Fix #28803 (GenParticles2HepMC with exotic LHE)

[mseidel42]

PR description:

Fixes crash in GenParticles2HepMCConverter (#28803) by adding fake beam particles for exotic LHE files with only one beam-particle candidate. This was used for NeutrinoGun samples (no beam particles) before.

PR validation:

• Processed NanoAod from problematic events
• Processed MiniAod events to produce Rivet histograms

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

Backport of #28876

[cmsbuild]

A new Pull Request was created by @intrepid42 (Markus Seidel) for CMSSW_10_2_X.

It involves the following packages:

GeneratorInterface/RivetInterface

@SiewYan, @efeyazgan, @mkirsano, @cmsbuild, @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.
@davidlange6, @silviodonato, @fabiocos you are the release manager for this.

cms-bot commands are listed here

[srimanob]

@intrepid42
Which workflow you are testing?
I assume we did not run Run-2 workflows anymore, I can have a look.

[mseidel42]

Hi Phat, I just tried your NanoAOD_PPS_cfg.py in CMSSW 11_1_X. I fixed the general Rivet problem there, so I will do the PR to master soon.

[qliphy]

please test

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-run-pr-tests/4589/console Started: 2020/02/11 10:01

[cmsbuild]

+1
Tested at: c1d6266
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-be271a/4589/summary.html
CMSSW: CMSSW_10_2_X_2020-02-10-2300
SCRAM_ARCH: slc6_amd64_gcc700

[cmsbuild]

Comparison job queued.

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-be271a/4589/summary.html

Comparison Summary:

• No significant changes to the logs found
• Reco comparison results: 0 differences found in the comparisons
• DQMHistoTests: Total files compared: 31
• DQMHistoTests: Total histograms compared: 3007491
• DQMHistoTests: Total failures: 3
• DQMHistoTests: Total nulls: 0
• DQMHistoTests: Total successes: 3007298
• DQMHistoTests: Total skipped: 190
• DQMHistoTests: Total Missing objects: 0
• DQMHistoSizes: Histogram memory added: 0.0 KiB( 30 files compared)
• Checked 129 log files, 14 edm output root files, 31 DQM output files

[qliphy]

+1

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_10_2_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. @davidlange6, @silviodonato, @fabiocos (and backports should be raised in the release meeting by the corresponding L2)

[silviodonato]

+1
please make a backport also for 10_6_x