Parton distributions

Abstract

Parton distributions, α(χ,μ(^₂) are essential ingredients for almost all theoretical calculations at hadron colliders. They give the number densities of the colliding par- tons (quarks and gluons) inside their parent hadrons at a given momentum fraction χand scale μ(^₂). The scale dependence of the parton distributions is given by DGLAP evolution, while the X dependence must be determined from a global analysis of deep-inelastic scattering (DIS) and related hard-scattering data. In Part I we introduce ‘doubly-unintegrateď parton distributions, fa(x, z, k(^₂),μ(^₂)), which additionally depend on the splitting fraction z and the transverse momentum (k) associated with the last evolution step. We show how these distributions can be used to calculate cross sections for inclusive jet production in DIS and compare the predictions to data taken at the HERA ep collider. We then calculate the transverse momentum distributions of พ and z bosons at the Tevatron pp collider and of Standard Model Higgs bosons at the forthcoming LHC. In Part II we study diffractive DIS, which is characterised by a large rapidity gap between the slightly deflected proton and the products of the virtual photon dissociation. We perform a novel QCD analysis of recent HERA data and extract diffractive parton distributions. The results of this analysis are used to investigate the effect of absorptive corrections in inclusive DIS. These absorptive corrections are due to the recombination of partons within the proton and are found to enhance the size of the gluon distribution at small X. We discuss the problem that the gluon distribution decreases with decreasing X at low scales while the sea quark distribution increases with decreasing X, whereas Regge theory predicts that both should have the same small-X behaviour. Our study hints at the possible importance of power corrections at low scales of around 1 GeV.