No description available.

No description available.

The production of a Higgs boson with central rapidity, decaying to bb and accompanied by two jets that are forward in rapidity is presented as an important class of events at the Large Hadron Collider. Specifically, we study central Z boson production accompanied by rapidity gaps on either side as a way to gauge Higgs production via weak boson fusion at the LHC. We analyse the possible backgrounds for the bb decay mode and show that these can be substantially reduced. Special attention is paid to the evaluation of the gap survival factor, which is the major source of theoretical uncertainty in the rate of H, Z and W central production events with rapidity gaps. More generally, in relation to Higgs plus forward jet production, we consider the hadronic radiation patterns for the generic process of bb plus two forward jet production at the LHC, where the (centrally produced) bb originate either from a Higgs, a Z or from standard QCD production processes. A numerical technique for evaluating the radiation patterns for non-trivial final states is introduced and shown to agree with the standard analytic results for more simple processes. Significant differences between the radiation patterns for the Higgs signal and the background processes are observed and quantified. This suggests that hadronic radiation patterns could be used as an additional diagnostic tool in Higgs searches in this channel at the LHC. We also study the applicability of an equivalent photon/gluon approximation in describing events with a central system accompanied by two forward jets.

We present a summary of the methods required to solve loop-integrals and their reduction to Master Integrals. We then present the expansion in d = 4 - 2e of the Master Integrals required for the two loop massless vertex diagrams with three off-shell legs. The results are analytic and contain a new class of two-dimensional harmonic polylogarithms, which match onto the allowed phase-space boundary for the 1→2 process. These Master Integrals are relevant for the QCD corrections to Н → V*V* (where V = W,Z) and for two-loop studies of the triple gluon (and quark-gluon) vertex. We consider multi-parton collinear limits of QCD amplitudes at tree level. Using the MHV formalism we specify the underlying analytic structure of the resulting multi- collinear splitting functions. We adapt the MHV-rules to enable us to derive splitting functions without the need to evaluate the full amplitude. We derive general results for these splitting functions that are valid for specific numbers of negative helicity partons and an arbitrary number of positive helicity partons (or vice versa). Our method can be used to find splitting amplitudes with higher numbers of negative helicity partons. We present new results describing the collinear limits of up to six gluons and up to four partons. These results will have applications in the evaluation of higher order corrections to QCD cross-sections and jet evolution.

The calculation of observables in gauge theories with massless particles such as QCD - by traditional methods is significantly complicated by the presence of soft and collinear singularities, collectively termed infrared divergences, in the scattering amplitudes. The aim of this thesis is to investigate calculational methods which produce finite results at the amplitude level. We discuss the origin of the infrared divergences and outline some previous approaches to constructing finite amplitudes. After reviewing the traditional method for performing calculations we see how incorrect assumptions result in the presence of infrared divergences and what steps must be taken in order to produce infrared finite results. We then investigate how these ideas could be applied to the calculation of specific amplitudes. We see that there are problems involved in applying this exact approach, but that it suggests the adoption of a workable, more pragmatic alternative. We use this method in an explicit example calculation of the contributing cross sections for the process e+e (^_)> jets at O(as). We demonstrate that we recover the same result as that obtained with standard field theory techniques. We then briefly discuss how this approach might be adapted to suit more complex calculations and, eventually, a completely numerical approach.

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.

This thesis describes the measurements of jet activity in the rapidity region between a dijet system formed in proton-proton collisions at a centre-of-mass energy of 7 TeV. The data used were collected by the ATLAS detector during 2010 at the Large Hadron Collider at CERN. A number of observables that probe additional quark and gluon radiation in the dijet topology are studied. The development and performance of the monitoring system for the ATLAS calorimeter high level trigger is described. The performance of the jet calibration and a study of the properties of jets in the forward calorimeter is also given. The fraction of events that survive a veto on jets with transverse momentum above a jet veto scale, Q0, in the rapidity region between the dijet system is measured for dijets with mean transverse momentum 50 < pT < 500 GeV and rapidity separation, ∆y, of up to six. The mean number of jets that have a transverse momentum above the jet veto scale in the rapidity region between the dijet system is also measured. These measurements are compared to state-of-the-art theoretical calculations from HEJ and POWHEG, and also compared to PYTHIA and HERWIG++ Monte Carlo generators. The results of a preliminary analysis of dijet events with a large rapidity separation are given. In this analysis azimuthal decorrelation variables are also been measured.

The top quark is the heaviest fundamental particle. Its correspondingly short lifetime implies that it will decay before top flavoured hadrons can form. This provides an opportunity to study the properties of a quark without the effects of hadronisation, which is unique in the standard model.Using data recorded by the D0 experiment at the Fermilab Tevatron collider, and corresponding to \unit[5.4]{fb^{-1}} of proton-antiproton collisions two analyses of the production and decay mechanism of top quarks are presented in this thesis. In the standard model the directions of the spin of top quarks produced in pairs by the strong interaction are expected to be correlated. In the first analysis, the strength of the correlation is extracted from the angles of the two leptons in the top quark and antiquark rest frames, yielding a correlation strength C=0.10\pm0.45. This is in agreement with the NLO QCD prediction within two standard deviations, but also in agreement with the no correlation hypothesis.In the second analysis, forward-backward asymmetries in top quark-antiquark production are measured using the charged leptons from top quark decays. We find that the angular distributions of \ell^{\mathrm{-}} relative to antiprotons and \ell^{+} relative to protons are in agreement with each other. Combining the two distributions and correcting for detector acceptance we obtain the forward-backward asymmetry \al=\left(5.8\pm5.1\mathrm{(stat)}\pm1.3\mathrm{(syst)}\right)\%, compared to the standard model prediction of \al\mathrm{(predicted)}=\left(4.7\pm0.1\right)\%.

No description available.

The production of the neutral strange particles Λ, Λ[bar] and K⁰_s at HERA has been studied with the ZEUS detector, using an integrated luminosity of 121pb^-1 . Differential cross-sections are measured in a High Q² DIS sample (Q² ≥ 25GeV²), a Low Q² DIS sample (5 ≤ Q² ≤ 25GeV² ) and a photoproduction sample. The photoproduction sample has Q² <= 1GeV² and at least two jets that satisfy E_T^jet > 5GeV and |η^jet| < 2.4. Cross-sections are measured as a function of the particle P_T and η, and additionally as a function of the event Q² and x in the DIS samples, and as a function of x_γ in the photoproduction sample. The DIS cross-sections are compared to the ARIADNE Monte Carlo, which broadly describes the data. The photoproduction cross-sections are compared to PYTHIA, where the agreement is less good, particularly at low x_γ. The baryon-antibaryon asymmetry, defined as (Λ-Λ[bar])/(Λ+Λ[bar]), is measured and always found to be consistent with zero, indicating little transfer of the Baryon Number from the beam proton to the Λ system in the detector region. The polarization of Λ and Λ[bar] is measured and also found to be always consistent with zero, indicating that there is no preferred frame in Λ production. The baryon-meson ratio (Λ + Λ[bar])/K⁰_s is studied, and is found to generally agree with ARIADNE in DIS, but give an excess over PYTHIA in photoproduction. The excess is particularly prominent at low x_γ in events with no clear jet structure. This is interpreted as hadron production in this region depending on a more-than-one-dimensional overlap of quark wave functions. An excess of strangeness in this region at low x_γ with no high E_T jets would be significant. It is investigated by taking the ratio of strange particles to charged tracks but no excess is seen.