Abstract
We show how an inaccurate determination of experimental uncertainty correlations in high-precision LHC measurements may undermine the reliability of the associated chi ^2. We formulate the problem rigorously, and devise a regularisation procedure that increases the stability of the chi ^2 by altering the covariance matrix of the measurement as little as possible. We apply the procedure to the NNPDF4.0 global analysis of parton distribution functions that utilises a large amount of LHC measurements. We find that the regularised chi ^2 of the NNPDF4.0 determination is lowered by about 3sigma , without significantly altering the resulting PDFs upon refitting.
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