Thanks for the report.
I have succeeded to reproduce the different behavior for MadWidth and MadEvent and indeed the associated matrix-element is different  in both cases.
I’m investigating to understand what the problem is and how to fix this.

Cheers and thanks,

Olivier


> On Nov 25, 2015, at 15:48, Nikolai Wyderka <email address hidden> wrote:
> 
> Public bug reported:
> 
> Hello!
> 
> We generated events from a FeynRule generated model. This model is based
> on RPV-MSSM and includes a Majorana fermion 'psi' (a singlet under the SM)
> and a much heavier charged colored scalar 'sigma'. The complete model
> files are attached.
> The 'psi'-particle can decay into three SM quarks by violating R-parity.
> The decay is mediated by an off-shell 'sigma' particle.  To study the
> hadronization after the decay of 'psi', we generated the process with
> MadGraph: psi > u d d. We then launched the event generator and examined
> the LHE - output.
> The resulting energy spectrum of up-type -quarks  does not have the
> expected shape — it increases with raising energy which is not a physical
> behavior.
> In this case it's also possible to analytically calculate the differential
> decay rate of the produced up-type quark yielding dN/dE ~ E^2*(M_psi-2*E)
> in the limit of massless SM quarks. At least the analytical calculation of
> the squared matrix element was also performed in [Baltz, Gondolo. Physical
> Review D, 57(5):2969, 1998] (for RPV neutralino decays) and disagrees with
> the result from MadGraph.
> Later we figured out that we could reproduce the MadGraph spectrum by
> changing the sign between the two contributing Feynman diagrams in the
> analytical calculation.
> Thus, the problem seems to appear in the calculation of the squared matrix
> element. We therefore analyzed the 'matrix1.f' file created in the
> 'SubProcesses -> P1_psi_udd' sub-folder of the MadGraph output. We were
> able to find the correct spectrum by either deleting all negative signs in
> front of the stored color factors in the 'COLOR DATA' table or by deleting
> all negative signs occurring in the definition of each 'JAMP' beneath this
> color table. We cannot say for sure what the actual source of the error
> is.
> Interestingly, the total decay width calculated with MadWidth and the
> command "compute_width psi" yields the correct value.
> Although we have already found a pragmatic solution to our problem, this
> issue becomes more important in the further treatment of the created
> model. We are trying to generate decays where the up-type quark is a top
> resulting in a five-body final state where an analytic expression for any
> differential decay rate is out of range and the correction of signs in the
> matrix element file seems to be too cumbersome. Therefore we would be very
> grateful for any kind of help.
> 
> We are using MadGraph5 v. 2.3.3
> 
> Steps to reproduce:
> 1. Put attachment's content into MadGraph's model directory
> 2. Start MadGraph, import model MinDecDM_Model
> 3. Generate psi > u d d, output and launch
> 4. wait......
> 5. Analyze the energy spectrum of the LHE event file
> 
> Sincerely,
> Nikolai Wyderka on behalf of Christopher Eckner and Michael Gustafsson
> 
> ** Affects: mg5amcnlo
>     Importance: Undecided
>         Status: New
> 
> ** Attachment added: "DM_model.tar"
>   https://bugs.launchpad.net/bugs/1519852/+attachment/4525288/+files/DM_model.tar
> 
> -- 
> You received this bug notification because you are subscribed to
> MadGraph5_aMC@NLO.
> https://bugs.launchpad.net/bugs/1519852
> 
> Title:
>  Bug (sign errors) in MadGraph's R-parity violating part
> 
> Status in MadGraph5_aMC@NLO:
>  New
> 
> Bug description:
>  Hello!
> 
>  We generated events from a FeynRule generated model. This model is based
>  on RPV-MSSM and includes a Majorana fermion 'psi' (a singlet under the SM)
>  and a much heavier charged colored scalar 'sigma'. The complete model
>  files are attached.
>  The 'psi'-particle can decay into three SM quarks by violating R-parity.
>  The decay is mediated by an off-shell 'sigma' particle.  To study the
>  hadronization after the decay of 'psi', we generated the process with
>  MadGraph: psi > u d d. We then launched the event generator and examined
>  the LHE - output.
>  The resulting energy spectrum of up-type -quarks  does not have the
>  expected shape — it increases with raising energy which is not a physical
>  behavior.
>  In this case it's also possible to analytically calculate the differential
>  decay rate of the produced up-type quark yielding dN/dE ~ E^2*(M_psi-2*E)
>  in the limit of massless SM quarks. At least the analytical calculation of
>  the squared matrix element was also performed in [Baltz, Gondolo. Physical
>  Review D, 57(5):2969, 1998] (for RPV neutralino decays) and disagrees with
>  the result from MadGraph.
>  Later we figured out that we could reproduce the MadGraph spectrum by
>  changing the sign between the two contributing Feynman diagrams in the
>  analytical calculation.
>  Thus, the problem seems to appear in the calculation of the squared matrix
>  element. We therefore analyzed the 'matrix1.f' file created in the
>  'SubProcesses -> P1_psi_udd' sub-folder of the MadGraph output. We were
>  able to find the correct spectrum by either deleting all negative signs in
>  front of the stored color factors in the 'COLOR DATA' table or by deleting
>  all negative signs occurring in the definition of each 'JAMP' beneath this
>  color table. We cannot say for sure what the actual source of the error
>  is.
>  Interestingly, the total decay width calculated with MadWidth and the
>  command "compute_width psi" yields the correct value.
>  Although we have already found a pragmatic solution to our problem, this
>  issue becomes more important in the further treatment of the created
>  model. We are trying to generate decays where the up-type quark is a top
>  resulting in a five-body final state where an analytic expression for any
>  differential decay rate is out of range and the correction of signs in the
>  matrix element file seems to be too cumbersome. Therefore we would be very
>  grateful for any kind of help.
> 
>  We are using MadGraph5 v. 2.3.3
> 
>  Steps to reproduce:
>  1. Put attachment's content into MadGraph's model directory
>  2. Start MadGraph, import model MinDecDM_Model
>  3. Generate psi > u d d, output and launch
>  4. wait......
>  5. Analyze the energy spectrum of the LHE event file
> 
>  Sincerely,
>  Nikolai Wyderka on behalf of Christopher Eckner and Michael Gustafsson
> 
> To manage notifications about this bug go to:
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