# ttbb process at LO/NLO

Dear MadGraph Team,

We have been trying to reproduce the ttbb process as indicated in Table 5 of your paper arXiv:1405.0301 [hep-ph]. We copy below the run_cardNLOtype.dat file that we have used for running MadGraph5_aMC@NLO (version 2.6.0). We have obtained the following results which are consistent with what you show in your table 5:

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NLO-type generation

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xs_LO = 6.14 +- 0.07 pb

Scale uncertainty = +62% -35%

PDF uncertainty = +3% -4%

xs_NLO = 15.08 +- 0.44 pb

Scale uncertainty = +38% -28%

PDF uncertainty = +3% -4%

By contrast, if we run MadGraph5 (version 2.6.0) with the run_cardLOtype.dat file (copied below), we obtain this result:

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LO-type generation

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xs_LO = 1.116 +- 0.008281 pb

We would like to understand why the two calculations at LO show that large differences. Specifically, we would like to ask you some questions:

1.- In order to obtain a cross section at LO, we are inclined to use the last result xs_LO = 1.116 pb. Is this the correct interpretation?

2.- To compute the K-factor we think that one has to compare xs_LO = 6.14 pb with xs_NLO = 15.08 pb, what would mean K=15.08/6.14=2.46. Is that right?

3.- If the result xs_LO = 1.116 pb is the one that we have to use at LO and the K-factor has been well computed in the way of point 2, can we use this K-factor to correct the LO cross section xs_LO = 1.116 pb?

4.- MadGraph5 does not show scale and PDF uncertainties, is there any way to know them?

Thank you very much for your help.

Ernesto Arganda.

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run_cardNLOtype.dat

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#******

# MadGraph5_aMC@NLO *

# *

# run_card.dat aMC@NLO *

# *

# This file is used to set the parameters of the run. *

# *

# Some notation/

# *

# Lines starting with a hash (#) are info or comments *

# *

# mind the format: value = variable ! comment *

# *

# Some of the values of variables can be list. These can either be *

# comma or space separated. *

#******

#

#******

# Running parameters

#******

#

#******

# Tag name for the run (one word) *

#******

tag_1 = run_tag ! name of the run

#******

# Number of LHE events (and their normalization) and the required *

# (relative) accuracy on the Xsec. *

# These values are ignored for fixed order runs *

#******

1000 = nevents ! Number of unweighted events requested

-1.0 = req_acc ! Required accuracy (-1=auto determined from nevents)

-1 = nevt_job! Max number of events per job in event generation.

! (-1= no split).

#******

# Normalize the weights of LHE events such that they sum or average to *

# the total cross section *

#******

average = event_norm ! valid settings: average, sum, bias

#******

# Number of points per itegration channel (ignored for aMC@NLO runs) *

#******

0.1 = req_acc_FO ! Required accuracy (-1=ignored, and use the

# These numbers are ignored except if req_acc_FO is equal to -1

5000 = npoints_FO_grid ! number of points to setup grids

4 = niters_FO_grid ! number of iter. to setup grids

10000 = npoints_FO ! number of points to compute Xsec

6 = niters_FO ! number of iter. to compute Xsec

#******

# Random number seed *

#******

0 = iseed ! rnd seed (0=assigned automatically=

#******

# Collider type and energy *

#******

1 = lpp1 ! beam 1 type (0 = no PDF)

1 = lpp2 ! beam 2 type (0 = no PDF)

6500.0 = ebeam1 ! beam 1 energy in GeV

6500.0 = ebeam2 ! beam 2 energy in GeV

#******

# PDF choice: this automatically fixes also alpha_s(MZ) and its evol. *

#******

lhapdf = pdlabel ! PDF set

23300 = lhaid ! If pdlabel=lhapdf, this is the lhapdf number. Only

! numbers for central PDF sets are allowed. Can be a list;

! PDF sets beyond the first are included via reweighting.

#******

# Include the NLO Monte Carlo subtr. terms for the following parton *

# shower (HERWIG6 | HERWIGPP | PYTHIA6Q | PYTHIA6PT | PYTHIA8) *

# WARNING: PYTHIA6PT works only for processes without FSR!!!! *

#******

HERWIG6 = parton_shower

1.0 = shower_scale_factor ! multiply default shower starting

#******

# Renormalization and factorization scales *

# (Default functional form for the non-fixed scales is the sum of *

# the transverse masses divided by two of all final state particles *

# and partons. This can be changed in SubProcesses/

# dynamical_

#******

False = fixed_ren_scale ! if .true. use fixed ren scale

False = fixed_fac_scale ! if .true. use fixed fac scale

91.118 = muR_ref_fixed ! fixed ren reference scale

91.118 = muF_ref_fixed ! fixed fact reference scale

-1 = dynamical_

! dynamical choices. Can be a list; scale choices beyond the

! first are included via reweighting

1.0 = muR_over_ref ! ratio of current muR over reference muR

1.0 = muF_over_ref ! ratio of current muF over reference muF

#******

# Reweight variables for scale dependence and PDF uncertainty *

#******

1.0, 2.0, 0.5 = rw_rscale ! muR factors to be included by reweighting

1.0, 2.0, 0.5 = rw_fscale ! muF factors to be included by reweighting

True = reweight_scale ! Reweight to get scale variation using the

! rw_rscale and rw_fscale factors. Should be a list of

! booleans of equal length to dynamical_

! specify for which choice to include scale dependence.

True = reweight_PDF ! Reweight to get PDF uncertainty. Should be a

! list booleans of equal length to lhaid to specify for

! which PDF set to include the uncertainties.

#******