Interplay of higer-twist and off-shell corrections in global QCD fits at large x

We will show how, in a global QCD analysis that includes DIS data on deuteron targets, the fit of the d-quark PDF and the extraction of the neutron structure function at large x values can be heavily biased by theoretical assumptions. In particular, we will focus on the interplay of higher-twist effects and off-shell PDF deformations in deuteron targets, and show that: (1) the available hard scattering data are sufficient to independently fit higher-twist corrections to proton and neutron structure functions, and (2) doing so removes most of the theoretical bias. The result is a stable fit of: the d/u quark ratio, that tends to a small but non-zero value as x tends to 1; the extracted free neutron F2 structure function; and (3) the off-shell PDF deformation function.