Action semantics of unified modeling language

The Uni ed Modeling Language or UML, as a visual and general purpose modeling language, has been around for more than a decade, gaining increasingly wide application and becoming the de-facto industrial standard for modeling software systems. However, the dynamic semantics of UML behaviours are only described in natural languages. Speci cation in natural languages inevitably involves vagueness, lacks reasonability and discourages mechanical language implementation. Such semi-formality of UML causes wide concern for researchers, including us. The formal semantics of UML demands more readability and extensibility due to its fast evolution and a wider range of users. Therefore we adopt Action Semantics (AS), mainly created by Peter Mosses, to formalize the dynamic semantics of UML, because AS can satisfy these needs advantageously compared to other frameworks. Instead of de ning UML directly, we design an action language, called ALx, and use it as the intermediary between a typical executable UML and its action semantics. ALx is highly heterogeneous, combining the features of Object Oriented Programming Languages, Object Query Languages, Model Description Languages and more complex behaviours like state machines. Adopting AS to formalize such a heterogeneous language is in turn of signi cance in exploring the adequacy and applicability of AS. In order to give assurance of the validity of the action semantics of ALx, a prototype ALx-to-Java translator is implemented, underpinned by our formal semantic description of the action language and using the Model Driven Approach (MDA). We argue that MDA is a feasible way of implementing this source-to-source language translator because the cornerstone of MDA, UML, is adequate to specify the static aspect of programming languages, and MDA provides executable transformation languages to model mapping rules between languages. We also construct a translator using a commonly-used conventional approach, in i which a tool is employed to generate the lexical scanner and the parser, and then other components including the type checker, symbol table constructor, intermediate representation producer and code generator, are coded manually. Then we compare the conventional approach with the MDA. The result shows that MDA has advantages over the conventional method in the aspect of code quality but is inferior to the latter in terms of system performance

Graph Neural Network Flavour Tagging and Boosted Higgs Measurements at the LHC

This thesis presents investigations into the challenges of, and potential improvements to, b-jet identification (b-tagging) at the ATLAS experiment at the Large Hadron Collider (LHC). The presence of b-jets is a key signature of many interesting physics processes such as the production of Higgs bosons, which preferentially decay to a pair of b-quarks. In this thesis, a particular focus is placed on the high transverse momentum regime, which is a critical region in which to study the Higgs boson and the wider Standard Model, but also a region within which b-tagging becomes increasingly difficult. As b-tagging relies on the accurate reconstruction of charged particle trajectories (tracks), the tracking performance is investigated and potential improvements are assessed. Track reconstruction becomes increasingly difficult at high transverse momentum due to the in- creased multiplicity and collimation of tracks, and also due to the presence of displaced tracks from the decay of a long-flying b-hadron. The investigations reveal that the quality selections applied during track reconstruction are suboptimal for b-hadron decay tracks inside high transverse momentum b-jets, motivating future studies into the optimisation of these selections. Two novel approaches are developed to improve b-tagging performance. Firstly, an algorithm which is able to classify the origin of tracks is used to select a more optimal set of tracks for input to the b-tagging algorithms. Secondly, a graph neural network (GNN) jet flavour tagging algorithm has been developed. This algorithm directly accepts jets and tracks as inputs, making a break from previous algorithms which relied on the outputs of intermediate taggers. The model is trained to simultaneously predict the jet flavour, track origins, and the spatial track-pair compatibility, and demonstrates marked improvements in b-tagging performance both at low and high transverse momenta. The closely related task of c-jet identification also benefits from this approach. Analysis of high transverse momentum H → bb decays, where the Higgs boson is produced in association with a vector boson, was performed using 139 fb−1 of 13 TeV proton-proton collision data from Run 2 of the LHC. This analysis provided first measurements of the V H, H → bb process in two high transverse momentum regions, and is described with a particular focus on the background modelling studies performed by the author

Search for resonant WZ production in the fully leptonic final state with the ATLAS detector

Diboson resonance searches are an essential test of electroweak symmetry breaking theories beyond the Standard Model (SM) of particle physics. Vector or scalar resonances decaying to dibosons are predicted by various models going beyond theSM, such as Grand Unified theories, Little Higgs models, Composite Higgs models or models with extended Higgs sector (such as Super Symmetry (SUSY) or two-Higgs-doublet models). This thesis presents a search for resonant WZ production in the fully leptonic decay channel ℓν ℓℓ (ℓ = e or μ) with two production modes : quark-antiquark fusion or vector-boson fusion. Using 36.1 fb−1 of recent data collected by the ATLAS detector in pp collisions delivered by the Large Hadron Collider (LHC) at 13 TeV, constraints are obtained on models going beyond the SM. Since this analysis considers the fully leptonic decays of the vector bosons, events with exactly three leptons in the final state and a substantial missing transverse energy are selected. One signal region is established for each production mode of the resonance, either by quark-antiquark or vector boson fusion. For heavy vectorial resonances produced by quark fusion the W and Z bosons are required to carry a substantial fraction of the resonance energy (pWT /mWZ > 0.35 and pZT /mWZ > 0.35)). Events with resonances produced by vector boson fusion are characterised by two jets with a large invariant mass and a large separation in pseudorapidity. Therefore, in the search for this production mode, events are required to have at least two jets with an invariant mass (mj j ) greater than 500 GeV and a separation in pseudorapidity ( ηj j ) of at least | ηj j | > 3.5. In each signal region the distribution of the invariant mass of the WZ system will then be examined to determine the presence or absence of new resonances that manifest themselves as localised excesses in the invariant mass of the diboson system (mWZ). No significant excess was observed in the signal regions. Limits have then been set on the cross section times branching ratio for a heavy vector resonance produced by either quark-antiquark or vector boson fusion. Additionally, limits on the coupling parameters and masses are obtained for a charged Higgs boson in the Georgi-Machacek produced by vector boson fusion.Les recherches de résonances di-boson constituent un test essentiel des théories de brisure de symétrie électrofaible au-delà du modèle standard (MS). Plusieurs scénarios, comme les théories de grande unification, les modèles Little Higgs, les modèles de Higgs Composés ou celles avec un secteur de Higgs élargi (par example SUSY ou le two-Higgsdoublet model), prédisent des résonances vectorielles ou scalaires. Cette thèse présente une recherche de résonances lourdes se désintégrant en WZ dans le canal leptonique WZ → ℓν ℓℓ (ℓ = e ou μ). Deux modes de production sont considérés : par fusion de quark-antiquark ou par fusion de boson vectoriels. Se basant sur les récentes données recueillies par le détecteur ATLAS lors de collisions pp à 13 TeV au LHC au cours des années 2015 et 2016, avec une luminosité intégrée de 36.1 fb−1, on établira des contraintes sur des modèles allant au-delà du MS de la physique des particules. Puisqu’on considère la désintégration leptonique des bosons WZ, on sélectionne les événements ayant trois leptons et une grande énergie transverse manquante. Des régions de signal sont choisies pour chaque mode de production : fusion par quarks ou par bosons vectoriels. Pour les résonances produites par fusion de quark, on considère des résonances vectorielles lourdes. On sélectionne donc des événements où les bosonW et Z portent une fraction importante de l’énergie de masse de la résonance (pWT /mWZ > 0.35 et pZT /mWZ > 0.35). Les résonances produites par fusion de bosons sont caractérisées par deux jets ayant une grande séparation en pseudorapidité et une grande masse invariante. Pour la région de signal, dans ce cas, on requiert alors au moins deux jets avec une masse invariante supérieure à mj j > 500 GeV et une grande séparation en pseudorapidité | ηj j | > 3.5. Pour les deux régions, la distribution en masse invariante du système WZ sera examinée pour déterminer la présence ou non de nouvelles résonances qui se manifesteraient par un excès localisé. En fin de compte, aucun excès significatif n’a été observé dans les régions de signal, ce qui permet d’établir des limites sur le produit de la section efficace et du rapport d’embranchement d’un boson massif vectoriel dans les deux canaux de production. Des contraintes sont également obtenues sur la masse et le couplage d’un boson de Higgs chargé du modèle Georgi-Machacek, produites par fusion de bosons vectoriels

Associated production of a single top quark and a Z boson in proton-proton collisions at a center-of-mass energy of 13 TeV using the ATLAS detector

The production of an electroweak single top quark produced in association with a Z boson is studied using 139fb^-1 of proton-proton collision data collected by the ATLAS experiment at the LHC over the period from 2015 to 2018, at a center-of-mass energy of √s = 13 TeV. This search is performed in the three lepton channel in which the Z boson decays leptonically and top quark decays semi-leptonically. Events are analyzed from a final state that contains three high-pt electrons and/or muons, two or three jets (one identified as coming from a b-quark), and substantial missing transverse energy. Major irreducible backgrounds come from WZ diboson and ttV + ttH + tWZ, with reducible backgrounds from tt + X and Z + jets events. A gradient boosting algorithm is trained and applied in order to separate signal and background distributions, with a binned likelihood fit applied to the discriminant produced by the algorithm. From this fit the null-hypothesis significance and cross-section measurement are extracted. The observed (expected) significance is 9.0(9.1)σ with an extracted cross-section of σ_tZq = 770 ± 108 (statistical) ± 53 (systematic) fb. This cross-section is compatible with prediction and therefore this measurement clears the threshold to declare discovery of tZq production

The Development of the ATLAS SCT as a Luminometer and the Search for SUSY Decays with Compressed Mass Spectra

The formulation of the Standard Model of particle physics (SM) is one of the greatest scientific achievements of the 20th century. It is, however, incomplete (for example, it lacks a dark matter candidate) as well as the fact that the hierarchy problem violates naturalness arguments. This has motivated the construction of particle accelerators to probe fundamental particles at increasingly high center-of-mass energies and luminosities, the LHC at CERN being the latest to continue this legacy. This thesis covers both the enhancement of luminosity measurements of pp collisions at ATLAS, underpinning the accuracy of all measurements made by the detector, and a search for one of the most theoretically viable extensions to the SM: supersymmetry. ATLAS uses mainly event-counting algorithms to measure luminosity, which break down at higher luminosities. If the ATLAS SemiConductor Tracker (SCT) can be employed as a luminometer using hit-counting algorithms, this issue may be mitigated. It is established here that the SCT can feasibly operate as a luminometer when recording two-or-more strip clusters with the standard binary readout mode (01X). Thus, the SCT can measure the luminosity with an accuracy within 10% of two of ATLAS’s existing luminometers: LUCID and TileCal. The discovery of the supersymmetric top (stop) would be fundamental for solving the hierarchy problem. An analysis of an experimentally challenging region of phase space, where stop decays have a compressed mass spectrum, complements the ATLAS one-lepton stop search using 13 TeV pp collisions at 139 fb−1 is presented. The aMT2 kinematic variable, designed to give a lower limit on pair-produced particle masses, is found to be effective at differentiating SUSY decays from the SM background, when used as an upper bound. No significant excess was observed above the Standard Model background and limits at 95% confidence level are set. Stop quarks are found to be excluded up to 500 GeV and mass splittings between the stop and the neutralino are found to be excluded up to 130 GeV, complimenting the exclusion limits found by other ATLAS searches for stop decays with one-lepton final states

Top-quark pair production in association with a Z boson in the final state with four charged leptons

The production cross section of a top-quark pair in association with a Z boson is measured in the channel with four reconstructed leptons, using 36.1 fb-1 of proton–proton collisions, collected with the ATLAS detector at the Large Hadron Collider at CERN. A profile likelihood fit is performed, using four signal regions and a control region for the ZZ background. A total of 48 events are observed in the signal regions and a ttZ cross section of σttZ = 1.07+0.28-0.25 (stat.)+0.10-0.09 (syst.)pb is extracted, which is compatible with the Standard Model calculation. Combined with other channels, the measured cross section is σttZ = 0.95 ± 0.08 , (stat.) ± 0.10 (syst.)pb. Data is unfolded in the 4l channel, to compute the differential cross section for pT(Z(ll)) and ΔΦZ(ll), variables sensitive to EFT operators, that modify the ttZ vertex

Verification of Branching-Time and Alternating-Time Properties for Exogenous Coordination Models

Information and communication systems enter an increasing number of areas of daily lives. Our reliance and dependence on the functioning of such systems is rapidly growing together with the costs and the impact of system failures. At the same time the complexity of hardware and software systems extends to new limits as modern hardware architectures become more and more parallel, dynamic and heterogenous. These trends demand for a closer integration of formal methods and system engineering to show the correctness of complex systems within the design phase of large projects. The goal of this thesis is to introduce a formal holistic approach for modeling, analysis and synthesis of parallel systems that potentially addresses complex system behavior at any layer of the hardware/software stack. Due to the complexity of modern hardware and software systems, we aim to have a hierarchical modeling framework that allows to specify the behavior of a parallel system at various levels of abstraction and that facilitates designing complex systems in an iterative refinement procedure, in which more detailed behavior is added successively to the system description. In this context, the major challenge is to provide modeling formalisms that are expressive enough to address all of the above issues and are at the same time amenable to the application of formal methods for proving that the system behavior conforms to its specification. In particular, we are interested in specification formalisms that allow to apply formal verification techniques such that the underlying model checking problems are still decidable within reasonable time and space bounds. The presented work relies on an exogenous modeling approach that allows a clear separation of coordination and computation and provides an operational semantic model where formal methods such as model checking are well suited and applicable. The channel-based exogenous coordination language Reo is used as modeling formalism as it supports hierarchical modeling in an iterative top-down refinement procedure. It facilitates reusability, exchangeability, and heterogeneity of components and forms the basis to apply formal verification methods. At the same time Reo has a clear formal semantics based on automata, which serve as foundation to apply formal methods such as model checking. In this thesis new modeling languages are presented that allow specifying complex systems in terms of Reo and automata models which yield the basis for a holistic approach on modeling, verification and synthesis of parallel systems. The second main contribution of this thesis are tailored branching-time and alternating time temporal logics as well as corresponding model checking algorithms. The thesis includes results on the theoretical complexity of the underlying model checking problems as well as practical results. For the latter the presented approach has been implemented in the symbolic verification tool set Vereofy. The implementation within Vereofy and evaluation of the branching-time and alternating-time model checker is the third main contribution of this thesis

New physics searches in the ZZ sector with the ATLAS experiment

This thesis investigates the prospects of measuring anomalous triple gauge boson couplings in the ATLAS detector at the Large Hadron Collider (LHC). The most general (V ZZ, V = Z, g) vertex is parametrised by four couplings, fV = Z,g i=4,5 , all of which are zero in the Standard Model. Non-zero couplings would manifest themselves as an excess of events in ZZ diboson channels, and, if observed, would be a direct probe of new physics beyond the Standard Model. A set of criteria are outlined to select events recorded by ATLAS in two such channels, ZZ to llll(l = e, mu) and ZZ to llnunu. With 1 fb-1 of integrated luminosity at a centre of mass energy of sqrt(s) = 7 TeV, ATLAS can expect to observe 10+/-1 events in the ZZ to llll channel, with 0.5+0.9-0.2 background events. In the ZZ to llnunu channel, 6.2+/-0.7 signal events are expected, with a background of 1.9+2.0-0.2 events. The expected sensitivity of ATLAS to non-zero anomalous couplings is calculated by performing extended, unbinned maximum-likelihood fits to the Z boson transverse momentum spectrum. For 1 fb-1 of integrated luminosity at sqrt(s) = 7 TeV, ATLAS has the potential to place constraints on the coupling parameters of |fZi | < 0.06 and |fig | < 0.07 at the 95% confidence level. These limits assume a form factor with a cutoff of Lambda_FF = 1.2 TeV. As a prelude to ZZ observation, criteria are defined to select Z to ll(l = e, mu) events in the first 315 nb-1 of ATLAS pp collision data at sqrt(s) = 7 TeV. In total 57 events are observed in the electron channel, with 109 in the muon channel, leading to cross section measurements of sigma(Z to ee) = 0.70+/-0.09 (stat)+/-0.10 (syst)+/-0.08 (lumi) pb and sigma(Z to mumu) = 0.90+/-0.09 (stat) +/-0.07 (syst) +/-0.10 (lumi) pb, both of which are consistent with the Standard Model predictions. In addition, this thesis presents a summary of developments made to the Data Acquisition (DAQ) system of the ATLAS Semiconductor Tracker (SCT). These include the construction of a test system, involving a scaled-down version of the entire SCT readout chain. The test system was subsequently used to develop a number of new DAQ features, including a hardware-based event simulator and monitoring framework.This work was made possible by the Science and Technology Research Council (STFC) under STFC student number PPA/S/S/2006/04325