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arXiv · 2507.22969

Fant\^omas Unconfined: global QCD fits with B\'ezier parameterizations

Lucas Kotz·Aurore Courtoy·T. J. Hobbs·Pavel Nadolsky·Fredrick Olness·Maximiliano Ponce-Chavez·Varada Purohit
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Abstract

Fant\^omas is a C++ toolkit for exploring the parametrization dependence of parton distribution functions (PDFs) and other correlator functions in quantum chromodynamics (QCD). Fant\^omas facilitates the generation of adaptable polynomial parametrizations for PDFs, called metamorphs, to find best-fit PDF solutions and quantify the epistemic uncertainty associated with the parametrizations during their fitting. The method employs B\'ezier curves as universal approximators for a variety of PDF shapes. Integrated into the xFitter framework for the global QCD analysis, Fant\^omas provides a foundation for general models of PDFs, while reducing the computational time compared to the approaches utilizing traditional polynomial parametrizations as well as providing an interpretable alternative to neural-network-based models. This paper outlines the structure and practical usage of the Fant\^omas toolkit, including its inputs, outputs, and implementation within xFitter. It also provides a practical example of using Fant\^omas for uncertainty quantification as well as the combination of PDF fits into a single ensemble.

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Lucas Kotz, Aurore Courtoy, T. J. Hobbs, Pavel Nadolsky, Fredrick Olness, Maximiliano Ponce-Chavez, Varada Purohit. 2025-07-30. Fant\^omas Unconfined: global QCD fits with B\'ezier parameterizations. https://arxiv.org/abs/2507.22969

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