Parton Distribution Functions and the role of forward region data

Abstract

Abstract. In the era of searching for new discoveries at the LHC, it is crucial to achieve a higher level of precision in understanding the proton structure which will allow for unambiguous interpretations of the high energy, luminous data ahead. The knowledge of proton's constituents comes mainly from the deep inelastic scattering data at HERA, complemented by the measurements from the fixed target, Tevatron, and now increasingly precise more data from LHC. A road-map that marks the most recent developments from the past and present experiments sensitive to the proton constituents will be presented here with an emphasis on the recent results using LHC data from the forward region. MotivationIt is a most exciting phase of particle physics where there are many crucial questions which point to the deficiencies of the Standard Model (SM) that need to be answered. These questions can be addressed by the long term plan of the LHC running in different stages for achieving ever higher energies and luminosity. This poses new challenges for the experimental set-up and for the understanding of quark-gluon dynamics of the incoming proton beam particles at the LHC in the search for new particles or new interaction effects. The goal is first to facilitate the discovery of new physics by comparing measurements with the expectations driven either by the current SM hypothesis or by testing various beyond-SM scenarios. Precision is crucial to control the dominant uncertainties in order to observe and later analyse possible deviations from expectations. On the experimental side, the current measurements are now available at sub percent precision (e.g. W, Z at 7 TeV, Z p T at 8 TeV), while the last decade has seen an enormous progress in achieving percent level precision on the theory calculations. Many of these are now available up to the next-to-next-to leading order (NNLO) in the truncated perturbative series of the strong coupling. In the following, an overview of the progress on improving the current limiting factor on precision, which arises from the limited knowledge of the proton's constituents, is presented. Parton Distribution FunctionsProton's Parton Distribution Functions (PDFs) are understood, at the leading order approximation in QCD, as the probability of finding a parton of a given flavour that carries a fraction x of the total proton's momentum. Once QCD corrections are included, PDFs become scheme dependent. Their shape and normalisation are very different for each flavour, reflecting the different underlying dynamics that a