auto detect first vertex with Higgs in children list

[yihui-lai]

PR description:

The current HTXS production-mode identification relies on inspecting GenEvent::vertices()[0] to determine the Higgs boson’s production topology. However, for certain MG5_aMC@NLO FxFx samples—such as TTH-Hto2G_Par-M-125_TuneCP5_13p6TeV_amcatnloFXFX-pythia8—the Higgs does not appear among the children of vertices()[0].

As a result, the HTXS Rivet module fails to assign a valid Higgs production mode, which propagates to a failure in HTXS categorization during MiniAOD→NanoAOD production. This PR updates the logic to automatically identify the first GenVertex that contains a Higgs boson among its outgoing particles, rather than assuming it is located at vertices()[0].

PR validation:

The updated logic was validated by running MiniAOD → NanoAOD workflows on both:

• MG5_aMC@NLO FxFx samples (e.g. problematic TTH FxFx datasets)
• POWHEG samples

In all tested samples, the HTXS production mode and category are now correctly assigned.

@mseidel42

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Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-49644/47184

Code check has found code style and quality issues which could be resolved by applying following patch(s)

• code-format:
  https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-49644/47184/code-format.patch
  e.g. curl -k https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-49644/47184/code-format.patch | patch -p1
  You can also run scram build code-format to apply code format directly

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Logs: https://cmssdt.cern.ch/SDT/code-checks/cms-sw-PR-49644/47185

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A new Pull Request was created by @yihui-lai for master.

It involves the following packages:

• GeneratorInterface/RivetInterface (generators)

@cmsbuild, @lviliani, @mkirsano, @sensrcn, @theofil can you please review it and eventually sign? Thanks.
@alberto-sanchez, @mkirsano this is something you requested to watch as well.
@ftenchini, @mandrenguyen, @sextonkennedy you are the release manager for this.

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Milestone for this pull request has been moved to CMSSW_16_1_X. Please open a backport if it should also go in to CMSSW_16_0_X.