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 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 momentum 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 d ¯-u ¯ asymmetry in the proton.