Workaround for gluino decay bug (76X)

[duanders]

Pythia8 ignores the first entry in the DECAY table when reading from an SLHA file. Moving the neutralino to the top of the table allows the gluino to decay correctly.

[cmsbuild]

A new Pull Request was created by @duanders (Dustin Anderson) for CMSSW_7_6_X.

Workaround for gluino decay bug (76X)

It involves the following packages:

Configuration/Generator

@vciulli, @cmsbuild, @covarell, @bendavid, @thuer can you please review it and eventually sign? Thanks.
@ghellwig, @Martin-Grunewald this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[covarell]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_6_X IBs or unless it breaks tests. This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[bendavid]

Not sure I fully understand the logic. If pythia8 ignores the first entry, why do you want to move this line to be the first entry?

[duanders]

@bendavid
The decay we want to enable is the gluino one, so we are just putting something else ahead of it in the list. I think it should work no matter what we put there.

I confirm from the event listing that the neutralinos are not decaying to anything -- their daughters are listed as "0".

[bendavid]

Have you discussed this issue already with the pythia authors? Is it specific to cmssw or a general pythia 8 problem? (Is it still a problem in CMSSW 76x where a more recent pythia version is present than in 71x?)

[duanders]

Yes -- we worked with Steve Mrenna to figure out the issue. As I understood it, the problem was with pythia8 and was unnoticed before now, but you can check with the authors what the exact scope of it was.