"Decay without corresponding particle in core process" explanation?

Asked by Philippe Giguere

Hi, I am trying to generate a hard process for proton collisions which gives rise to susy particles, which subsequently decay to a neutralino and a SM particle (to conserve R-parity). Here is a sample input and the corresponding error:

Command "generate p p > susy susy, susy > n1 sm " interrupted in sub-command:
"generate p p > susy susy, susy > n1 sm" with error:
InvalidCmd : Decay without corresponding particle in core process. Please check your process definition.

What's odd is that MadGraph generates all the process: all the hard process, and all the right decays, but then proceeds to give me this error. It's as if MadGraph is insisting that the final states be one of the states involved in the hard process, but I don't understand why that would be necessary, and it's definitely not what I'm trying to achieve.

for completeness here are the multiparticle labels I'm using:

susy = go ul cl t1 ur cr t2 dl sl b1 dr sr b2 ul~ cl~ t1~ ur~ cr~ t2~ dl~ sl~ b1~ dr~ sr~ b2~ sve svm svt el- mul- ta1- er- mur- ta2- sve~ svm~ svt~ el+ mul+ ta1+ er+ mur+ ta2+ x1+ x2+ n1 n2 n3 n4 x1- x2-

sm = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b t~ b~ w+ z w- ta- ta+

p = g u c d s u~ c~ d~ s~

Thank you for any input.

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Johan Alwall (johan-alwall) said :
#1

Hello Philippe,

The reason for this error is that *not* all SUSY 2->2 processes are generated by the command
generate p p > susy susy
You will therefore have particles in your decay multiparticle label that are not part of any core process, and MadGraph then assumes that you have made a mistake.

The reason is that, when you don't include coupling orders in your process, MadGraph 5 will generate only the leading processes according to the coupling hierarchy of the model. In mssm (as in sm) the coupling hierarchy is QCD=1, QED=2 (which means, roughly, that 2 orders of QCD corresponds to 1 order of QED). This is crucial when you generate e.g. multijet processes, and want to avoid getting diagrams with photon, W and Z boson exchange which give a negligible contribution but take (a lot of) extra time in the phase space integration.

The easiest way to fix this is by simply adding an order specification in your core process:
generate p p > susy susy QED=2 QCD=2, susy > n1 sm
(meaning that you allow a maximum of 2 QED vertices and 2 QCD vertices in the diagrams of the processes).

However, this way you will get a (not too large, in this case) number of negligibly contributing diagrams in e.g. squark pair production.

So in this case, I think it was good that this error was displayed, since you might otherwise not have noticed that you are missing processes that you intended to include. In general, it might be a bit too restrictive however, so we will change the behavior (from error to warning) in a future version of MadGraph.

Many thanks for bringing this case to our attention, all the best,
Johan

Revision history for this message
Johan Alwall (johan-alwall) said :
#2

Hello again Philippe,

I have pushed a new version where I replaced the error with a warning, please see branch
lp:~maddevelopers/madgraph5/1.5.6
(this change will become official once we release v. 1.5.6).

With the new version, you can simply divide your set of particles into two, depending on whether they are charged under QCD or not:

import model mssm
define susyqcd = go ul cl t1 ur cr ul~ cl~ t1~ ur~ cr~
define susyqed = sve svm svt el- mul- ta1- er- mur- ta2- sve~ svm~ svt~ el+ mul+ ta1+ er+ mur+ ta2+ x1+ x2+ n1 n2 n3 n4 x1- x2-
define sm = g u c d s u~ c~ d~ s~ a ve vm vt e- mu- ve~ vm~ vt~ e+ mu+ t b t~ b~ w+ z w- ta- ta+
generate p p > susyqcd susyqcd, susyqcd > n1 sm @0
add process p p > susyqcd susyqed, susyqcd > n1 sm, susyqed > n1 sm @1
add process p p > susyqed susyqed, susyqed > n1 sm @3

The automatic order counting will then be correct for each of the samples, and you will get all (and only) the processes you are actually interested in.

All the best,
Johan

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