmll cut require to have *
# different lepton masses for each flavor! *
#*********************************************************************
 0.0 = mmjj ! min invariant mass of a jet pair
 0.0 = mmbb ! min invariant mass of a b pair
 0.0 = mmaa ! min invariant mass of gamma gamma pair
 0.0 = mmll ! min invariant mass of l+l- (same flavour) lepton pair
 -1.0 = mmjjmax ! max invariant mass of a jet pair
 -1.0 = mmbbmax ! max invariant mass of a b pair
 -1.0 = mmaamax ! max invariant mass of gamma gamma pair
 -1.0 = mmllmax ! max invariant mass of l+l- (same flavour) lepton pair
 {} = mxx_min_pdg ! min invariant mass of a pair of particles X/X~ (e.g. {6:250})
 {'default': False} = mxx_only_part_antipart ! if True the invariant mass is applied only
                       ! to pairs of particle/antiparticle and not to pairs of the same pdg codes.
#*********************************************************************
# Minimum and maximum invariant mass for all letpons *
#*********************************************************************
 0.0 = mmnl ! min invariant mass for all letpons (l+- and vl)
 -1.0 = mmnlmax ! max invariant mass for all letpons (l+- and vl)
#*********************************************************************
# Minimum and maximum pt for 4-momenta sum of leptons *
#*********************************************************************
 0.0 = ptllmin ! Minimum pt for 4-momenta sum of leptons(l and vl)
 -1.0 = ptllmax ! Maximum pt for 4-momenta sum of leptons(l and vl)
#*********************************************************************
# Inclusive cuts *
#*********************************************************************
 0.0 = ptheavy ! minimum pt for at least one heavy final state
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 0.0 = xptl ! minimum pt for at least one charged lepton
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#*********************************************************************
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#*********************************************************************
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#*********************************************************************
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 0.0 = ptl3min ! minimum pt for the third lepton in pt
 0.0 = ptl4min ! minimum pt for the fourth lepton in pt
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 -1.0 = ptl2max ! maximum pt for the second lepton in pt
 -1.0 = ptl3max ! maximum pt for the third lepton in pt
 -1.0 = ptl4max ! maximum pt for the fourth lepton in pt
#*********************************************************************
# Control the Ht(k)=Sum of k leading jets *
#*********************************************************************
 0.0 = htjmin ! minimum jet HT=Sum(jet pt)
 -1.0 = htjmax ! maximum jet HT=Sum(jet pt)
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 0.0 = ht2min ! minimum Ht for the two leading jets
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 -1.0 = ht2max ! maximum Ht for the two leading jets
 -1.0 = ht3max ! maximum Ht for the three leading jets
 -1.0 = ht4max ! maximum Ht for the four leading jets
#***********************************************************************
# Photon-isolation cuts, according to hep-ph/9801442 *
# When ptgmin=0, all the other parameters are ignored *
# When ptgmin>0, pta and draj are not going to be used *
#***********************************************************************
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 1.0 = xn ! n parameter of eq.(3.4) in hep-ph/9801442
 1.0 = epsgamma ! epsilon_gamma parameter of eq.(3.4) in hep-ph/9801442
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#*********************************************************************
# WBF cuts *
#*********************************************************************
 0.0 = xetamin ! minimum rapidity for two jets in the WBF case
 0.0 = deltaeta ! minimum rapidity for two jets in the WBF case
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#***********************************************************************
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#*********************************************************************
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#*********************************************************************
 4 = maxjetflavor ! Maximum jet pdg code
#*********************************************************************
#
#*********************************************************************
# Store info for systematics studies *
# WARNING: Do not use for interference type of computation *
#*********************************************************************
   False = use_syst ! Enable systematics studies
#
systematics = systematics_program ! none, systematics [python], SysCalc [depreceted, C++]
['--mur=0.5,1,2', '--muf=0.5,1,2', '--pdf=errorset'] = systematics_arguments ! see: https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Systematics#Systematicspythonmodule
# Syscalc is deprecated but to see the associate options type'update syscalc'
]]>
</MGRunCard>
<slha>
######################################################################
## PARAM_CARD AUTOMATICALY GENERATED BY MG5 ####
######################################################################
###################################
## INFORMATION FOR CKMBLOCK
###################################
BLOCK CKMBLOCK #
      1 2.277360e-01 # cabi
###################################
## INFORMATION FOR DARKCOUPLING
###################################
BLOCK DARKCOUPLING #
      1 3.870000e-04 # gsmu
      2 0.000000e+00 # gsel
      3 0.000000e+00 # gsta
      4 0.000000e+00 # gchi
###################################
## INFORMATION FOR HET
###################################
BLOCK HET #
      1 0.000000e+00 # gsyy
###################################
## INFORMATION FOR MASS
###################################
BLOCK MASS #
      1 5.040000e-03 # md
      2 2.550000e-03 # mu
      3 1.010000e-01 # ms
      4 1.270000e+00 # mc
      5 4.700000e+00 # mb
      6 1.720000e+02 # mt
      11 5.110000e-04 # me
      13 1.056600e-01 # mmu
      15 1.777000e+00 # mta
      23 9.118760e+01 # mz
      25 1.250000e+02 # mh
      1001 1.000000e-03 # ms0
      1101 5.200000e+01 # mnul
      3001 1.000000e-01 # mchi
      12 0.000000e+00 # ve : 0.0
      14 0.000000e+00 # vm : 0.0
      16 0.000000e+00 # vt : 0.0
      21 0.000000e+00 # g : 0.0
      22 0.000000e+00 # a : 0.0
      24 8.093919e+01 # w+ : cmath.sqrt(mz__exp__2/2. + cmath.sqrt(mz__exp__4/4. - (aew*cmath.pi*mz__exp__2)/(gf*sqrt__2)))
###################################
## INFORMATION FOR NUCLEON
###################################
BLOCK NUCLEON #
      1 3.028177e-01 # gan
###################################
## INFORMATION FOR SMINPUTS
###################################
BLOCK SMINPUTS #
      1 1.370400e+02 # aewm1
      2 1.166370e-05 # gf
      3 1.184000e-01 # as
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## INFORMATION FOR YUKAWA
###################################
BLOCK YUKAWA #
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      5 4.700000e+00 # ymb
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      13 1.056600e-01 # ymm
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DECAY 25 4.070000e-03 # wh
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DECAY 22 0.000000e+00 # a : 0.0
DECAY 1101 0.000000e+00 # n : 0.0
DECAY 3001 0.000000e+00 # ~chi : 0.0
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BLOCK QNUMBERS 1101 # n
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      2 2 # number of spin states (2s+1)
      3 1 # colour rep (1: singlet, 3: triplet, 8: octet)
      4 1 # particle/antiparticle distinction (0=own anti)
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BLOCK QNUMBERS 3001 # ~chi
      1 0 # 3 times electric charge
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      4 1 # particle/antiparticle distinction (0=own anti)
###################################
## INFORMATION FOR QNUMBERS 1001
###################################
BLOCK QNUMBERS 1001 # s0
      1 0 # 3 times electric charge
      2 1 # number of spin states (2s+1)
      3 1 # colour rep (1: singlet, 3: triplet, 8: octet)
      4 0 # particle/antiparticle distinction (0=own anti)
</slha>
<MGGenerationInfo>
# Number of Events : 10000
# Integrated weight (pb) : 0.765535
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</LesHouchesEvents>

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 : #5

Hi Olivier,

Thanks a lot for your reply!

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.

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 : #7

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

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 : #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

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:
> https://answers.launchpad.net/mg5amcnlo/+question/671061
>
> Andrea requested more information:
> 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
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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 : #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

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:
> https://answers.launchpad.net/mg5amcnlo/+question/671061
>
> 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
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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