W boson + QCD Jet events, produced in 1.8 TeV proton-antiproton collisions and measured by the Collider Detector at Fermilab (CDF), were used to measure the center-of-mass production angle of the W + jet system, and were also used to place limits on the production of excited quark states. The center-of-mass production angular distribution agrees well with leading order and next-to-leading order QCD predictions. Excited quark states were searched for in the reaction q + g --> q* --> q + W. Upper limits on the q* cross section, as a function of the q* mass, are shown. Comparison with a theoretical prediction for q* production excludes excited quark states with a mass in the range 150--530 GeV/c2, at 95% confidence.

Jet fragmentation properties have been studied in collisions of protons and antiprotons at a center-of-mass energy of 1.8 TeV, using the Collider Detector at Fermilab (CDF). The fractional momentum distribution of charged particles within jets is presented and compared with Monte-Carlo predictions. With increasing di-jet invariant mass from 60 to 200 GeV/c2 the fragmentation is observed to soften as predicted by scale breaking effects in Quantum Chromodynamics (QCD). The charged multiplicity in the jet core is observed to rise with di-jet invariant mass. 57 refs.

The production of jets in association with a Z/?* boson is an example of an important class of processes at hadron colliders, namely vector boson + jet (V + jet) production. Comparisons of measurements of this class of processes with theory predictions constitute an important, fundamental test of the Standard Model of particle physics, and of the theory of QCD in particular. While having a smaller cross section than other V +jet processes, Z/?*(→ e+e-) + jets production, with Z/?* → e+e-/?+?-, has a distinct experimental signature allowing for measurements characterized by low backgrounds and a direct, precise measurement of the properties of the decay products of the Z/?* boson. In this thesis, several new measurements of the properties of jets produced in association with a Z/?* boson in p$ar{p}$ collisions at √s = 1.96 TeV are presented. The cross section for Z/?*(→ e+e-) + N jet production (N ≤ 3) is measured, differential in the transverse momentum of the Nth jet in the event, normalized to the inclusive Z/?* cross section. Also, the cross section for Z/?*(→e+e-) + N jets (N ≥ 1) is measured, differential in the difference in azimuthal angle between the di-electron system and any jet in the event, normalized to unity. The data used in the measurements were collected by the D0 experiment located at the Tevatron Collider of the Fermi National Accelerator Laboratory and correspond to an integrated luminosity of 1.04 fb-1. The measured jet transverse momentum spectra are compared with the predictions of perturbative calculations at the next-to-leading order in the strong coupling constant. Given the low sensitivity of the calculations to model parameters, these comparisons represent a stringent test of perturbative QCD. One of the main goals currently being pursued in particle physics is the discovery of the only particle predicted by the Standard Model which has so far no been detected experimentally, namely the Higgs boson. It is assumed that the ATLAS and CMS experiments located at the Large Hadron Collider (LHC), a proton-proton collider at √s = 14 TeV, will be able to detect the Higgs boson, or rule out its existence, within the next few years. The collisions delivered by the LHC will also be used to perform a long range of searches for other new particles, for instance particles predicted by models based on the principle of supersymmetry. The associated production of vector bosons with jets has relatively large production rates at the LHC and can produce a long list of different final states which can include charged leptons, missing transverse energy, as well as light- and heavy-flavour jets. This makes V + jet production a major source of background events to many searches for new particles. Most techniques used for estimating the expected number of background events to searches rely on passing the stable final-state particles of simulated hadron collisions generated using a so-called event generator code, through a simulation of the experimental detector system. The development of event generators which are capable of reliably predicting the properties of jets produced in association with a core process, e.g. the production of a vector boson, has been the subject of a large amount of research activity during the last ten years. These efforts have led to the appearance of the CKKW and MLM algorithms which are implemented in several event generators, among them SHERPA and ALPGEN + PYTHIA. The large data sample collected by the D0 experiment during Run II offers an excellent opportunity for validating these new event generators against experimental measurements of V + jet production. As argued above, the Z/?*(→ e+e-) + jets process offers the combination of a clean experimental signature and large production rates, making it the process of choice for these studies.

Characteristics of multi-particle production in proton-proton collisions at sqrt(s) = 7 TeV are studied as a function of the charged-particle multiplicity, N[ch]. The produced particles are separated into two classes: those belonging to jets and those belonging to the underlying event. Charged particles are measured with pseudorapidity abs(eta) 2.4 and transverse momentum pt 0.25 GeV. Jets are reconstructed from charged-particles only and required to have pt> 5 GeV. The distributions of jet pt, average pt of charged particles belonging to the underlying event or to jets, jet rates, and jet shapes are presented as functions of N[ch] and compared to the predictions of the PYTHIA and HERWIG event generators. Predictions without multi-parton interactions fail completely to describe the N[ch]-dependence observed in the data. For increasing N[ch], PYTHIA systematically predicts higher jet rates and harder pt spectra than seen in the data, whereas HERWIG shows the opposite trends. At the highest multiplicity, the data-model agreement is worse for most observables, indicating the need for further tuning and/or new model ingredients.