# Improved description of the HERA data with a new simple PDF parametrization

## TL;DR

This paper introduces a new, more flexible PDF parametrization that significantly improves the fit quality to HERA data, especially at small-x, highlighting the importance of parametrization choices in collider physics.

## Contribution

It presents a novel PDF parametrization with higher small-x flexibility, implemented in xFitter, resulting in a better fit to HERA data and revealing biases in previous models.

## Key findings

- Reduced chi-squared by over 60 units with only four additional parameters
- Inclusion of small-x resummation further improves fit quality by 30 units
- Identifies parametrization bias in default xFitter at small-x

## Abstract

A new parametrization for the parton distribution functions with a higher flexibility in the small-$x$ region is presented. It has been implemented in the xFitter open-source PDF fitting tool, and compared to the default xFitter parametrization, used for the determination of the HERAPDF set. It has been found that the combined inclusive HERA I+II data can be described 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 more parameters. Our result highlights a significant parametrization bias in the default xFitter parametrization at small $x$, which would lead to even more dramatic effects when used for higher energy colliders, where the small-$x$ region is more relevant. We also find that the inclusion of small-$x$ resummation leads to a further reduction by approximately 30 extra units in $\chi^2$. In this contribution, we review the results of the recent paper "A new simple PDF parametrization: improved description of the HERA data" (arXiv:1902.11125).