Hardcoded Proton Mass in Inclusive Kinematics Reconstruction

[rahmans1]

Bug: Hardcoded Proton Mass in Inclusive Kinematics Reconstruction

Not sure if it's a bug or a feature....

Affected Files

• src/algorithms/reco/InclusiveKinematicsDA.cc:79
• src/algorithms/reco/InclusiveKinematicsElectron.cc:64
• src/algorithms/reco/InclusiveKinematicsJB.cc:63
• src/algorithms/reco/InclusiveKinematicsSigma.cc:79
• src/algorithms/reco/InclusiveKinematicsESigma.cc:87

Description

All reconstructed inclusive kinematics methods hardcode the proton mass (PDG 2212)
when computing W and ν, regardless of the actual beam hadron species. This produces
incorrect results for ion beams (e.g. Au, Pb, He-3).

Example (DA method)

// InclusiveKinematicsDA.cc:79-85
static const auto m_proton = m_particleSvc.particle(2212).mass;  // hardcoded
const auto nu_da = Q2_da / (2. * m_proton * x_da);
const auto W_da  = sqrt(m_proton * m_proton + 2 * m_proton * nu_da - Q2_da);

What Works Correctly

The beam 4-momentum construction already uses the actual PDG mass of the beam particle:

// InclusiveKinematicsDA.cc:46-49
const auto& pi_particle = (*mc_beam_protons)[0];
const PxPyPzEVector pi(round_beam_four_momentum(
    pi_particle.getMomentum(),
    m_particleSvc.particle(pi_particle.getPDG()).mass,  // correct
    {41.0, 100.0, 275.0}, m_crossingAngle));

So x, Q2, and y are computed correctly. Only W and ν are affected.

Reference: Correct Pattern

InclusiveKinematicsTruth.cc:55 already does this correctly:

const auto pi_mass = m_particleSvc.particle(pi_coll[0].getPDG()).mass;

Proposed Fix

Replace the hardcoded lookup with the actual beam particle PDG mass in each affected
file, following the pattern used in InclusiveKinematicsTruth:

// Before
static const auto m_proton = m_particleSvc.particle(2212).mass;

// After
const auto m_beam = m_particleSvc.particle(pi_particle.getPDG()).mass;

Then substitute m_beam for m_proton in the W and ν expressions.

Note: For nuclear ion beams, whether to use the full ion mass or the per-nucleon
mass in W and ν is a physics convention that should be confirmed with collaborators.

Impact Summary

| Variable | Reconstructed methods | Truth method |
|----------|-----------------------|--------------|
| x, Q2, y | Correct               | Correct      |
| W, ν     | Wrong for ion beams   | Correct      |
| ```      |                       |              |