Angular ordering and parton showers for non-global QCD observables

We study the mismatch between a full calculation of non-global single-logarithms in the large-N_c limit and an approximation based on free azimuthal averaging, and the consequent angular-ordered pattern of soft gluon radiation in QCD. We compare the results obtained in either case to those obtained from the parton showers in the Monte Carlo event generators HERWIG and PYTHIA, with the aim of assessing the accuracy of the parton showers with regard to such observables where angular ordering is merely an approximation even at leading-logarithmic accuracy and which are commonly employed for the tuning of event generators to data.Comment: 20 pages, 8 figure

Dijet rates with symmetric E_t cuts

We consider dijet production in the region where symmetric cuts on the transverse energy, $E_t$, are applied to the jets. In this region next-to--leading order calculations are unreliable and an all-order resummation of soft gluon effects is needed, which we carry out. Although, for illustrative purposes, we choose dijets produced in deep inelastic scattering, our general ideas apply additionally to dijets produced in photoproduction or $\gamma \gamma$ processes and should be relevant also to the study of prompt di-photon $E_t$ spectra in association with a recoiling jet, in hadron-hadron processesComment: 28 pages, 4 figure

Aspects of power corrections in hadron-hadron collisions

The program of understanding inverse-power law corrections to event shapes and energy flow observables in e+ e- annihilation to two jets and DIS (1+1) jets has been a significant success of QCD phenomenology over the last decade. The important extension of this program to similar observables in hadron collisions is not straightforward, being obscured by both conceptual and technical issues. In this paper we shed light on some of these issues by providing an estimate of power corrections to the inter-jet E_t flow distribution in hadron collisions using the techniques that were employed in the e+ e- annihilation and DIS cases.Comment: 15 pages, 1 figure, uses JHEP3.cl

Semi-numerical resummation of event shapes

For many event-shape observables, the most difficult part of a resummation in the Born limit is the analytical treatment of the observable's dependence on multiple emissions, which is required at single logarithmic accuracy. We present a general numerical method, suitable for a large class of event shapes, which allows the resummation specifically of these single logarithms. It is applied to the case of the thrust major and the oblateness, which have so far defied analytical resummation and to the two-jet rate in the Durham algorithm, for which only a subset of the single logs had up to now been calculated.Comment: 29 pages, 7 figures. Version 2 adds some clarifications, a reference, as well as corrections to the subleading fixed-order coefficients and to figures 4 and

The aT distribution of the Z boson at hadron colliders

We provide the first theoretical study of a novel variable, $a_T$, proposed in Ref.[1] as a more accurate probe of the region of low transverse momentum $p_T$, for the $Z$ boson $p_T$ distribution at hadron colliders. The $a_T$ is the component of $p_T$ transverse to a suitably defined axis. Our study involves resummation of large logarithms in $a_T$ up to the next-to--leading logarithmic accuracy and we compare the results to those for the well-known $p_T$ distribution, identifying important physical differences between the two cases. We also test our resummed result at the two-loop level by comparing its expansion to order $\alpha_s^2$ with the corresponding fixed-order results and find agreement with our expectations.Comment: 30 pages, 3 figures, JHEP class included. Final version published in JHE

A general method for the resummation of event-shape distributions in e⁺ e− annihilation

We present a novel method for resummation of event shapes to next-to-next-to-leading-logarithmic (NNLL) accuracy. We discuss the technique and describe its implementation in a numerical program in the case of e + e − collisions where the resummed prediction is matched to NNLO. We reproduce all the existing predictions and present new results for oblateness and thrust major

A Minimalist Approach to Clitics and Clitic Doubling in Spanish

This dissertation addresses questions concerning cliticisation within the framework of the minimalist approach as presented, among others, by Chomsky (1995a). Chapter 1 presents the basic tenets underlying the minimalist approach. It also briefly discusses the historical developments within generative grammar which have led to the adoption of a minimalist approach. Chapter 2 presents general background on cliticisation and clitic doubling, including theories such as the Movement and Base generation theories which have been advanced to account for the behaviour of clitics. The theory of Clitic Voices presented by Sportiche (1992), a synthesis of previous analyses, is also presented and adopted as providing a more principled account of clitics in clitic-doubling constructions. In Chapter 3 the traditional classification of clitics in terms of their Case properties is abandoned in favour of a more fundamental one based on other features of clitics such as [± person] and [± number]. This characterisation leads to the postulation of a maximum of two clitic phrases for Spanish, each associated with different feature compositions. This feature system takes into account both phi-features and aspectual features of the predicate associated with the clitics. This proposal allows a unified analysis of a number of constructions, previously viewed as distinct, under the more general umbrella of clitic doubling. Chapter 4 provides an account of the alternation between enclisis and proclisis found in Spanish as well as other Romance languages. This alternation is explained by reference to the features present on the verbal host which trigger movement in cases of enclisis. In an extension of the analysis of the relation between a clitic and its host, an account of interpolation in Old Spanish is also discussed. Chapter 5 discusses the restrictions apparent in instances of clitic doubling with respect to the features of the doubling element, e.g. [± pronominal], [± human], and [± specific]. A parallel is drawn between these restrictions and similar patterns found in ergative languages. Finally, Chapter 6 shows how the conclusions reached in this thesis can be seen to apply to broader concerns of language acquisition and language impairment

Probing the low transverse momentum domain of Z production with novel variables

The measurement of the low transverse momentum region of vector boson production in Drell-Yan processes has long been invaluable to testing our knowledge of QCD dynamics both beyond fixed-order in perturbation theory as well as in the non-perturbative region. Recently the D\O\ collaboration have introduced novel variables which lead to improved measurements compared to the case of the standard QT variable. To complement this improvement on the experimental side, we develop here a complete phenomenological study dedicated in particular to the new \phi* variable. We compare our study, which contains the state-of-the-art next-to-next-to-leading resummation of large logarithms and a smooth matching to the full next-to-leading order result, to the experimental data and find excellent agreement over essentially the entire range of \phi*, even without direct inclusion of non-perturbative effects. We comment on our findings and on the potential for future studies to constrain non-perturbative behaviour.Comment: 20 pages, 7 figures. Version accepted for publication in JHEP. A figure with comparison to RESBOS has been adde

Non-global logarithms and jet algorithms in high-pT jet shapes

We consider jet-shape observables of the type proposed recently, where the shapes of one or more high-pT jets, produced in a multi-jet event with definite jet multiplicity, may be measured leaving other jets in the event unmeasured. We point out the structure of the full next-to-leading logarithmic resummation specifically including resummation of non-global logarithms in the leading-Nc limit and emphasising their properties. We also point out differences between jet algorithms in the context of soft gluon resummation for such observables.Comment: 22 pages, 4 figures. Title and a few words changed. Several typos corrected. Version accepted by JHE

Quark masses in Higgs production with a jet veto

We study the impact of finite mass effects due to top and bottom loops in the jet-veto distribution for Higgs production. We discuss the appearance of non-factorizing logarithms in the region p t,veto ≳ m b . We study their numerical impact and argue that these terms can be treated as a finite remainder. We therefore detail our prescription for resumming the jet-vetoed cross section and for assessing its uncertainty in the presence of finite mass effects. Resummation for the jet-veto, including mass effects, has been implemented in the public code JetVHeto