Comment 2 for bug 1519852

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
>
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