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arXiv · 2609.02794

MadSpin2: automated spin-entangled decays of heavy resonances via the spin-density matrix formalism

Spyros Argyropoulos·Valentin Durupt·Olivier Mattelaer
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Abstract

The simulation of processes that contain multiple particles from the decay of unstable resonances becomes computationally challenging both at leading order and beyond. This paper presents MADSPIN2, a new version of MADSPIN, a dedicated tool tackling this problem, primarily for events generated with the MG5AMC event generator. While still based on the Frixione-Laenen-Motylinski-Webber formalism, the new version leverages spin-density matrices to lift most of the limitations of the previous implementation. It supports loop-induced processes, N -body decays and samples defined by amplitude-level interference between distinct hard-process amplitudes. It also introduces the multiple-pole approximation and a resonance-helicity decomposition, including off-diagonal helicity-interference contributions. These developments substantially broaden the phenomenological scope of MADSPIN, enabling applications that were previously inaccessible within the framework, several of which are demonstrated in this work.

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Spyros Argyropoulos, Valentin Durupt, Olivier Mattelaer. 2026-09-02. MadSpin2: automated spin-entangled decays of heavy resonances via the spin-density matrix formalism. https://arxiv.org/abs/2609.02794

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