Backport of #29567 (biased tau decayer) to CMSSW_9_4_X

[mseidel42]

PR description:

Backport of #29567 by @smrenna to require at least one tau decaying only to leptons after heavy resonance decays:

Pythia8 has the capability to define a decay handler for specific particles through DecayHandler.
This request is a special tau decayer that can be used to bias tau pair decays to contain at least one lepton. It is meant to be used with Powheg samples containing taus so that no Powheg events are lost.
Since DecayHandler is meant to decay one particle at a time, while the standard Pythia tau decayer decays the correlated pair, part of the decay history is saved and then used to fill the products of the second (correlated) tau.

PR validation:

Validation plots for W and Z, things look as expected: https://mseidel.web.cern.ch/mseidel/cms/PDG_TAUS/

In particular, here is the number of tau->charged lepton decays in Z events:

As expected, 0 lepton is depleted completely, and the ratio betwen 1 and 2 leptons stays the same.

To check spin correlations, here is a plot of the dPhi between the two charged leptons, in case of both taus decaying to charged leptons:

if this PR is a backport please specify the original PR and why you need to backport that PR:

Backport of #29567, needed for 2017 low-PU MC

[cmsbuild]

A new Pull Request was created by @intrepid42 (Markus Seidel) for CMSSW_9_4_X.

It involves the following packages:

GeneratorInterface/Pythia8Interface

@SiewYan, @efeyazgan, @mkirsano, @cmsbuild, @GurpreetSinghChahal, @agrohsje, @alberto-sanchez, @qliphy can you please review it and eventually sign? Thanks.
@alberto-sanchez, @agrohsje, @mkirsano this is something you requested to watch as well.
@silviodonato, @dpiparo you are the release manager for this.

cms-bot commands are listed here

[qliphy]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-run-pr-tests/6243/console Started: 2020/05/12 11:31

[cmsbuild]

+1
Tested at: e2b2565
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-2bb40e/6243/summary.html
CMSSW: CMSSW_9_4_X_2020-05-10-0000
SCRAM_ARCH: slc6_amd64_gcc630

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-2bb40e/6243/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 27
• DQMHistoTests: Total histograms compared: 2721493
• DQMHistoTests: Total failures: 104
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 2721227
• DQMHistoTests: Total skipped: 162
• DQMHistoTests: Total Missing objects: 0

[qliphy]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_9_4_X IBs (tests are also fine) and once validation in the development release cycle CMSSW_11_1_X is complete. This pull request will now be reviewed by the release team before it's merged. @silviodonato, @dpiparo (and backports should be raised in the release meeting by the corresponding L2)

[silviodonato]

+1