Top-quark pair production at hadron colliders

In this thesis we investigate several phenomenologically important properties of top-quark pair production at hadron colliders. We calculate double differential cross sections in two different kinematical setups, pair invariant-mass (PIM) and single-particle inclusive (1PI) kinematics. In pair invariant-mass kinematics we are able to present results for the double differential cross section with respect to the invariant mass of the top-quark pair and the top-quark scattering angle. Working in the threshold region, where the pair invariant mass M is close to the partonic center-of-mass energy \sqrt{\hat{s}}, we are able to factorize the partonic cross section into different energy regions. We use renormalization-group (RG) methods to resum large threshold logarithms to next-to-next-to-leading-logarithmic (NNLL) accuracy. On a technical level this is done using effective field theories, such as heavy-quark effective theory (HQET) and soft-collinear effective theory (SCET). The same techniques are applied when working in 1PI kinematics, leading to a calculation of the double differential cross section with respect to transverse-momentum pT and the rapidity of the top quark. We restrict the phase-space such that only soft emission of gluons is possible, and perform a NNLL resummation of threshold logarithms. The obtained analytical expressions enable us to precisely predict several observables, and a substantial part of this thesis is devoted to their detailed phenomenological analysis. Matching our results in the threshold regions to the exact ones at next-to-leading order (NLO) in fixed-order perturbation theory, allows us to make predictions at NLO+NNLL order in RG-improved, and at approximate next-to-next-to-leading order (NNLO) in fixed order perturbation theory. We give numerical results for the invariant mass distribution of the top-quark pair, and for the top-quark transverse-momentum and rapidity spectrum. We predict the total cross section, separately for both kinematics. Using these results, we analyze subleading contributions to the total cross section in 1PI and PIM originating from power corrections to the leading terms in the threshold expansions, and compare them to previous approaches. We later combine our PIM and 1PI results for the total cross section, this way eliminating uncertainties due to these corrections. The combined predictions for the total cross section are presented as a function of the top-quark mass in the pole, the minimal-subtraction (MS), and the 1S mass scheme. In addition, we calculate the forward-backward (FB) asymmetry at the Tevatron in the laboratory, and in the ttbar rest frames as a function of the rapidity and the invariant mass of the top-quark pair at NLO+NNLL. We also give binned results for the asymmetry as a function of the invariant mass and the rapidity difference of the ttbar pair, and compare those to recent measurements. As a last application we calculate the charge asymmetry at the LHC as a function of a lower rapidity cut-off for the top and anti-top quarks.Diese Arbeit beschäftigt sich mit mehreren phänomenologisch relevanten Eigenschaften der Top-Quark Paarproduktion an Hadronen-Beschleunigern. Wir berechnen doppelt differentielle Wirkungsquerschnitte mit Hilfe zweier unterschiedlicher kinematischer Rahmenbedingungen, “pair invariant-mass” (PIM) und “single-particle inclusive” (1PI) Kinematik. Innerhalb PIM sind wir in der Lage Ergebnisse für den doppelt differentiellen Wirkungsquerschnitt (WQ) bzgl. der invarianten Masse des Top-Antitop Paares und des Top-Quark Streuwinkels zu geben. In dem Schwellbereich wo die invariante Masse M des Top-Antitop Paares fast der partonischen Schwerpunktsenergie \sqrt{\hat{s}} entspricht, sind wir in der Lage den WQ in verschiedene Energiebereiche zu faktorisieren. Mit Methoden der Renormierungsgruppe (RG) resummieren wir numerisch große Logarithmen auf “next-to-next-to-leading- logarithmic” (NNLL) Ordnung. Um dies zu erzielen wenden wir effektive Feldtheorien wie “soft-collinear effective theory” (SCET) und “heavy-quark effective theory” (HQET) an. Mit den gleichen Methoden berechnen wir innerhalb der 1PI Kinematik den doppelt differentiellen WQ bzgl. des transversalen Impulses p⊥ und der Rapidität des Top-Quarks. Wird beschränken den Phasenraum so, dass nur Emissionen von niederenergetischen Gluonen möglich ist und resummieren Schwelbereichlogarithmen auf NNLL. Die analytischen Ausdrücke die wir erhalten ermöglichen es uns genaue Prognosen für mehrere Observablen zu machen und ein großer Teil dieser Arbeit beschäftigt sich damit diese im Detail zu studieren. Wir kombinieren unsere Schwellbereichresultate mit den allgemeingültigen “next-to-leading order” (NLO) Ergebnissen. Dies erlaubt es uns Vorhersagen auf NLO+NNLL in RG-verbesserter und auf approximierter “next-to-next-to-leading or- der” (NNLO) in “fixed order” Störungstheorie zu machen. Wir präsentieren Ergebnisse für das invariante-Masse Spektrum des Top-Antitop Paares sowie für das p⊥ und Rapiditäts Spektrum des Top-Quarks. Wir berechnen den totalen WQ getrennt in beiden kinematischen Rahmenbedingungen welches es uns ermöglicht die Relevanz nichtführender Beiträge im Schwellbereich innerhalb PIM und 1PI zu analysieren und mit vorherigen Berechnungen zu vergleichen. Gegen Ende dieser Arbeit kombinieren wir unsere PIM und 1PI Vorhersagen für den totalen WQ was unter anderem zu einer Reduktion des systematischen Fehlers durch die oben genannten Beiträge führt. Die kombinierten Ergebnisse für den totalen WQ werden als Funktion der Top-Quark Masse im “pole”, “minimal-subtraktion” und 1S Schema gegeben. Des Weiteren Berechnen wir die “forward-backward” (FB) Asymmetrie am Tevatron im Laborsystem und im ttbar Ruhesystem als Funktion der Rapidität sowie der invarianten Masse des Top-Antitop Paares auf NLO+NNLL. Unsere Prognosen für die Asymmetrie als Funktion der invarianten Masse und der Rapiditätsdifferenz des Top-Antitop Paares unterteilen wir in verschiedene Wertebereiche und vergleichen dies mit neuen Messungen. Als letzte Anwendung unserer analytischen Resultate berechnen wir die Ladungs-Asymmetrie am LHC als Funktion einer unteren Grenze der Rapidität des Top und Antitop Quarks.177 S

Renormalization-Group Improved Prediction for Higgs Production at Hadron Colliders

We use renormalization-group methods in effective field theory to improve the theoretical prediction for the cross section for Higgs-boson production at hadron colliders. In addition to soft-gluon resummation at NNNLL, we also resum enhanced contributions of the form (C_A\pi\alpha_s)^n, which arise in the analytic continuation of the gluon form factor to time-like momentum transfer. This resummation is achieved by evaluating the matching corrections arising at the Higgs-boson mass scale at a time-like renormalization point \mu^2<0, followed by renormalization-group evolution to \mu^2>0. We match our resummed result to NNLO fixed-order perturbation theory and give numerical predictions for the total production cross section as a function of the Higgs-boson mass. Resummation effects are significant even at NNLO, where our improved predictions for the cross sections at the Tevatron and the LHC exceed the fixed-order predictions by about 13% and 8%, respectively, for m_H=120 GeV. We also discuss the application of our technique to other time-like processes such as Drell-Yan production, e^+ e^- --> hadrons, and hadronic decays of the Higgs boson.Comment: 35 pages, 6 figures; v2: update to MSTW2008 PDFs, detailed comparison with moment-space formalism; v3: typo in equation (A.3) correcte

Forward-Backward and Charge Asymmetries in the Standard Model

This talk reviews the Standard Model predictions for the top-quark forward backward and charge asymmetries measured at the Tevatron and at the LHC.Comment: 8 pages, 2 figures. Proceedings of CKM 2012, the 7th International Workshop on the CKM Unitarity Triangle, University of Cincinnati, USA, 28 September - 2 October 201

Updated Predictions for Higgs Production at the Tevatron and the LHC

We present updated predictions for the total cross section for Higgs boson production through gluon fusion at hadron colliders. In addition to renormalization-group improvement at next-to-next-to-next-to-leading logarithmic accuracy, we incorporate the two-loop electroweak corrections, which leads to the most precise predictions at present. Numerical results are given for Higgs masses between 115 GeV and 200 GeV at the Tevatron with \sqrt{s}=1.96 TeV and the LHC with \sqrt{s}=7-14 TeV.Comment: 8 pages, 2 figures. v2: combined PDF+alpha_s uncertainties included; results using NNPDF2.0 added; upgrade CTEQ6.6->CT1

Enabling Logic Computation Between Ta/CoFeB/MgO Nanomagnets

Dipolar coupled magnets proved to have the potential to be capable of successfully performing digital computation in a highly parallel way. For that, nanomagnet-based computation requires precise control of the domain wall nucleation from a well-localized region of the magnet. Co/Pt and Co/Ni multilayer stacks were successfully used to demonstrate a variety of computing devices. However, Ta/CoFeB/MgO appears more promising, thanks to the lower switching field required to achieve a full magnetization reversal, reduced thickness (less than 10 nm), and its compatibility with magnetic tunnel junctions. In this work, the switch of the information is achieved through the application of a magnetic field, which allows to scale more the nanomagnets with respect to current-driven magnetization reversal-based devices and to go toward 3-D structures. We experimentally demonstrate that Ga ions can be used to tune the energy landscape of the structured magnets to provide signal directionality and achieve a distinct logic computation. We prove that it is possible to define the artificial nucleation center (ANC) in different structures with two irradiation steps and that this approach can enable logic computation in ultrathin films by dipolar interaction. Moreover, different from previous studies, the results coming from the irradiation analysis are then used for real logic devices. We present the experimental demonstration of a set of fully working planar inverters, showing that it is possible to reach a coupling field between the input and the output, which is strong enough to reliably implement logic operations. Micromagnetic simulations are used to study the nucleation center's effectiveness with respect to its position in the magnet and to support the experiments. Our results open the path to the development of more efficient nanomagnet-based logic circuits

Experimental Demonstration of a Rowland Spectrometer for Spin Waves

We experimentally demonstrate the operation of a spin-wave Rowland spectrometer. In the proposed device geometry, spin waves are coherently excited on a diffraction grating and form an interference pattern that spatially separates spectral components of the incoming signal. The diffraction grating was created by focused-ion-beam irradiation, which was found to locally eliminate the ferrimagnetic properties of YIG, without removing the material. We found that in our experiments spin waves were created by an indirect mechanism, by exploiting nonlinear resonance between the grating and the coplanar waveguide. Our work paves the way for complex spin-wave optic devices -- chips that replicate the functionality of integrated optical devices on a chip-scale.Comment: 7 pages, 5 figures, presented at Joint European Magnetic Symposia (JEMS) 202

Controlling Domain-Wall Nucleation in Ta/CoFeB/MgO Nanomagnets via Local Ga+ Ion Irradiation

Comprehensive control of the domain wall nucleation process is crucial for spin-based emerging technologies ranging from random-access and storage-class memories over domain-wall logic concepts to nanomagnetic logic. In this work, focused Ga+ ion-irradiation is investigated as an effective means to control domain-wall nucleation in Ta/CoFeB/MgO nanostructures. We show that analogously to He+ irradiation, it is not only possible to reduce the perpendicular magnetic anisotropy but also to increase it significantly, enabling new, bidirectional manipulation schemes. First, the irradiation effects are assessed on film level, sketching an overview of the dose-dependent changes in the magnetic energy landscape. Subsequent time-domain nucleation characteristics of irradiated nanostructures reveal substantial increases in the anisotropy fields but surprisingly small effects on the measured energy barriers, indicating shrinking nucleation volumes. Spatial control of the domain wall nucleation point is achieved by employing focused irradiation of pre-irradiated magnets, with the diameter of the introduced circular defect controlling the coercivity. Special attention is given to the nucleation mechanisms, changing from a Stoner-Wohlfarth particle's coherent rotation to depinning from an anisotropy gradient. Dynamic micromagnetic simulations and related measurements are used in addition to model and analyze this depinning-dominated magnetization reversal

RG-improved single-particle inclusive cross sections and forward-backward asymmetry in ttˉt\bar t production at hadron colliders

We use techniques from soft-collinear effective theory (SCET) to derive renormalization-group improved predictions for single-particle inclusive (1PI) observables in top-quark pair production at hadron colliders. In particular, we study the top-quark transverse-momentum and rapidity distributions, the forward-backward asymmetry at the Tevatron, and the total cross section at NLO+NNLL order in resummed perturbation theory and at approximate NNLO in fixed order. We also perform a detailed analysis of power corrections to the leading terms in the threshold expansion of the partonic hard-scattering kernels. We conclude that, although the threshold expansion in 1PI kinematics is susceptible to numerically significant power corrections, its predictions for the total cross section are in good agreement with those obtained by integrating the top-pair invariant-mass distribution in pair invariant-mass kinematics, as long as a certain set of subleading terms appearing naturally within the SCET formalism is included.Comment: 55 pages, 14 figures, 6 table

Association of growth with neurodevelopment in extremely low gestational age infants: a population-based analysis.

To assess the association between postnatal growth and neurodevelopment at the age of 2 years in extremely low gestational age newborns (ELGAN, < 28 weeks' gestation). Retrospective population-based cohort study including all live born ELGAN in 2006-2012 in Switzerland. Growth parameters (weight, length, head circumference, body mass index) were assessed at birth, at hospital discharge home, and 2-year follow-up (FU2). Unadjusted and adjusted regression models assessed associations between growth (birth to hospital discharge and birth to FU2) and neurodevelopment at FU2. A total of 1244 infants (mean GA 26.5 ± 1.0 weeks, birth weight 853 ± 189 g) survived to hospital discharge and were included in the analyses. FU2 was documented for 1049 (84.3%) infants. The mean (± SD) mental and a psychomotor development index at 2FU were 88.9 (± 18.0) and 86.9 (± 17.7), respectively. Moderate or severe neurodevelopmental impairment was documented in 23.2% of patients. Changes of z-scores between birth and discharge and between birth and FU2 for weight were - 1.06 (± 0.85) and - 0.140 (± 1.15), for length - 1.36 (± 1.34), and - 0.40 (± 1.33), for head circumference - 0.61 (± 1.04) and - 0.76 (± 1.32) as well as for BMI 0.22 (± 3.36) and - 0.006 (± 1.45). Unadjusted and adjusted analyses showed that none of the four growth parameters was significantly associated with any of the three outcome parameters of neurodevelopment. This was consistent for both time intervals. CONCLUSION In the present population-based cohort of ELGAN, neither growth between birth and hospital discharge nor between birth and FU2 were significantly associated with neurodevelopment at age of 2 years. WHAT IS KNOWN • Studies assessing the association between growth and neurodevelopment in extremely low gestational age newborns (28 weeks' gestation) show conflicting results. WHAT IS NEW • Neither growth between birth and hospital discharge nor between birth and corrected age of 2 years were significantly associated with neurodevelopment at age of 2 years. • The role of postnatal growth as a predictor of neurodevelopmental outcome during infancy might be smaller than previously assumed