Unexpected behaviour of iseed in scripted runs

Asked by Baptiste Ravina on 2021-02-04

Using vanilla MG v2.8.2, I ran the following two scripts:

generate p p > t t~ QED=0 QCD=2
output test_nodecay
launch test_nodecay
    set nevents 1000
    set iseed 12345
launch test_nodecay
    set nevents 1000
    set iseed 12345
launch test_nodecay
    set nevents 1000
    set iseed 12345
launch test_nodecay -i
    print_results --path=./xsec_nodecay.txt --format=short

generate p p > t t~ QED=0 QCD=2, t > w+ b, t~ > w- b~
output test_decay
launch test_decay
    set nevents 1000
    set iseed 12345
launch test_decay
    set nevents 1000
    set iseed 12345
launch test_decay
    set nevents 1000
    set iseed 12345
launch test_decay -i
    print_results --path=./xsec_decay.txt --format=short

The first script, which doesn't decay the tops, produces the following cross sections:
run_01 tag_1 503.307 1.75095850431 1000
run_02 tag_1 503.307 1.75095850431 1000
run_03 tag_1 503.307 1.75095850431 1000

as expected, given the random seed is the same in each run.

On the other hand, adding the decay chain as in the second script:
run_01 tag_1 484.54 1.6126228715 1000
run_02 tag_1 482.07 1.51073567367 1000
run_03 tag_1 482.07 1.51073567367 1000

So it seems that something is at play here, with the decay syntax, that somehow ignores the choice of iseed in the first run?

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Last query:
2021-02-04
Last reply:
2021-02-05

I'm closing this question, since this will be track as a bug.

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