First Monte Carlo Global QCD Analysis of Pion Parton Distributions

We perform the first global QCD analysis of parton distribution functions (PDFs) in the pion, combining pi 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 cany a significantly higher pion momentum fraction, similar to 30%, than that inferred from Drell-Yan data alone, with sea quarks carrying a somewhat smaller fraction, similar to 15%, at the input scale. Within the same effective theory framework, the chiral splitting function and pion PDFs can be used to describe the (d) over bar - (u) over bar asymmetry in the proton.