Fixed target Drell-Yan data and NNLO QCD fits of parton distribution functions

We discuss the influence of fixed-target Drell-Yan data on the extraction of parton distribution functions at next-to-next-to-leading order (NNLO) in QCD. When used in a parton distribution fit, the Drell-Yan (DY) data constrain sea quark distributions at large values of Bjorken x. We find that not all available DY data are useful for improving the precision of parton distribution functions (PDFs) obtained from a fit to the deep inelastic scattering (DIS) data. In particular, some inconsistencies between DIS-based parton distribution functions and DY data for large values of dilepton rapidity are found. However, by selecting a sample of the DY data that is both representative and consistent with the DIS data, we are able to perform a combined PDF fit that significantly improves the precision of nonstrange quark distributions at large values of x. The NNLO QCD corrections to the DY process are crucial for improving the precision. They reduce the uncertainty of the theoretical prediction, making it comparable to the experimental uncertainty in DY cross-sections over a broad range of x.