Bhabha scattering and multilepton processes

Hi,

- Did you notice that the energy cut was applied in the center of mass frame?
I guess that you have to adapt your cut accordingly.

They are no mandatory cut so you do not need to put a pt cut.

> Madgraph (using again an apparently mandatory cut on the pt) I obtained, as a final cross section value, one of the fake value than CalcHEP has discarded. How is it possible?

Just bad luck I guess...

> 3) CalcHEP is unable to generate process with 6 leptons in the final state? Is Madgraph capable to do it?

Yes we can

Cheers,

Olivier

> On 10 Apr 2020, at 16:52, Gabriele Martelli <email address hidden> wrote:
>
> New question #689819 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/689819
>
> Hello!! I'm trying to generate e+ e- > n(e+ e-) processes, where n goes from1 to 3, at low energy (positron beam = 550 MeV) and with the electron target at rest.
> The first process I was trying to simulate is the Bhabha scattering. In order to do so, I changed the EM coupling constant from 1/128 to 1/137 'cause I want only the photon as a mediator boson. After that, I generated the process, with a 50 MeV cut on the minimun energy of the final leptons in order to prevent the cross section from diverging, but the generation failed.
>
> Since the first failure, some questions have occurred to me and I hope to find the answers with you:
>
> 1) After many tries I saw that if I put a cut on the traverse momentum the generations will not fail but I will obtain false cross sections (I have CalcHEP cross section values for comparison). Is the cut on the transverse momentum mandatory on Madgraph? Is there a way to put it to zero and obtain a real value of cross section?
>
> 2) When I generate a process on CalcHEP, if it is complicate it will take some time to obtain a real value. It will show you some fake values and then the real one.
> I tried the e+ e- > 2(e+e-) on CalcHEP and after some trials I obtained the expected value of cross section. On Madgraph (using again an apparently mandatory cut on the pt) I obtained, as a final cross section value, one of the fake value than CalcHEP has discarded. How is it possible?
>
> 3) CalcHEP is unable to generate process with 6 leptons in the final state? Is Madgraph capable to do it?
>
> To help you better understand my problems, I paste here my param_card and my run_ card from the Bhabha process(note that I use the model sm).
>
> PARAM_CARD
>
> ######################################################################
> ## PARAM_CARD AUTOMATICALY GENERATED BY MG5 FOLLOWING UFO MODEL ####
> ######################################################################
> ## ##
> ## Width set on Auto will be computed following the information ##
> ## present in the decay.py files of the model. ##
> ## See arXiv:1402.1178 for more details. ##
> ## ##
> ######################################################################
>
> ###################################
> ## INFORMATION FOR MASS
> ###################################
> Block mass
> 4 1.270000e+00 # MC
> 5 4.700000e+00 # MB
> 6 1.720000e+02 # MT
> 11 5.110000e-04 # Me
> 13 1.056600e-01 # MM
> 15 1.777000e+00 # MTA
> 23 9.118800e+01 # MZ
> 25 1.250000e+02 # MH
> ## Dependent parameters, given by model restrictions.
> ## Those values should be edited following the
> ## analytical expression. MG5 ignores those values
> ## but they are important for interfacing the output of MG5
> ## to external program such as Pythia.
> 1 0.000000e+00 # d : 0.0
> 2 0.000000e+00 # u : 0.0
> 3 0.000000e+00 # s : 0.0
> 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 8.093557e+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 SMINPUTS
> ###################################
> Block sminputs
> 1 1.370000e+02 # aEWM1
> 2 1.166390e-05 # Gf
> 3 1.180000e-01 # aS
>
> ###################################
> ## INFORMATION FOR WOLFENSTEIN
> ###################################
> Block wolfenstein
> 1 2.253000e-01 # lamWS
> 2 8.080000e-01 # AWS
> 3 1.320000e-01 # rhoWS
> 4 3.410000e-01 # etaWS
>
> ###################################
> ## INFORMATION FOR YUKAWA
> ###################################
> Block yukawa
> 4 1.270000e+00 # ymc
> 5 4.200000e+00 # ymb
> 6 1.645000e+02 # ymt
> 11 5.110000e-04 # yme
> 13 1.056600e-01 # ymm
> 15 1.777000e+00 # ymtau
>
> ###################################
> ## INFORMATION FOR DECAY
> ###################################
> DECAY 6 1.508336e+00 # WT
> DECAY 15 2.270000e-12 # WTau
> DECAY 23 2.441404e+00 # WZ
> DECAY 24 2.047600e+00 # WW
> DECAY 25 6.382339e-03 # WH
> ## Dependent parameters, given by model restrictions.
> ## Those values should be edited following the
> ## analytical expression. MG5 ignores those values
> ## but they are important for interfacing the output of MG5
> ## to external program such as Pythia.
> DECAY 1 0.000000e+00 # d : 0.0
> DECAY 2 0.000000e+00 # u : 0.0
> DECAY 3 0.000000e+00 # s : 0.0
> DECAY 4 0.000000e+00 # c : 0.0
> DECAY 5 0.000000e+00 # b : 0.0
> DECAY 11 0.000000e+00 # e- : 0.0
> DECAY 12 0.000000e+00 # ve : 0.0
> DECAY 13 0.000000e+00 # mu- : 0.0
> DECAY 14 0.000000e+00 # vm : 0.0
> DECAY 16 0.000000e+00 # vt : 0.0
> DECAY 21 0.000000e+00 # g : 0.0
> DECAY 22 0.000000e+00 # a : 0.0
>
>
> RUN_CARD
>
> #*********************************************************************
> # 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 *
> #*********************************************************************
> 0 = lpp1 ! beam 1 type
> 0 = lpp2 ! beam 2 type
> 0.55 = ebeam1 ! beam 1 total energy in GeV
> 0 = 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
> #*********************************************************************
> 15.0 = bwcutoff ! (M+/-bwcutoff*Gamma)
> #*********************************************************************
> # Standard Cuts *
> #*********************************************************************
> # Minimum and maximum pt's (for max, -1 means no cut) *
> #*********************************************************************
> 0.0 = ptl ! minimum pt for the charged leptons
> -1.0 = ptlmax ! maximum pt for the charged leptons
> {} = 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
> 0.05 = 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 = etal ! max rap for the charged leptons
> 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 = drll ! min distance between leptons
> -1.0 = drllmax ! max distance between leptons
> #*********************************************************************
> # Minimum and maximum invariant mass for pairs *
> #*********************************************************************
> 0.0 = mmll ! min invariant mass of l+l- (same flavour) lepton 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 = xptl ! minimum pt for at least one charged lepton
> #*********************************************************************
> # Control the pt's of leptons sorted by pt *
> #*********************************************************************
> 0.0 = ptl1min ! minimum pt for the leading lepton in pt
> 0.0 = ptl2min ! minimum pt for the second lepton in pt
> -1.0 = ptl1max ! maximum pt for the leading lepton in pt
> -1.0 = ptl2max ! maximum pt for the second lepton in pt
> #*********************************************************************
> # 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'#*********************************************************************
> # Additional hidden parameters
> #*********************************************************************
> ['--mur=0.5,1,2', '--muf=0.5,1,2', '--pdf=errorset'] = systematics_arguments # Choose the argment to pass to the systematics command. like --mur=0.25,1,4. Look at the help of the systematics function for more details.
>
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hello Mr Mattaelaer!!

1) I didn't notice that the energy cut was applied in the center of mass frame, my mistake. I have to apply this cut only on the energy of the final state leptons. I am still a novice in using Madgraph, do you have any suggestion on how to implement such cut?

2) I'm sorry but I don't understand how this can be bad luck. I give you another example: I have a theoretical value of the e+e- -> 2(e+e-) cross section without cuts. When I compute this process in CalcHEP, I obtain (after many evaluations in the same run) the expected value. On Madgraph, even if I compute this process many times, I will still obtain the same fake value, as a final result, that can be seen during the CalcHEP computation.
To make the idea clear I show you some numbers:

expected value: 1.29948E+08 pb

CalcHEP value: 1.290934E+08 pb

Madgraph value: 2.68829660329e+11 pb

Cheers,

Gabriele Martelli

Hi,

> 1) I didn't notice that the energy cut was applied in the center of mass
> frame, my mistake. I have to apply this cut only on the energy of the
> final state leptons. I am still a novice in using Madgraph, do you have
> any suggestion on how to implement such cut?

My advise would be then to write in the run_card the minimum energy possible in that frame.
(in principle you can put 0 but I will try to avoid it if possible)

Then you will have to implement your own cut (in fortran) by editing the file
SubProcesses/dummy_functions.f

> 2) I'm sorry but I don't understand how this can be bad luck.

I do not think that the fact that you see the same value within Calchep and madgraph has any meaning that's it.
> expected value: 1.29948E+08 pb
>
> CalcHEP value: 1.290934E+08 pb
>
> Madgraph value: 2.68829660329e+11 pb

This probably just means that you do not have the correct cut setup in madgraph (which would not suprise me due to the first point above)

Cheers,

Olivier

> On 11 Apr 2020, at 14:09, Gabriele Martelli <email address hidden> wrote:
>
> Question #689819 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/689819
>
> Status: Answered => Open
>
> Gabriele Martelli is still having a problem:
> Hello Mr Mattaelaer!!
>
> 1) I didn't notice that the energy cut was applied in the center of mass
> frame, my mistake. I have to apply this cut only on the energy of the
> final state leptons. I am still a novice in using Madgraph, do you have
> any suggestion on how to implement such cut?
>
> 2) I'm sorry but I don't understand how this can be bad luck. I give you another example: I have a theoretical value of the e+e- -> 2(e+e-) cross section without cuts. When I compute this process in CalcHEP, I obtain (after many evaluations in the same run) the expected value. On Madgraph, even if I compute this process many times, I will still obtain the same fake value, as a final result, that can be seen during the CalcHEP computation.
> To make the idea clear I show you some numbers:
>
> expected value: 1.29948E+08 pb
>
> CalcHEP value: 1.290934E+08 pb
>
> Madgraph value: 2.68829660329e+11 pb
>
> Cheers,
>
> Gabriele Martelli
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.