[Rivet] [Pythia8] Missing of Matrix Element particles with status 23 cut

[Torbjörn Sjöstrand]

Hello Yichen,

Indeed you have to watch out which information is physical and what not.

On the technical side, some of the status codes that are used internally
in Pythia are converted to the HepMC standard ones when the event is
stored in HepMC. Some other codes, that are not clashing with HepMC,
are retained. If you present an explicit example it may be easier to
understand what is your problem.

Best, Torbjörn

On 2018-03-06 21:09, Andy Buckley wrote:
> Hi Yichen,
> 
> One of the major design features of Rivet is to avoid unphysical search 
> criteria like this. Different generators write hard processes with 
> different status codes and different topologies, so there is no portable 
> way to look for their particles. More importantly, there is nothing 
> physically distinct about "the matrix element" from the rest of the 
> perturbative simulation: you are in danger of "measuring" an artefact of 
> the calculation, rather than something physical.
> 
> On your specific question, I don't know where the problem is, since 
> you've not supplied any example code. But if you are using the Rivet 
> FinalState, for example, then the name gives a hint as to why there are 
> no matrix element particles in it! I guess that you are doing something 
> else, though, if you were able to get what you wanted with Pythia6. 
> Rivet doesn't do any special treatment of generator internal status codes.
> 
> Andy
> 
> *Dr Andy Buckley, Lecturer / Royal Society University Research Fellow*
> Particle Physics Experiment Group, University of Glasgow
> 	
> 
> On Mar 6 2018, at 6:13 pm, Yichen <yichen.li at cern.ch> wrote:
> 
> 
>     Dear Pythia8/Rivet experts,
> 
>     I'm using Rivet to analyze a MG5+Pythia8 ttbar+photon sample.
>     And I'm trying to get the matrix element particles using status==23 cut.
>     But it turns out that in many cases, "status==23" can't capture all the
>     matrix element particles!
>     (I did similar study for Pythia6, where all matrix element particles can
>     be identified with status==3)
> 
>     Do you know what could be the source of the problem?
>     I'm not sure if it's due to Pythia8 or Rivet. So I send this email to
>     both mailing lists.
>     And please let me know if you need more information!
> 
>     Thanks a lot!
> 
>     Yichen Li
> 
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