MadSpin2: automated spin-entangled decays of heavy resonances via the spin-density matrix formalism
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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BibTeX
@misc{argyropoulos2026madspin2,
title = {{MadSpin2: automated spin-entangled decays of heavy resonances via the spin-density matrix formalism}},
author = {Spyros Argyropoulos and Valentin Durupt and Olivier Mattelaer},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2609.02794},
eprint = {2609.02794},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2609.02794},
}
RIS
TY - UNPB
TI - MadSpin2: automated spin-entangled decays of heavy resonances via the spin-density matrix formalism
AU - Spyros Argyropoulos
AU - Valentin Durupt
AU - Olivier Mattelaer
PY - 2026
DA - 2026/09/02
PB - arXiv
DO - 10.48550/arxiv.2609.02794
UR - https://arxiv.org/abs/2609.02794
AB - 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.
ER -