We perform the first global QCD analysis of parton distribution functions (PDFs) in the pion, combining π A Drell-Yan data with leading neutron electroproduction from HERA within a Monte Carlo approach based on nested sampling. Inclusion of the HERA data allows the pion PDFs to be determined down to much lower values of x, with relatively weak model dependence from uncertainties in the chiral splitting function. The combined analysis reveals that gluons carry a significantly higher pion

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... fraction, ∼ 30%, than that inferred from Drell-Yan data alone, with sea quarks carrying a somewhat smaller fraction, ∼ 15%, at the input scale. Within the same effective theory framework, the chiral splitting function and pion PDFs can be used to describe the d̅-u̅ asymmetry in the proton.