Zero cross sect. with no cuts

Asked by Michele Tammaro on 2020-07-08

Hello,

I am relatively new with MadGraph. I am generating the process

ve e- > ve e- a /z

through a new model created with FeynRules. Since I want to look at low energy neutrinos scattering on electrons at rest, I set the electron energy = 0.511 MeV and I put no cuts on the final energy (run_card below). For neutrino energies below 0.8 MeV, MadGraph always returns me with the zero cross section error message, while it works fine for higher energies. As an example, I tried also the neutral current process ve e- > ve e- (a Z exchange) with electron at rest, same cuts and neutrino with .5 MeV energy, and gives me the same error message.

Am I missing some cuts or something else in the run_card?

Thanks,
Michele

#*********************************************************************
# MadGraph5_aMC@NLO *
# *
# run_card.dat MadEvent *
# *
# 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 *
#*********************************************************************
#
#*********************************************************************
# Tag name for the run (one word) *
#*********************************************************************
  tag_1 = run_tag ! name of the run
#*********************************************************************
# Number of events and rnd seed *
# Warning: Do not generate more than 1M events in a single run *
#*********************************************************************
  10000 = nevents ! Number of unweighted events requested
  0 = iseed ! rnd seed (0=assigned automatically=default))
#*********************************************************************
# Collider type and energy *
# lpp: 0=No PDF, 1=proton, -1=antiproton, 2=photon from proton,
# 3=photon from electron 4
#********************************************************************6
     0 = lpp1 ! beam 1 type
     0 = lpp2 ! beam 2 type
     5e-4 = ebeam1 ! beam 1 total energy in GeV
     5.11e-4 = ebeam2 ! beam 2 total energy in GeV
#*********************************************************************
# Beam polarization from -100 (left-handed) to 100 (right-handed) *
#*********************************************************************
     0.0 = polbeam1 ! beam polarization for beam 1
     0.0 = polbeam2 ! beam polarization for beam 2

#*********************************************************************
# PDF CHOICE: this automatically fixes also alpha_s and its evol. *
#*********************************************************************
     nn23lo1 = pdlabel ! PDF set
     230000 = lhaid ! if pdlabel=lhapdf, this is the lhapdf number
# To see heavy ion options: type "update ion_pdf"
#*********************************************************************
# Renormalization and factorization scales *
#*********************************************************************
 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
#*********************************************************************
# Type and output format
#*********************************************************************
  False = gridpack !True = setting up the grid pack
  -1.0 = time_of_flight ! threshold (in mm) below which the invariant livetime is not written (-1 means not written)
  average = event_norm ! average/sum. Normalization of the weight in the LHEF
# To see MLM/CKKW merging options: type "update MLM" or "update CKKW"

#*********************************************************************
#
#*********************************************************************
# handling of the helicities:
# 0: sum over all helicities
# 1: importance sampling over helicities
#*********************************************************************
   0 = nhel ! using helicities importance sampling or not.
#*********************************************************************
# Generation bias, check the wiki page below for more information: *
# 'cp3.irmp.ucl.ac.be/projects/madgraph/wiki/LOEventGenerationBias' *
#*********************************************************************
 None = bias_module ! Bias type of bias, [None, ptj_bias, -custom_folder-]
 {} = bias_parameters ! Specifies the parameters of the module.
#
#*******************************
# Parton level cuts definition *
#*******************************
#
#
#*********************************************************************
# BW cutoff (M+/-bwcutoff*Gamma) ! Define on/off-shell for "$" and decay
#*********************************************************************
  -1.0 = bwcutoff ! (M+/-bwcutoff*Gamma)
#*********************************************************************
# Standard Cuts *
#*********************************************************************
# Minimum and maximum pt's (for max, -1 means no cut) *
#*********************************************************************
 0.0 = pta ! minimum pt for the photons
 0.0 = ptl ! minimum pt for the charged leptons
 0.0 = misset ! minimum missing Et (sum of neutrino's momenta)
 -1.0 = ptamax ! maximum pt for the photons
 -1.0 = ptlmax ! maximum pt for the charged leptons
 -1.0 = missetmax ! maximum missing Et (sum of neutrino's momenta)
 {} = pt_min_pdg ! pt cut for other particles (use pdg code). Applied on particle and anti-particle
 {} = pt_max_pdg ! pt cut for other particles (syntax e.g. {6: 100, 25: 50})
#*********************************************************************
# Minimum and maximum E's (in the center of mass frame) *
#*********************************************************************
  0.0 = ej ! minimum E for the jets
  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
  -1.0 = ejmax ! maximum E for the jets
 -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
 {} = e_min_pdg ! E cut for other particles (use pdg code). Applied on particle and anti-particle
 {} = e_max_pdg ! E cut for other particles (syntax e.g. {6: 100, 25: 50})

#*********************************************************************
# Maximum and minimum absolute rapidity (for max, -1 means no cut) *
#*********************************************************************
 -1.0 = etaa ! max rap for the photons
 -1.0 = etal ! max rap for the charged leptons
 0.0 = etajmin ! min rap for the jets
 0.0 = etaamin ! min rap for the photons
 0.0 = etalmin ! main rap for the charged leptons
 {} = eta_min_pdg ! rap cut for other particles (use pdg code). Applied on particle and anti-particle
 {} = eta_max_pdg ! rap cut for other particles (syntax e.g. {6: 2.5, 23: 5})
#*********************************************************************
# Minimum and maximum DeltaR distance *
#*********************************************************************
 0.0 = dral ! min distance between gamma and lepton
 -1.0 = dralmax ! maxdistance between gamma and lepton
#*********************************************************************
# Minimum and maximum invariant mass for pairs *
#*********************************************************************
 {} = 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 *
 #*********************************************************************
#*********************************************************************
# Inclusive cuts *
#*********************************************************************
#*********************************************************************
# maximal pdg code for quark to be considered as a light jet *
# (otherwise b cuts are applied) *
#*********************************************************************
 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'

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Michele Tammaro
Solved:
2020-07-09
Last query:
2020-07-09
Last reply:
2020-07-09

Hi,

They are indeed an hidden cut associated to the default dynamical scale (preventing issue with running at low scale). So I would suggest to just run at fix scale.

Cheers,

Olivier

> On 8 Jul 2020, at 18:50, Michele Tammaro <email address hidden> wrote:
>
> dynamical_scale_choice

Hi,

Also I see that you have

#*********************************************************************
# Beam polarization from -100 (left-handed) to 100 (right-handed) *
#*********************************************************************
    0.0 = polbeam1 ! beam polarization for beam 1
    0.0 = polbeam2 ! beam polarization for beam 2

The neutrino are typically fully polarised (but in some exotic model).
Since a couple of version, we now set polarised beam as default for neutrino in initial state
so either you are using an old version, or you have changed that on purpose (in which case it is probably fine)

Cheers,

Olivier

> On 8 Jul 2020, at 18:50, Michele Tammaro <email address hidden> wrote:
>
> dynamical_scale_choice

Michele Tammaro (mekeee) said : #3

Hi Oliver,

thank you for the quick response. Yes I changed the polarization on purpose but did not affect the results in general.

How do I implement the running at fix scale? Should I change "False" to "True" in the renormalization part (below)?

Thanks again,
Michele

#*********************************************************************
# Renormalization and factorization scales *
#*********************************************************************
 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

> thank you for the quick response. Yes I changed the polarization on
> purpose but did not affect the results in general.

If your neutrino is only left handed it should give you a factor of 2.

> How do I implement the running at fix scale? Should I change "False" to
> "True" in the renormalization part (below)?

Yes, I would also set the same value for fixed_fac_scale

Cheers,

Olivier

> On 8 Jul 2020, at 20:45, Michele Tammaro <email address hidden> wrote:
>
> Question #691759 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/691759
>
> Michele Tammaro posted a new comment:
> Hi Oliver,
>
> thank you for the quick response. Yes I changed the polarization on
> purpose but did not affect the results in general.
>
> How do I implement the running at fix scale? Should I change "False" to
> "True" in the renormalization part (below)?
>
> Thanks again,
> Michele
>
> #*********************************************************************
> # Renormalization and factorization scales *
> #*********************************************************************
> 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
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Michele Tammaro (mekeee) said : #5

Hi,

thanks. Sadly, fixing the scale does not seem to solve the problem.

Best,
Michele

Can you send me your model by email (<email address hidden> <mailto:<email address hidden>>) and a link to this thread
I will take a look

Cheers,

Olivier

> On 8 Jul 2020, at 23:11, Michele Tammaro <email address hidden> wrote:
>
> Question #691759 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/691759
>
> Michele Tammaro posted a new comment:
> Hi,
>
> thanks. Sadly, fixing the scale does not seem to solve the problem.
>
> Best,
> Michele
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hi,

Thanks for the model.
The issue was related to a fix to prevent numerical issue with small invariant mass. I have update the condition to avoid it to trigger in such cases (which is much more safe).

I have also noticed a wrong warning about polbeam.

You will find the patches here (and also a link to download the associated build)
https://bazaar.launchpad.net/~mg5core1/mg5amcnlo/2.7.4/revision/297

Thanks a lot for this report,

Olivier

Michele Tammaro (mekeee) said : #8

Hi Olivier,

thank you very much! The revision seems to have fixed it, I tried to run at low energy and it works. To be noted, the results I am having with higher energies are orders of magnitude bigger than before, was it to be expected?

Thanks again,
Michele

yes,

I tested the 1MeV case, and noted that of the three channel of integration two of them was returning 0 cross-section before the patch and only one was returning non zero cross-section.
The patch makes all of them having non zero contribution which is what we want since you do not have any cuts. Obviously this means that the cross-section increases.

Cheers,

Olivier

> On 9 Jul 2020, at 21:50, Michele Tammaro <email address hidden> wrote:
>
> Question #691759 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/691759
>
> Michele Tammaro posted a new comment:
> Hi Olivier,
>
> thank you very much! The revision seems to have fixed it, I tried to run
> at low energy and it works. To be noted, the results I am having with
> higher energies are orders of magnitude bigger than before, was it to be
> expected?
>
> Thanks again,
> Michele
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Michele Tammaro (mekeee) said : #10

Thanks again!

Michele