Theory Seminar - Aurore Courtoy

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Speaker: Aurore Courtoy (University of Mexico - UNAM)

Title: Sampling uncertainties and biases in global QCD fits

Abstract: The global analyses of parton distribution functions (PDFs) enclose various sources of uncertainties, ranging from data uncertainty, higher-order corrections to specific methodological choices. Indeed, in such fits of PDFs, a large part of the estimated uncertainty on the PDFs originates from the choices of parametric functional forms and fitting methodology. The latter can be underestimated with common PDF ensembles in high-stake measurements at the Large Hadron Collider and Tevatron. This is best understood when these uncertainties are viewed as arising from sampling of allowed PDF solutions in a multidimensional parametric space. This is related to the big data paradox, coming from recent statistical studies of large-scale population surveys and quasi-Monte Carlo integration methods. More experimental data do not automatically raise the accuracy of PDFs, instead close attention to the data quality and sampling of possible PDF solutions is as essential. To test if the sampling of the PDF uncertainty of an experimental observable is truly representative of all acceptable solutions, we introduce a technique ("a hopscotch scan") based on a combination of parameter scans and stochastic sampling. Using this technique, we show the need for sufficiently complete sampling of PDF functional forms and choices of the experiments.