can not generate event for ee>zh with Higgs decay

Asked by Hao Zhang on 2021-02-02

Hi,

When we generate the Zh events with Higgs and Z decay, we find a strange behavior which is hard to understand. We use the standard model with default run_card.dat and param_card.dat. For the next three processes:
(1) e+ e- > z h, h> b b~ a
(2) e+ e- > z h, z > l+ l-, h > b b~ a
(3) e+ e- > z h, z > l+ l-, h > b b~ a (change the pole mass of the b-quark to be 0 in the param_card.dat)
We find that MG can generate Feynman diagrams for all of these processes but can only calculate (1) and (3). For (2), when we generate events, it shows:
"INFO: load configuration from /Users/haozhang/HEP_Tools/MG5_aMC_v2_8_2/ee_zh_llbba/Cards/me5_configuration.txt
INFO: load configuration from /Users/haozhang/HEP_Tools/MG5_aMC_v2_8_2/input/mg5_configuration.txt
INFO: load configuration from /Users/haozhang/HEP_Tools/MG5_aMC_v2_8_2/ee_zh_llbba/Cards/me5_configuration.txt
Using default text editor "vi". Set another one in ./input/mg5_configuration.txt
generate_events -f --multicore test01 --nb_core=4
Generating 10000 events with run name test01
survey test01
INFO: compile directory
compile Source Directory
Using random number seed offset = 27
INFO: Running Survey
Creating Jobs
Working on SubProcesses
INFO: P1_ll_zh_z_ll_h_bbxa
INFO: Idle: 1, Running: 1, Completed: 0 [ current time: 17h56 ]
INFO: Idle: 0, Running: 0, Completed: 2 [ 0.35s ]
INFO: Idle: 0, Running: 0, Completed: 2 [ 0.35s ]
INFO: End survey
refine 10000
Creating Jobs
INFO: Refine results to 10000
INFO: Generating 10000.0 unweighted events.
sum of cpu time of last step: 0 second
INFO: Effective Luminosity 1.2e+103 pb^-1
INFO: need to improve 0 channels
Survey return zero cross section.
   Typical reasons are the following:
   1) A massive s-channel particle has a width set to zero.
   2) The pdf are zero for at least one of the initial state particles
      or you are using maxjetflavor=4 for initial state b:s.
   3) The cuts are too strong.
   Please check/correct your param_card and/or your run_card.
Zero result detected: See https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-14
quit
INFO:
"

Does anyone know what is the problem here? Thank you very much!
Best regards,

Hao

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2021-02-03
Last query:
2021-02-03
Last reply:
2021-02-02

Hi,

Indeed this is weird.
I will look at that but i want to already make some point.

Now setting a mass to zero should be done at the model level and not in the param_card.
I do not think that it impacts you here but you can have a lot of nasty issue when setting a mass to zero via the param_card.

Independently of this, your cross-section should be infinite for your third syntax no?
I do not think that you can avoid to set separation cut and pt cut on the photon.
This is certainly mandatory when you have massless b, but likely necessary as well for the massive b as well.

I have tested your first syntax with simple cuts
set cut_decays T
set pta 10
set etaa -1
set drab 0.1

and this is night and day between the two generations. I would strongly suggest to use cuts even for the first syntax.

Cheers,

Olivier

> On 2 Feb 2021, at 11:01, Hao Zhang <email address hidden> wrote:
>
> New question #695319 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/695319
>
> Hi,
>
> When we generate the Zh events with Higgs and Z decay, we find a strange behavior which is hard to understand. We use the standard model with default run_card.dat and param_card.dat. For the next three processes:
> (1) e+ e- > z h, h> b b~ a
> (2) e+ e- > z h, z > l+ l-, h > b b~ a
> (3) e+ e- > z h, z > l+ l-, h > b b~ a (change the pole mass of the b-quark to be 0 in the param_card.dat)
> We find that MG can generate Feynman diagrams for all of these processes but can only calculate (1) and (3). For (2), when we generate events, it shows:
> "INFO: load configuration from /Users/haozhang/HEP_Tools/MG5_aMC_v2_8_2/ee_zh_llbba/Cards/me5_configuration.txt
> INFO: load configuration from /Users/haozhang/HEP_Tools/MG5_aMC_v2_8_2/input/mg5_configuration.txt
> INFO: load configuration from /Users/haozhang/HEP_Tools/MG5_aMC_v2_8_2/ee_zh_llbba/Cards/me5_configuration.txt
> Using default text editor "vi". Set another one in ./input/mg5_configuration.txt
> generate_events -f --multicore test01 --nb_core=4
> Generating 10000 events with run name test01
> survey test01
> INFO: compile directory
> compile Source Directory
> Using random number seed offset = 27
> INFO: Running Survey
> Creating Jobs
> Working on SubProcesses
> INFO: P1_ll_zh_z_ll_h_bbxa
> INFO: Idle: 1, Running: 1, Completed: 0 [ current time: 17h56 ]
> INFO: Idle: 0, Running: 0, Completed: 2 [ 0.35s ]
> INFO: Idle: 0, Running: 0, Completed: 2 [ 0.35s ]
> INFO: End survey
> refine 10000
> Creating Jobs
> INFO: Refine results to 10000
> INFO: Generating 10000.0 unweighted events.
> sum of cpu time of last step: 0 second
> INFO: Effective Luminosity 1.2e+103 pb^-1
> INFO: need to improve 0 channels
> Survey return zero cross section.
> Typical reasons are the following:
> 1) A massive s-channel particle has a width set to zero.
> 2) The pdf are zero for at least one of the initial state particles
> or you are using maxjetflavor=4 for initial state b:s.
> 3) The cuts are too strong.
> Please check/correct your param_card and/or your run_card.
> Zero result detected: See https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/FAQ-General-14
> quit
> INFO:
> "
>
> Does anyone know what is the problem here? Thank you very much!
> Best regards,
>
> Hao
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hao Zhang (haozhang-pkuphy) said : #2

Hi Olivier,

Thank you very much! In fact, I have already checked different cuts (to avoid any IR diversity). I did not mention it because the strangest point is that (1) works but (2) doesn't with the same cut... Sorry for not mention that.

Best regards,

Hao

Ok this was a very concerning bug.

So I have release two urgent bug fix release
1. version 2.8.3.2 : https://launchpad.net/mg5amcnlo/2.0/2.8.x/+download/MG5_aMC_v2.8.3.2.tar.gz
2. version 2.9.1.2 : https://launchpad.net/mg5amcnlo/2.0/2.9.x/+download/MG5_aMC_v2.9.1.2.tar.gz
(2.9 version is still very new, so I decide to backport the fix to 2.8 version as well even if in theory we do not do that)

Thanks so much to have find this and for the report,

Cheers,

Olivier

> On 2 Feb 2021, at 14:30, Hao Zhang <email address hidden> wrote:
>
> Question #695319 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/695319
>
> Hao Zhang posted a new comment:
> Hi Olivier,
>
> Thank you very much! In fact, I have already checked different cuts (to
> avoid any IR diversity). I did not mention it because the strangest
> point is that (1) works but (2) doesn't with the same cut... Sorry for
> not mention that.
>
> Best regards,
>
> Hao
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Hao Zhang (haozhang-pkuphy) said : #4

Hi Olivier,

Thank you vert much!
Best regards,

Hao

Hao Zhang (haozhang-pkuphy) said : #5

Thanks Olivier Mattelaer, that solved my question.