fix: prevent unphysical boost in UndoAfterBurner when only one beam found

[wdconinc]

Fix MCParticlesHeadOnFrameNoBeamFX overflow for particle gun events

Stacked on: #2887 (acts-event-data-seed-deprecated-declarations)

Problem

Single-electron gun events with secondary neutrons produce extreme momentum values (~10³⁰⁸ GeV) in MCParticlesHeadOnFrameNoBeamFX, causing CI failures in capybara histogram generation:

• https://github.com/eic/EICrecon/actions/runs/32655851820/job/97236980632?pr=2887

Root Cause

The fallback logic (lines 67-75) incorrectly treats secondary hadrons from particle gun events as beam particles:

1. Electron gun event produces secondary neutron (E=0.94 GeV) from showering
2. Fallback finds neutron → sets hasBeamHadron=true, leaves hasBeamLepton=false
3. Old check if (!hasBeamHadron && !hasBeamLepton) only returns if BOTH are false
4. Code proceeds with:
   • e_beam = (0,0,0,0) (uninitialized)
   • h_beam = (0.024, 0, 0.94, 0.94) (from secondary neutron)
5. Boost calculation: β = (-0.024/0.94, 0, -0.94/0.94) = (-0.026, 0, -1.0)
6. β² = 1.0006 > 1 → unphysical (faster than light) → extreme Lorentz factors → 10³⁰⁸ GeV

Solution

One-character fix: Change && to || on line 98.

// Old (buggy):
if (!hasBeamHadron && !hasBeamLepton) {  // Only returns if BOTH false
  return;
}

// New (correct):
if (!hasBeamHadron || !hasBeamLepton) {  // Returns if EITHER false
  return;
}

This is consistent with the check at line 62 and prevents unphysical boost calculations.

Effect

• Particle gun events (with or without secondary hadrons): MCParticlesHeadOnFrameNoBeamFX will be empty (no valid collision frame)
• DIS events (both beams present): Boost transformation applied correctly as before

Testing

Verified with rec_e_1GeV_20GeV_craterlake.edm4eic.root:

• Before fix: Events 7, 11, 61, 95 have momentum ~10³⁰⁸ GeV or NaN
• Input MCParticles: Normal momentum ~1.4 GeV
• Expected after fix: No extreme values (empty output collection for gun events)

[Copilot]

Pull request overview

This PR updates eicrecon::UndoAfterBurner to avoid unphysical boosts/rotations when processing particle-gun MC inputs, which previously could generate extreme momentum values in MCParticlesHeadOnFrameNoBeamFX and break downstream CI validation (capybara histograms).

Changes:

• Detect presence of incoming beam lepton/hadron and add an early “identity transform” path intended for particle-gun events.
• Remove the prior “fake beam particle” fallback logic that attempted to synthesize beam four-vectors.
• Keep the existing boost/rotation workflow for collision events where both beam particles are found.

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[wdconinc]

Still failed, but now likely due to having used a ref artifact that has unphysicals... https://github.com/eic/EICrecon/actions/runs/32684922352/job/97312267237?pr=2891#step:16:160

[veprbl]

cc @ajentsch FYI