Parametrization dependence and Delta Chi-squared in parton distribution fitting

Understand

Parton distributions functions (PDFs), which are essential to the interpretation of data from high energy colliders, are measured by representing them as functional forms containing many parameters.

• Those parameters are determined by fitting a wide variety of experimental data.
• The best-fit PDF set is obtained by minimizing the standard $\chi^2$ measure of fit quality.
• The uncertainty range is estimated in the Hessian method by regarding as acceptable, all fits for which $\chi^2$ lies within $\Delta\chi^2$ of its minimum.