Infrared QCD resummations at hadron colliders

In this thesis we study two different processes at hadron colliders: Z-boson production and dijet production with a jet veto. Our calculations focus on the resummation of logarithmically enhanced contributions coming from soft and/or collinear gluon emission. For Z-boson production, we calculate the cross-section distribution in aT , a novel variable proposed by Vesterinen and Wyatt as a more accurate probe of the Z at low transverse momentum pT . The observable aT is defined as the component of pT perpendicular to an experimentally convenient axis: the axis with respect to which the two final-state leptons (from a Z leptonic decay) have equal transverse momenta. Our study involves the resummation of large logarithms in aT /pT up to next-to-leading accuracy. We then compare the resulting distributions in aT to the well-known pT distribution, identifying important physical differences between the two cases. We also test our resummed result at the two-loop level by comparing its expansion with a fixed-order calculation and find agreement with our expectations. Besides, we study dijet production with a veto in the inter-jet rapidity region in proton-proton collisions. We resum the leading logarithms in the ratio of the transverse momentum of the leading jets and the veto scale and we match this result to leading-order QCD matrix elements, taking into account energy-momentum conservation effects. We compare our theoretical predictions to experimental data measured by the ATLAS collaboration and find good agreement, although our results are affected by large theoretical uncertainties.