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We introduce a new parametrization for the parton distribution functions(PDFs) designed to be flexible in the small-x region. We implement it in thexFitter open-source PDF fitting tool, and compare it to the default xFitterparametrization, widely used for many PDF studies, and notably for the HERAPDFdetermination. We find that we can describe the combined inclusive HERA I+IIdata using NNLO theory with a significantly higher quality than HERAPDF2.0: the$\chi^2$ is reduced by more than 60 units, having used only four moreparameters. Our result highlights a significant parametrization bias in thedefault xFitter parametrization at small x, which would lead to even moredramatic effects when used for higher energy colliders, where the small-xregion is more relevant. We also find that the inclusion of small-xresummation, that was shown in previous studies to lead to similar improvementsin the fit quality, further reduces the $\chi^2$ by approximately 30 extraunits.

A new simple PDF parametrization: improved description of the HERA data