Scheme dependence, leading order and higher twist studies of MRST partons

We extend a recent global analysis of nucleon parton distributions carried out at next-to-leading order (NLO) in the MS scheme to provide distributions in the so-called DIS scheme. We pay particular attention to the translation of the heavy quark distributions in going from the MS to the DIS scheme. We repeat the global analysis at leading order (LO) and comment on the major effects produced when going from LO to NLO. Finally we include in the global analysis a freely parameterised form of possible higher twist contributions to make an exploratory study of the size of these effects as a function of x.