[guitargeek]

This Pull request:

Rewrites the rf709 Barlow-Beeston tutorial with the same building blocks that HistFactory uses, and deprecates RooParamHistFunc and RooHistConstraint for removal in ROOT 6.46.

Changes or fixes:

RooParamHistFunc and RooHistConstraint duplicate what the HistFactory classes already provide for the Barlow-Beeston method. Over the last years, this has repeatedly caused confusion about which classes to use and how they differ. After this PR there is one recommended approach: ParamHistFunc scaling a RooHistFunc, with one RooPoisson constraint per bin, like in the models that HistFactory builds.

• rf709_BarlowBeeston (C++ and Python) now builds its models this way, so it also serves as an introduction to how HistFactory models work. As in HistFactory, the gammas of empty template bins are fixed and not constrained. Otherwise they would be flat directions of the likelihood, which breaks HESSE.
• The likelihood is the same as before: the old and new NLLs agree to about 1e-12 at random parameter points. The printed results therefore change only within the fit tolerance, for example Barlow-Beeston Asig = 0.8503 ± 0.2358 before and 0.8506 ± 0.2358 after. The new version also stops printing "Minimized function has error status". That message came from RooHistConstraint, which returns the product of all per-bin Poisson terms. The product underflows to zero away from the minimum, because the default CPU backend takes the log of that value instead of calling getLogVal().
• Deprecation: both classes get R__DEPRECATED(6, 46, ...) messages that point to the HistFactory alternative and to the tutorial. Their docs get \deprecated notes, and there is a release note. The attribute is deliberately not guarded by ROOFIT_BUILDS_ITSELF. That definition is exported with ROOT::RooFitCore and is also used when building the RooFit module, so a guarded deprecation would never warn CMake or interpreter users. Instead, the warnings are silenced locally where RooFit itself still uses the classes: RooHistConstraint, RooLagrangianMorphFunc, codegen, and the unit test.
• The release note acknowledges that these classes may be stored in RooWorkspaces written to disk, and asks affected users to reach out via a GitHub issue or the ROOT forum. Until the removal, such workspaces can still be read and fitted without warnings.

Checklist:

• tested changes locally
• updated the docs (if necessary)

Tested locally:

• the rf709 tutorials (C++ interpreted and with ACLiC, and Python), plus the rf711 and rf712 tutorials;
• testRooFit and testCodegen;
• a full clang build, with no warnings;
• GCC syntax checks of every file that uses the deprecated classes.

Assisted-by: claude-code:claude-opus-5-5. Claude Code compared the old and new tutorials and drafted the follow-up commits: the empty-bin fix, the Python version, the replacement of rf709, and the deprecation.

Closes #24003

[github-actions]

Test Results

    21 files      21 suites   3d 7h 51m 47s ⏱️
 3 885 tests  3 883 ✅ 0 💤 2 ❌
72 220 runs  72 218 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit 1a7e64f.

♻️ This comment has been updated with latest results.

[cburgard]

Deprecating this class would resolve a few of the complications we have with HS3. I'm all for it 👍