[cburgard]

Note

Implements HS3 PR #131, merged on 2026-11-12: fixed domain axes with the key value.

This Pull request:

Writes and reads parameters that only take a single value, e.g. a named constant like the Z mass, as a fixed domain axis with that value (HS3 #131). It replaces the implicit rule that a parameter without a domain is constant.

"domains": [{"name": "default_domain", "type": "product_domain", "axes": [
   {"name": "x", "min": 80.0, "max": 100.0},
   {"name": "mZ", "value": 91.188}
]}]

Background

Since #22283 (July 2026, not released), RooFit has written a named RooConstVar as a parameter without a domain, and read every parameter without a domain back as a RooConstVar (config().importNoDomainParametersAsRooConstVars). HS3 #131 originally proposed to make this the standard: a parameter not listed in any domain is constant. In the review, the HS3 maintainers preferred an explicit construction, which is what #131 was merged with: a parameter that only takes one value says so in its domain with a fixed axis, and leaving out a domain doesn't make a parameter constant. Since no ROOT release has the implicit rule, no compatibility with it is kept.

Changes or fixes:

What	How
Export	A named RooConstVar is written as a fixed domain axis {"name": ..., "value": <value>} in the default domain, and no longer to the parameter points. Constant RooRealVars are unchanged: a range in the domain, "const": true in the parameter points. Literal RooConstVars are still written as numbers.
Import	A fixed axis gives a RooConstVar, whichever domain has it. It needs no entry in a parameter point.
Errors	The axis also has min, max, nbins, edges or categories; the value is not a number; two domains give the parameter different single values, or one a single value and another a range; a parameter point gives the parameter a different value. The same value in a parameter point is accepted, and "const": false stays the same as no "const".
Parameters without domain	Read as RooRealVar, as before #22283: constant with "const": true, otherwise free, with a warning that HS3 requires a domain or "const": true. config().importNoDomainParametersAsRooConstVars is removed.
Older files	ROOT versions before #22283 marked named RooConstVars with the attribute is_const_var in misc.ROOT_internal. Such parameters are read as RooConstVar again (#22283 had dropped this).

Tests

• New FixedDomainAxis: import as RooConstVar, export and stable round trip, the same value in a parameter point, every error above, a parameter without domain (constant, or free with a warning), and the legacy attribute.
• RooGaussianConstVarSigmaExport, RooConstVarCollectionProxyExport and ConstRooRealVarStaysDistinctFromRooConstVar expect the fixed axis instead of a missing domain.
• ParametersOnlyInParameterPoints expects parameters without domain and without "const": true to be free, with the warning, instead of constant.
• The tests that switched importNoDomainParametersAsRooConstVars off now run with the default behaviour.

Behaviour changes users may notice

• Named RooConstVars appear as fixed domain axes instead of "const": true entries in the parameter points.
• A parameter without domain is read as a RooRealVar, not as a RooConstVar. Documents written by ROOT master since [HS3] Bugfixes to HS3 histfactoy_dist handling and other minor bugs #22283 rely on the implicit rule; their RooConstVars come back as constant RooRealVars.
• RooJSONFactoryWSTool::config().importNoDomainParametersAsRooConstVars no longer exists.

Checklist:

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

Tests run (Linux, Ubuntu 26.04, GCC 15.2, RelWithDebInfo, -Droofit=ON -Dtesting=ON): testRooFitHS3 113 → 114 tests (1 new), testHistFactory 60, all passing. clang-format clean.

AI assistance: Claude Code (claude-opus-5-5) implemented the changes, the tests and this description under review of the author. The commit carries Assisted-by: claude-code:claude-opus-5-5.

🤖 Generated with Claude Code

[guitargeek]

LGTM!