Parton Distributions at High X

We extract the ratio of the down (d) and up (u) parton distribution functions (PDF’s) from the ratio of NMC deuteron and proton structure function F d 2 /F p 2 , using corrections for nuclear binding effects in the deuteron, which are extracted from the nuclear dependence of SLAC F2 data. Significant corrections to the d quark distribution in standard PDF’s are required, especially at high x. The corrected d/u ratio is in agreement with the QCD prediction of 0.2 at x = 1. The predictions for the recent CDF W asymmetry data using PDF’s with the corrected d/u ratio give much better agreement at large rapidity. Using the updated d/u ratio and the most recent world average for αs, we perform a NLO global fit to all DIS data for F2 and R, and estimate the size of the higher twist contributions using both a renormalon model and an empirical model. We find that with the updated value of αs, the magnitude of the higher twist terms is half the value of previous analysis. With the inclusion of target mass and higher twist corrections, the standard NLO PDF’s with the updated d/u ratio describe the SLAC F2 data up to x = 1.0. When the analysis is repeated in NNLO, we find that the additional NNLO contributions to R account for most of the higher twist effects extracted in the NLO fit. The analysis in NNLO indicates that the higher twist effects in R, F2 and xF3 (e.g. GLS sum rule) are very small. ∗ To be published in proceedings of the 6th International Workshop on Deep Inelastic Scattering and QCD, Brussels, Apr. 1998. Email: [email protected]