Add dt_hit / dt_ptoca linear fit information for hits on a track

[michaelmackenzie]

The straw hit time vs. track expected time at the hit position has a non-unit slope when the wrong mass hypothesis is assumed. This slope information can be used in PID, as is relevant to PR Mu2e/MLTrain#9. This adds a reco data product with this information and a producer module to create a collection of these products for input KalSeed or KalSeedPtr collections.

[FNALbuild]

Hi @michaelmackenzie,
You have proposed changes to files in these packages:

• ParticleID
• RecoDataProducts

which require these tests: build.

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⌛ The following tests have been triggered for e7ca770: build (Build queue - API unavailable)

About FNALbuild. Code review on Mu2e/Offline.

[FNALbuild]

☔ The build tests failed for e7ca770.

Test	Result	Details
test with	✅	Command did not list any other PRs to include
merge	✅	Merged e7ca770 at 3d84f93
build (prof)	✅	Log file. Build time: 04 min 18 sec
ceSimReco	✅	Log file.
g4test_03MT	✅	Log file.
transportOnly	✅	Log file.
POT	✅	Log file.
g4study	✅	Log file.
cosmicSimReco	✅	Log file.
cosmicOffSpill	✅	Log file.
ceSteps	✅	Log file.
ceDigi	✅	Log file.
muDauSteps	✅	Log file.
ceMix	✅	Log file.
rootOverlaps	✅	Log file.
g4surfaceCheck	✅	Log file.
trigger	✅	Log file.
check_cmake	❌	Log file. Return Code 1.
FIXME, TODO	✅	TODO (0) FIXME (0) in 2 files
clang-tidy	➡️	2 errors 9 warnings
whitespace check	✅	no whitespace errors found

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[oksuzian]

Possible Improvements

The following are concrete, diff-anchored points worth addressing before merge:

1. Weight formula looks physically wrong / risks division-by-zero.
   ParticleID/src/TrackDtDt_module.cc:60-62

   const double t_unc  = hit._udresid / hit._dvel;
   const double weight = 1./t_unc;

   _udresid is the unbiased distance residual value (see TrkStrawHitSeed.hh:186), not an uncertainty — so residual/velocity is not a time uncertainty. A time error would more plausibly come from a variance term (e.g. sqrt(_udresidmvar)/_dvel, or _utocavar). As written, weight can be negative (residuals are signed) and 1/t_unc blows up when _udresid == 0 or _dvel == 0 (the legacy BTrk constructor sets _dvel = 0). Please confirm the intended uncertainty and guard against zero/negative weights.

2. LsqSums2 accessors are non-const, but you pass seed by const-ref. This compiles only because fitter is a local non-const. Still, consider that dof_ = fitter.qn() - 2 can go negative for tracks with <2 hits, and chisq_ = chi2Dof()*dof_ will then be meaningless. Add a guard for qn() < 3 (skip or set sentinel values).

3. Struct members are uninitialized when set from the fit. KalSeedDtDt's default ctor zero-inits, but fitTrackDtDt creates a result and sets 5 fields — fine today, but if any LsqSums2 accessor returns NaN (e.g. det()==0 for degenerate input), it propagates silently. A quick isnan check before emplace would harden the product.

4. Minor: #include <iostream> guarded diagnostics use std::cout; consider mf::LogInfo/MessageLogger for consistency with framework conventions, though many Mu2e modules do use std::cout under diagLevel.