Backport option to override GEN decays with FastSim decays

[sbein]

PR description:

This PR backports the option to only use the FastSim decayer when specified by a flag, which is necessary for FastSim to avoid large inaccuracies in the modeling of SM event processes.

This change is needed for the use of FastSim by PAGs other than SUS and EXO, e.g.,. TOP/HIG with Run 2 UL campaigns. The changes replace earlier changes which affect only decay chains of exotic particles, and extends the behavior to cover all decay chains. Future development may include tweaking the FastSim decayer to be as identical to FullSim as possible.

PR validation:

Distributions of all quantities in the NANO were examined for their impact in ttbar events [1]. Almost all changes are desirable, and some are very much needed, e.g., the MET [2]

[1] https://www.desy.de/~beinsam/FastSim/Nano/6July2022/TTbar/
[2] https://www.desy.de/~beinsam/FastSim/Nano/6July2022/TTbar/CaloMET_pt.png

If this PR is a backport please specify the original PR and why you need to backport that PR. If this PR will be backported please specify to which release cycle the backport is meant for:

This PR is a backport of #38813, which should be merged for all future FastSim productions in CMSSW_10_6_X, e.g., UL requests.

[cmsbuild]

A new Pull Request was created by @sbein (Sam Bein) for CMSSW_10_6_X.

It involves the following packages:

• FastSimulation/SimplifiedGeometryPropagator (fastsim)

@cmsbuild, @ssekmen, @civanch, @mdhildreth, @sbein can you please review it and eventually sign? Thanks.
@matt-komm this is something you requested to watch as well.
@perrotta, @dpiparo, @qliphy, @rappoccio you are the release manager for this.

cms-bot commands are listed here

[sbein]

please test

[sbein]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_10_6_X IBs after it passes the integration tests and once validation in the development release cycle CMSSW_12_5_X is complete. This pull request will now be reviewed by the release team before it's merged. @perrotta, @dpiparo, @qliphy, @rappoccio (and backports should be raised in the release meeting by the corresponding L2)

[cmsbuild]

+1

Summary: https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-4633f7/26736/summary.html
COMMIT: 488ce63
CMSSW: CMSSW_10_6_X_2022-08-10-1100/slc7_amd64_gcc700
User test area: For local testing, you can use /cvmfs/cms-ci.cern.ch/week1/cms-sw/cmssw/39022/26736/install.sh to create a dev area with all the needed externals and cmssw changes.

Comparison Summary

Summary:

• No significant changes to the logs found
• Reco comparison results: 1172 differences found in the comparisons
• DQMHistoTests: Total files compared: 35
• DQMHistoTests: Total histograms compared: 3215686
• DQMHistoTests: Total failures: 2904
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3212448
• DQMHistoTests: Total skipped: 334
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 34 files compared)
• Checked 143 log files, 29 edm output root files, 35 DQM output files
• TriggerResults: no differences found

[qliphy]

+1