cross section zero when using the gridpack

Hi

I am curious about the following, normally when I generate the gridpack for intense processes I get a non-zero cross section with a zero number of events. Consider for instance the process

generate p p > l- vl~ j j b b~ QED<=99 QCD<=99

the message after the generation of the gridpack is

Cross-section : 1.909 +- 0.004413 pb
     Nb of events : 0

Then when I use the run.sh script for let us say 10 events I get

combine_events
store_events
  === Results Summary for run: GridRun_1 tag: tag_1 ===

     Cross-section : 0 +- 0 pb
     Nb of events : 10

decay_events -from_cards
DONE
write ./events.lhe.gz

So, null cross section with 10 events, normally I do not worry about this and I assume for this number of events the cross section reported during the generation of the gridpack, i.e. 1.909 +- 0.004413 pb. So why is the cross section reported as zero in the end. And why there are zero events at the beginning?

Here I copy the run card used:

#*********************************************************************
# Collider type and energy *
# lpp: 0=No PDF, 1=proton, -1=antiproton, *
# 2=elastic photon of proton/ion beam *
# +/-3=PDF of electron/positron beam *
# +/-4=PDF of muon/antimuon beam *
#*********************************************************************
     1 = lpp1 ! beam 1 type
     1 = lpp2 ! beam 2 type
     6500.0 = ebeam1 ! beam 1 total energy in GeV
     6500.0 = ebeam2 ! beam 2 total energy in GeV
# To see polarised beam options: type "update beam_pol"

#*********************************************************************
# PDF CHOICE: this automatically fixes alpha_s and its evol. *
# pdlabel: lhapdf=LHAPDF (installation needed) [1412.7420] *
# iww=Improved Weizsaecker-Williams Approx.[hep-ph/9310350] *
# eva=Effective W/Z/A Approx. [2111.02442] *
# edff=EDFF in gamma-UPC [eq.(11) in 2207.03012] *
# chff=ChFF in gamma-UPC [eq.(13) in 2207.03012] *
# none=No PDF, same as lhapdf with lppx=0 *
#*********************************************************************
     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
#*********************************************************************
  True = 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"

#*********************************************************************
#
#*********************************************************************
# Phase-Space Optimization strategy (basic options)
#*********************************************************************
   0 = nhel ! using helicities importance sampling or not.
                             ! 0: sum over helicity, 1: importance sampling
   1 = sde_strategy ! default integration strategy (hep-ph/2021.00773)
                             ! 1 is old strategy (using amp square)
        ! 2 is new strategy (using only the denominator)
# To see advanced option for Phase-Space optimization: type "update psoptim"
#*********************************************************************
# Customization (custom cuts/scale/bias/...) *
# list of files containing fortran function that overwrite default *
#*********************************************************************
  = custom_fcts ! List of files containing user hook function
#*******************************
# Parton level cuts definition *
#*******************************
  0.0 = dsqrt_shat ! minimal shat for full process
#
#
#*********************************************************************
# 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) *
#*********************************************************************
 20.0 = ptj ! minimum pt for the jets
 20.0 = ptb ! minimum pt for the b
 10.0 = ptl ! minimum pt for the charged leptons
 0.0 = misset ! minimum missing Et (sum of neutrino's momenta)
 -1.0 = ptjmax ! maximum pt for the jets
 -1.0 = ptbmax ! maximum pt for the b
 -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})
#
# For display option for energy cut in the partonic center of mass frame type 'update ecut'
#
#*********************************************************************
# Maximum and minimum absolute rapidity (for max, -1 means no cut) *
#*********************************************************************
 5.0 = etaj ! max rap for the jets
 5.0 = etab ! max rap for the b
 2.5 = etal ! max rap for the charged leptons
 0.0 = etabmin ! min rap for the b
 0.0 = etalmin ! min 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 *
#*********************************************************************
 0.4 = drjj ! min distance between jets
 0.4 = drbb ! min distance between b's
 0.4 = drbj ! min distance between b and jet
 0.4 = drjl ! min distance between jet and lepton
 0.4 = drbl ! min distance between b and lepton
 -1.0 = drjjmax ! max distance between jets
  1.8 = drbbmax ! max distance between b's
 -1.0 = drbjmax ! max distance between b and jet
 -1.0 = drjlmax ! max distance between jet and lepton
 -1.0 = drblmax ! max distance between b and lepton
#*********************************************************************
# Minimum and maximum invariant mass for pairs *
#*********************************************************************
 0.0 = mmjj ! min invariant mass of a jet pair
 100.0 = mmbb ! min invariant mass of a b pair
 -1.0 = mmjjmax ! max invariant mass of a jet pair
 150.0 = mmbbmax ! max invariant mass of a b 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 / neutrino *
 # for pair of lepton includes only same flavor, opposite charge
 #*********************************************************************
#*********************************************************************
# Inclusive cuts *
#*********************************************************************
 0.0 = xptj ! minimum pt for at least one jet
 0.0 = xptb ! minimum pt for at least one b
 #*********************************************************************
 # Control the pt's of the jets sorted by pt *
 #*********************************************************************
 0.0 = ptj1min ! minimum pt for the leading jet in pt
 0.0 = ptj2min ! minimum pt for the second jet in pt
 -1.0 = ptj1max ! maximum pt for the leading jet in pt
 -1.0 = ptj2max ! maximum pt for the second jet in pt
 0 = cutuse ! reject event if fails any (0) / all (1) jet pt cuts
 #*********************************************************************
 # 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)
 0.0 = ihtmin !inclusive Ht for all partons (including b)
 -1.0 = ihtmax !inclusive Ht for all partons (including b)
 0.0 = ht2min ! minimum Ht for the two leading jets
 0.0 = ht3min ! minimum Ht for the three leading jets
 0.0 = ht4min ! minimum Ht for the four leading jets
 -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
 #*********************************************************************
 # 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
#*********************************************************************
# 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