MadGraph5_aMC@NLO

Is it possible to use Pythia to shower the partons from a 1 to N decay process

Asked by yicongsui

Hi,

I am trying to use MG5_aMC 2.5.4 and pythia 8.2 to generate a Xf > ve b b~ . I write a new model that the darkmatter Xf could decay to neutrino and Higgs. Two questions:

Firstly, can i use madgraph to generate such process? If i set the beam energy in run_card to be exactly the mass of the darkmatter particle, does this represent the case of a dark matter particle at rest decaying?

Secondly, in my test run, pythia 8 has been producing 0 output. I check the PY8_log.txt file and find these two useful lines:

 PYTHIA Error in Pythia::init: cannot handle this beam combination
 PYTHIA Abort from Pythia::init: checkBeams initialization failed

Does this mean pythia cannot handle the showering for such 1 to N decay case? If it could, does this error occur because of my mistake in the setup?

Best

Yicong

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi,

> Firstly, can i use madgraph to generate such process?

Yes you can.

> If i set the beam energy in run_card to be exactly the mass of the darkmatter particle, does this represent the case of a dark matter particle at rest decaying?

The beam energy is not used in madgraph for such computation. All the event produce will be in the rest frame of the original particles.

> Secondly, in my test run, pythia 8 has been producing 0 output. I check the PY8_log.txt file and find these two useful lines:

Looks like that mad graph incorrectly writes the <init> block for such type of event which prevent pythia8 to work correctly on such events.
I will fix this. For the time being, the solution is to modify this <init> information by hand.

Here is an example of the one created by MG5aMC (for h > b b~)
<init>
2212 2212 6.500000e+03 6.500000e+03 0 0 247000 247000 -4 1
5.390800e-03 1.038500e-11 5.390800e-03 1
<generator name='MadGraph5_aMC@NLO' version='2.5.4'>please cite 1405.0301 </generator>
</init>
And here is the correct one (again for h> b b~):
<init>
25 0 1.2500000e+02 0.000000e+03 0 0 247000 247000 -4 1
5.390800e-03 1.069900e-11 5.390800e-03 1
<generator name='MadGraph5_aMC@NLO' version='2.5.4'>please cite 1405.0301 </generator>
</init>

To limit the change that are needed, you can set some of the value correctly in the run_card
(Setting ebeam1 to the mass of the particle, and ebeam2 to 0, and setting lpp1 and lpp2 to 0

I will check what is happening and try to have it fix for the next version of the code.

Cheers,

Olivier

> On 3 May 2017, at 06:53, yicongsui <email address hidden> wrote:
>
> New question #630911 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/630911
>
> Hi,
>
> I am trying to use MG5_aMC 2.5.4 and pythia 8.2 to generate a Xf > ve b b~ . I write a new model that the darkmatter Xf could decay to neutrino and Higgs. Two questions:
>
> Firstly, can i use madgraph to generate such process? If i set the beam energy in run_card to be exactly the mass of the darkmatter particle, does this represent the case of a dark matter particle at rest decaying?
>
> Secondly, in my test run, pythia 8 has been producing 0 output. I check the PY8_log.txt file and find these two useful lines:
>
> PYTHIA Error in Pythia::init: cannot handle this beam combination
> PYTHIA Abort from Pythia::init: checkBeams initialization failed
>
>
> Does this mean pythia cannot handle the showering for such 1 to N decay case? If it could, does this error occur because of my mistake in the setup?
>
> Best
>
> Yicong
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#2

Hi,

Here is a link to a patch that seems to make the trick.
http://bazaar.launchpad.net/~mg5core2/mg5amcnlo/2.5.5/diff/282?context=3

Cheers,

Olivier
> On 3 May 2017, at 10:48, Olivier Mattelaer <email address hidden> wrote:
>
> Question #630911 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/630911
>
> Status: Open => Answered
>
> Olivier Mattelaer proposed the following answer:
> Hi,
>
>> Firstly, can i use madgraph to generate such process?
>
> Yes you can.
>
>> If i set the beam energy in run_card to be exactly the mass of the
> darkmatter particle, does this represent the case of a dark matter
> particle at rest decaying?
>
>
> The beam energy is not used in madgraph for such computation. All the event produce will be in the rest frame of the original particles.
>
>
>> Secondly, in my test run, pythia 8 has been producing 0 output. I check the PY8_log.txt file and find these two useful lines:
>
> Looks like that mad graph incorrectly writes the <init> block for such type of event which prevent pythia8 to work correctly on such events.
> I will fix this. For the time being, the solution is to modify this <init> information by hand.
>
> Here is an example of the one created by MG5aMC (for h > b b~)
> <init>
> 2212 2212 6.500000e+03 6.500000e+03 0 0 247000 247000 -4 1
> 5.390800e-03 1.038500e-11 5.390800e-03 1
> <generator name='MadGraph5_aMC@NLO' version='2.5.4'>please cite 1405.0301 </generator>
> </init>
> And here is the correct one (again for h> b b~):
> <init>
> 25 0 1.2500000e+02 0.000000e+03 0 0 247000 247000 -4 1
> 5.390800e-03 1.069900e-11 5.390800e-03 1
> <generator name='MadGraph5_aMC@NLO' version='2.5.4'>please cite 1405.0301 </generator>
> </init>
>
> To limit the change that are needed, you can set some of the value correctly in the run_card
> (Setting ebeam1 to the mass of the particle, and ebeam2 to 0, and setting lpp1 and lpp2 to 0
>
> I will check what is happening and try to have it fix for the next
> version of the code.
>
> Cheers,
>
> Olivier
>
>> On 3 May 2017, at 06:53, yicongsui <email address hidden> wrote:
>>
>> New question #630911 on MadGraph5_aMC@NLO:
>> https://answers.launchpad.net/mg5amcnlo/+question/630911
>>
>> Hi,
>>
>> I am trying to use MG5_aMC 2.5.4 and pythia 8.2 to generate a Xf > ve b b~ . I write a new model that the darkmatter Xf could decay to neutrino and Higgs. Two questions:
>>
>> Firstly, can i use madgraph to generate such process? If i set the beam energy in run_card to be exactly the mass of the darkmatter particle, does this represent the case of a dark matter particle at rest decaying?
>>
>> Secondly, in my test run, pythia 8 has been producing 0 output. I check the PY8_log.txt file and find these two useful lines:
>>
>> PYTHIA Error in Pythia::init: cannot handle this beam combination
>> PYTHIA Abort from Pythia::init: checkBeams initialization failed
>>
>>
>> Does this mean pythia cannot handle the showering for such 1 to N decay case? If it could, does this error occur because of my mistake in the setup?
>>
>> Best
>>
>> Yicong
>>
>> --
>> You received this question notification because you are an answer
>> contact for MadGraph5_aMC@NLO.
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
yicongsui (trilobite) said : 		#3

Thanks Olivier Mattelaer, that solved my question.