Attributes saved to HepMC3

The HepMC3 output of KKMCee is augmented with spin-aware attributes at both the event and particle levels. This lets you validate spin modeling (including a helicity-like approximation) directly in your analysis — no generator re-runs required.

A snapshot from the hepmc3 output file is shown below:

Enable HepMC3 ASCII output: set KKMCee input parameter 100 = 1.
Access: use HepMC3::ReaderAscii to parse each record into a HepMC3::GenEvent.
Vertex code: The identifiers preceding the new attributes correspond to the respective vertex codes.

Event level attributes:

1)SpinWT and SpinWThelApprox

SpinWT: the full spin weight computed from the complete spin–correlation tensor R.
SpinWThelApprox: the approximated spin weight, obtained by projecting polarimetric vectors onto the preffered direction.

\[wt_{\text{appx}} = \frac{1}{4}\sum_{m,n=\pm}R_{mn}(1+nh_{z}^+)(1+mh_{z}^-)\]

where:

\(R_{m,n} = s^{m}\, R \, s^{n}\)
\(h_{z}^+,\, h_{z}^-\) are the projection of two polarimetric vectors for \(\tau^+\), \(\tau^-\) respectively onto prefered direction.

Per \(\tau\) attributes

2) approximateHelicity:

For each event, the helicity tags \(m,n \in \{+1, -1\}\) to \(\tau^-\) and \(\tau^+\) by sampling from the helicity posterior

\[p(m,n|event) \propto R_{mn} (1+mh_{z}^+)(1+nh_{z}^-)\]

normalized over (m,n). The stored per particle attribute approximateHelicity is thus a probabilistic tag, not a ground-truth helicity, which can be defined only for massless fermions.

3) polarimetricInLabFrame:

For each \(\tau^\pm\), the corresponding polarimetric four-vector \(h^\pm\). They are defined in the laboratory frame. These vectors encode the full decay-channel analysing power and permit offline reweighting to alternative production dynamics without modifying the decay simulation.