Pythia outputUsage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement

Asked by Christopher Chang

I have been trying to generate one-loop diagrams (for a range of different models generated with FeynRules) and output pythia, yet have hit the error:

Traceback (most recent call last):
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/extended_cmd.py", line 1544, in onecmd
    return self.onecmd_orig(line, **opt)
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/extended_cmd.py", line 1493, in onecmd_orig
    return func(arg, **opt)
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/master_interface.py", line 304, in do_output
    self.cmd.do_output(self, line, *args, **opts)
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/madgraph_interface.py", line 8182, in do_output
    self.export(nojpeg, main_file_name, group_processes, args)
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/master_interface.py", line 318, in export
    return self.cmd.export(self, *args, **opts)
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/madgraph_interface.py", line 8340, in export
    exporter = self._curr_exporter.generate_process_directory(\
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 1774, in generate_process_directory
    process_exporter_pythia8.generate_process_files()
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 226, in generate_process_files
    self.write_process_cc_file(writers.CPPWriter(filename))
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 300, in write_process_cc_file
    self.get_process_function_definitions()
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 1105, in get_process_function_definitions
    color_amplitudes = [me.get_color_amplitudes() for me in \
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 1105, in <listcomp>
    color_amplitudes = [me.get_color_amplitudes() for me in \
  File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/loop/loop_helas_objects.py", line 2399, in get_color_amplitudes
    raise self.PhysicsObjectError("Usage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement")
madgraph.core.base_objects.PhysicsObject.PhysicsObjectError: Usage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement

I can see that the function get_color_amplitudes() in loop_helas_object.py simply returns this error to prevent usage of this. However, I can also see that below this, there are functions 'get_born_color_amplitudes' and 'get_loop_color_amplitudes'. Is there a choice of settings that will allow Pythia to be output for a loop process, or, is Pythia not capable of handling these at all?

The easiest way to reproduce the error is to just use the loop_sm model:

import model loop_sm
generate g g > h > t t~ g [noborn=QCD]
output pythia8 PythiaLOOP

Any help would be greatly appreciated,
Christopher

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

Hi,

I do not think that such type of computation are handled/supported in pythia8.
Additionnally I do not think that we are ready to have the loop computation in a pure C++ form.

Cheers,

Olivier

> On 8 Jul 2021, at 08:11, Christopher Chang <email address hidden> wrote:
>
> New question #697905 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697905
>
> I have been trying to generate one-loop diagrams (for a range of different models generated with FeynRules) and output pythia, yet have hit the error:
>
> Traceback (most recent call last):
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/extended_cmd.py", line 1544, in onecmd
> return self.onecmd_orig(line, **opt)
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/extended_cmd.py", line 1493, in onecmd_orig
> return func(arg, **opt)
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/master_interface.py", line 304, in do_output
> self.cmd.do_output(self, line, *args, **opts)
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/madgraph_interface.py", line 8182, in do_output
> self.export(nojpeg, main_file_name, group_processes, args)
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/master_interface.py", line 318, in export
> return self.cmd.export(self, *args, **opts)
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/interface/madgraph_interface.py", line 8340, in export
> exporter = self._curr_exporter.generate_process_directory(\
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 1774, in generate_process_directory
> process_exporter_pythia8.generate_process_files()
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 226, in generate_process_files
> self.write_process_cc_file(writers.CPPWriter(filename))
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 300, in write_process_cc_file
> self.get_process_function_definitions()
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 1105, in get_process_function_definitions
> color_amplitudes = [me.get_color_amplitudes() for me in \
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/iolibs/export_cpp.py", line 1105, in <listcomp>
> color_amplitudes = [me.get_color_amplitudes() for me in \
> File "/home/s4358844/MG5_aMC_v3.1.0/MG5_aMC_v3_1_0/madgraph/loop/loop_helas_objects.py", line 2399, in get_color_amplitudes
> raise self.PhysicsObjectError("Usage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement")
> madgraph.core.base_objects.PhysicsObject.PhysicsObjectError: Usage of get_color_amplitudes is not allowed in a LoopHelasMatrixElement
>
> I can see that the function get_color_amplitudes() in loop_helas_object.py simply returns this error to prevent usage of this. However, I can also see that below this, there are functions 'get_born_color_amplitudes' and 'get_loop_color_amplitudes'. Is there a choice of settings that will allow Pythia to be output for a loop process, or, is Pythia not capable of handling these at all?
>
> The easiest way to reproduce the error is to just use the loop_sm model:
>
> import model loop_sm
> generate g g > h > t t~ g [noborn=QCD]
> output pythia8 PythiaLOOP
>
> Any help would be greatly appreciated,
> Christopher
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Christopher Chang (chrisjchang) said :
#2

Thank you for this answer.