Validation of the Whizard EVA recoil scheme

Validate the recoil scheme implemented in Whizard for generating transverse-momentum kicks in the Equivalent Vector Boson Approximation, in order to determine whether it reliably reproduces the kinematics of the leading-transverse-momentum final-state particle.

Background

The Equivalent Vector Boson Approximation assumes that the emitted vector bosons are collinear with the incoming beams and therefore does not fully reproduce hard-recoil kinematics. In the di-Higgs study, the kinematics of the particle leading in transverse momentum is found to be less accurately described than that of the subleading particle.

Whizard provides an optional recoil scheme intended to generate a transverse-momentum kick and thereby improve the kinematic description. The thesis explicitly states that this implementation has not yet been validated, making its reliability an unresolved technical issue.

References

Whizard allows users to generate a $p_{\perp,1}$ kick in the EVA in the so-called recoil scheme, but this option remains to be validated.

— Modelling electroweak interactions at lepton colliders - from the Z pole to the highest energies  (2608.18788 - Mękała, 19 Aug 2026) in Section 6.4, subsection “Di-Higgs”