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Showing 71 to 80 of 81 (0.029 seconds)Spelling suggestions: "subject:"electroweak"" "subject:"eletroweak""
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Measurement of Electroweak Gauge Boson Scattering in the Channel pp → W ± W ± jj with the ATLAS Detector at the Large Hadron ColliderGumpert, Christian 27 February 2015 (has links)
Particle physics deals with the elementary constituents of our universe and their interactions. The electroweak symmetry breaking mechanism in the Standard Model of Particle Physics is of paramount importance and it plays a central role in the physics programmes of current high-energy physics experiments at the Large Hadron Collider. The study of scattering processes of massive electroweak gauge bosons provides an approach complementary to the precise measurement of the properties of the recently discovered Higgs boson. Owing to the unprecedented energies achieved in proton-proton collisions at the Large Hadron Collider and the large amount of data collected, experimental studies of these processes become feasible for the first time. Especially the scattering of two W± bosons of identical electric charge is considered a promising process for an initial study due to its distinct experimental signature. In the course of this work, 20.3 fb−1 of proton-proton collision data recorded by the ATLAS detector at a centre-of-mass energy of √s = 8 TeV are analysed. An analysis of the production of two W± bosons of identical electric charge in association with two jets, pp → W ± W ± jj, is conducted in the leptonic decay channel of the W± bosons.
Thereby, emphasis is put on the development of methods for the estimation of experimental backgrounds as well as on the optimisation of the event selection. As a result of this work, first experimental evidence for the existence of the aforementioned process is established with an observed significance of 4.9. Based on the number of observed events in the selected phase space the extracted fiducial cross section is σ(fid) = (2.3 ± 0.5(stat.) +0.4/−0.3 (sys.)) fb which is in agreement with the prediction of the Standard Model of σ(fid,SM) = (1.6 ± 0.2) fb. Of particular theoretical interest are electroweak contributions to the pp → W ± W ± jj process due to their sensitivity to the nature of the electroweak symmetry breaking mechanism. Criteria for a dedicated event selection are investigated and implemented in the analysis with the goal of enhancing the sensitivity to these contributions. First experimental evidence for the presence of electroweak contributions to the pp → W ± W ± jj process can be claimed with an observed significance of 4.1. The cross section extracted in the selected phase space region is found to be σ(fid) = (1.7 +0.5/−0.4 (stat.) ± 0.3(sys.)) fb which is 1.3 standard deviations above the theoretical prediction of the Standard Model of σ(fid,SM) = (1.0 ± 0.1) fb.
A variety of extensions to the Standard Model predict modifications to the electroweak gauge sector. In the context of the electroweak chiral Lagrangian, which serves as an effective approximation of these theories in the energy regime E = 1 − 3 TeV, anomalous contributions to the quartic WWWW gauge coupling can be described by the parameters α4 and α5 . The selection of events is optimised again to enhance the sensitivity to these two parameters. On the basis of the number of events observed in this phase space region, the following one-dimensional confidence intervals at the 95% confidence level are derived: −0.09 ≤ α4 ≤ 0.10 and −0.15 ≤ α5 ≤ 0.15. At present, these limits represent the most stringent constraints on contributions from new physics processes to the quartic WWWW gauge coupling.
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Narušení elektroslabé symetrie dynamickým generováním hmot kvarků a leptonů / Electroweak symmetry breaking by dynamically generated masses of quarks and leptonsSmetana, Adam January 2013 (has links)
Title: Electroweak symmetry breaking by dynamically generated masses of quarks and leptons Author: Adam Smetana Department: Institute of Particle and Nuclear Physics, Faculty of Mathematics and Physics, Charles University Supervisor: Ing. Jiří Hošek, DrSc., Department of Theoretical Physics, Nuclear Physics Institute, Academy of Sciences of the Czech Republic Abstract: The aim of the thesis is to study models of the electroweak symmetry breaking caused by dynamically generated masses of quarks and lep- tons. (1) We perform the basic analysis whether the main underlying idea, that the masses of only known fermions can provide the elec- troweak symmetry breaking, is actually feasible. For that we elaborate a two-composite-Higgs-doublet model of the top-quark and neutrino condensation. The model suggests rather large number, O(100), of right-handed neutrinos. (2) We analyze the model of strong Yukawa dynamics where the dynamical fermion mass generation is provided by exchanges of new elementary massive complex doublet scalar fields. We focus on solving the coupled Schwinger-Dyson equations for fermion and scalar self-energies by means of approximative methods. We doc- ument that strongly hierarchical mass spectra can be reproduced. (3) We elaborate the flavor gauge model where the dynamical fermion mass...
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New Physics Probes at Present/Future Hadron Colliders via Vh ProductionEnglert, Philipp 26 April 2023 (has links)
In dieser Arbeit nutzen wir Effektive Feldtheorien, genauer gesagt die SMEFT, um BSM-Effekte modellunabhängig zu parametrisieren.
Wir demonstrieren die Relevanz von Präzisionsmessungen sowohl an aktuellen als auch an zukünftigen Hadronenbeschleunigern durch die Untersuchung von Vh-Dibosonen-Prozessen. Diese Prozesse ermöglichen uns die Untersuchung einer Reihe von Dimension-6-Operatoren, die BSM-Effekte erzeugen, die mit der Schwerpunktsenergie wachsen. Im Besonderen betrachten wir die leptonischen Zerfallskanäle der Vektorbosonen und zwei verschiedene Zerfallsmodi des Higgs-Bosons, den Diphoton-Kanal und den h->bb-Kanal.
Der Diphoton-Kanal zeichnet sich durch eine saubere Signatur aus, die mit relativ einfachen Mitteln sehr gut von den relevanten Hintergründen unterschieden werden kann. Aufgrund der geringen Rate dieses Higgs-Zerfallskanals werden diese Prozesse allerdings erst für die Untersuchung von BSM-Effekten am FCC-hh relevant. Dank des großen h->bb Verzweigungsverhältnisse liefert der Vh(->bb)-Kanal bereits eine kompetitive Sensitivität für BSM-Effekte am LHC. Jedoch leidet dieser Kanal unter großen QCD-induzierten Hintergründen, weswegen ausgefeiltere Analysetechniken nötig sind, um dieses Niveau an BSM-Sensitivität zu erreichen. Wir leiten die erwarteten Schranken für die zuvor erwähnten Operatoren für den Vh(->gamma gamma)-Kanal am FCC-hh und für den Vh(->bb)-Kanal am LHC Run 3, HL-LHC und FCC-hh her.
Unsere Studie des Vh(->bb)-Kanals zeigt, dass die Extraktion von Schranken für BSM-Operatoren an Hadronenbeschleunigern eine höchst nicht-triviale Aufgabe sein kann.
Algorithmen des Maschinellen Lernens können potenziell nützlich zur Analyse solch komplexer Event-Strukturen sein. Wir leiten Schranken her, indem wir Boosted Decision Trees zur Signal-Hintergrund Klassifizierung benutzen und und vergleichen sie mit den Schranken aus der zuvor diskutierten Cut-and-Count Analyse. Wir finden eine leichte Verbesserung von O(einige %) für die verschiedenen Operatoren. / In this thesis, we utilise the framework of Effective Field Theories, more specifically the Standard Model Effective Field Theory, to parameterise New-Physics effects in a model-independent way.
We demonstrate the relevance of precision measurements both at current and future hadron colliders by studying Vh-diboson-production processes. These processes allow us to probe a set of dimension-6 operators that generate BSM effects growing with the center-of-mass energy. More specifically, we consider the leptonic decay channels of the vector bosons and two different decay modes of the Higgs boson, the diphoton channel and the hadronic h->bb channel.
The diphoton channel is characterised by a clean signature that can be separated very well from the relevant backgrounds with relatively simple methods. However, due to the small rate of this Higgs-decay channel, these processes will only become viable to probe New-Physics effects at the FCC-hh. Thanks to the large h->bb branching ratio, the Vh(->bb) channel already provides competitive sensitivity to BSM effects at the LHC. However, it suffers from large QCD-induced backgrounds that require us to use more sophisticated analysis techniques to achieve this level of BSM sensitivity. We derive the expected bounds on the previously mentioned dimension-6 operators from the Vh(->gamma gamma) channel at the FCC-hh and from the Vh(->bb) channel at the LHC Run 3, HL-LHC and FCC-hh.
Our study of the Vh(->bb) channel demonstrates that extracting bounds on BSM operators at hadron colliders can be a highly non-trivial task. Machine-Learning algorithms can potentially be useful for the analysis of such complex event structures. We derive bounds using Boosted Decision Trees for the signal-background classification and compare them with the ones from the previously discussed cut-and-count analysis. We find a mild improvement of O(few %) across the different operators.
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Constraints on the Fourth-Generation Quark Mixing Matrix from Precision Flavour ObservablesMenzel, Andreas 27 February 2017 (has links)
Das Standardmodell einer zusätzlichen sequentiellen Fermiongeneration (SM4) war 2012 auf Basis eines Fits an elektroschwache Präzisionsobservable und die Higgs-Signalstärken mit einer Signifikanz von 5.3 sigma ausgeschlossen worden. Komplementär dazu wurden in der vorliegenden Arbeit Fits des SM4 an eine Kombination eines typischen Satzes von Flavour-Observablen mit den Ergebnissen des zuvor durchgeführten Elektroschwachen Präzisionsfits durchgeführt. Im SM3-Kontext extrahierte Größen wurden gemäß ihrer Bedeutung im SM4 reinterpretiert und die angepassten theoretischen Ausdrücke angegeben. Die resultierenden Einschränkungen der CKM-Matrix des SM4, ihrer potentiell CP-verletzenden Phasen sowie der Masse des up-type-Quarks der 4. Generation t'' werden angegeben. Zum Vergleich des SM4 mit dem SM3 werden die erreichten chi^2-Werte genutzt. chi^2=15.53 im SM4 und 9.56 im SM3 passen fast vollkommen zu einer gleich guten Beschreibung der Experimente durch beide Modelle, wobei das SM3 aber sechs Freiheitsgrade mehr besitzt. Außerdem wurden die Vorhersagen des SM3 und des SM4 für die Dimyon-Ladungsasymmetrie ASL mit experimentellen Werten verglichen. Die Vorhersage des SM3 ist ca. 2 sigma vom experimentellen Wert entfernt, die des SM4 ca. 3 sigma.\par Die Ergebnisse deuten nicht darauf hin, dass die Signifikanz des 2012 erreichten Ausschlusses des SM4 durch die Hinzunahme von Flavour-Observablen zu den damals verwendeten elektroschwachen Präzisionsobservablen und Higgs-Querschnitten bedeutend verringert würde.\par Es konnte jedoch keine genaue quantitative Aussage über die Auswirkungen der Flavourobservablen auf diese Signifikanz getroffen werden, weil das Programm CKMfitter likelihood-ratio-Berechnung nur durchführen kann, wenn sich eines der untersuchten Modelle durch Fixierung von Parametern aus dem anderen ergibt (nested models), was hier nicht der Fall ist. / The Standard Model extended by an additional sequential generation of Dirac fermions (SM4) was excluded with a significance of 5.3 sigma in 2012. This was achieved in a combined fit of the SM4 to Electroweak Precision Observables and signal strengths of the Higgs boson. This thesis complements this excludion by a fit of the SM4 to a typical set of Flavour physics observables and the results of the previously performed Electroweak Precision fit. Quantities extracted in an SM3 framework are reinterpreted in SM4 terms and the adapted theoretical expressions are given. The resultant constraints on the SM4''s CKM matrix, its potentially CP-violating phases and the mass of the new up-type quark t'' are given. To compare the relative performance of the SM4 and the SM3, this work uses the chi^2 values achieved in the fit. The values of 15.53 for the SM4 and 9.56 for the SM4 are almost perfectly consistent with both models describing the experimental data equally well with the SM3 having six degrees of freedom more. The dimuon charge asymmetry ASL was not used as a fit input because the interpretation of its measurement was subject to debate at the time when the fits were produced, but its prediction in the fit was used as an additional test of the SM4. The SM3''s prediction differs from the experimental values by about 2 sigma, and the SM4''s prediction by about 3 sigma. \par In summary, these results do not suggest that any significant reduction of the 5.3 sigma exclusion could be achieved by combining the Electroweak Precision Observables and Higgs inputs with Flavour physics data. However, the exact effect of the Flavour physics input on the significance of the SM4''s exclusion cannot be given at this point because the CKMfitter software is currently not able to perform a statistically stringent likelihood comparison of non-nested models.
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Measurement of the Standard Model W⁺W⁻ production cross-section using the ATLAS experiment on the LHC / Mesure de la section efficace de production des bosons W⁺W⁻ dans l'experience ATLAS au LHCZeman, Martin 02 October 2014 (has links)
Les mesures de sections efficaces de production di-bosons constituent une partie importante du programme de physique au Large Hadron Collider (Grand collisionneur de hadrons) au CERN. Ces analyses de physique offrent la possibilité d'explorer le secteur électrofaible du modèle standard à l'échelle du TeV et peuvent être une indication de l'existence de nouvelles particules au-delà du modèle standard. L'excellente performance du LHC dans les années 2011 et 2012 a permis de faire les mesures très compétitives. La thèse donne un aperçu complet des méthodes expérimentales utilisées dans la mesure de la section efficace de production de W⁺W⁻ dans les collisions proton-proton au √s = 7 TeV et 8 TeV. Le texte décrit l'analyse en détail, en commençant par l'introduction du cadre théorique du modèle standard et se poursuit avec une discussion détaillée des méthodes utilisées dans l'enregistrement et la reconstruction des événements de physique dans une expérience de cette ampleur. Les logiciels associés (online et offline) sont inclus dans la discussion. Les expériences sont décrites en détail avec en particulier une section détaillée sur le détecteur ATLAS. Le dernier chapitre de cette thèse présente une description détaillée de l'analyse de la production de bosons W dans les modes de désintégration leptoniques utilisant les données enregistrées par l'expérience ATLAS pendant les années 2011 et 2012 (Run I). Les analyses utilisent 4,6 fb⁻¹ de données enregistrées à √s = 7 TeV et 20,28 fb⁻¹ enregistré à 8 TeV. La section efficace mesurée expérimentalement pour la production de bosons W dans l'expérience ATLAS est plus grande que celle prédite par le modèle standard à 7 TeV et 8 TeV. La thèse se termine par la présentation des résultats de mesures différentielles de section efficace. / Measurements of di-boson production cross-sections are an important part of the physics programme at the CERN Large Hadron Collider. These physics analyses provide the opportunity to probe the electroweak sector of the Standard Model at the TeV scale and could also indicate the existence of new particles or probe beyond the Standard Model physics. The excellent performance of the LHC through years 2011 and 2012 allowed for very competitive measurements. This thesis provides a comprehensive overview of the experimental considerations and methods used in the measurement of the W⁺W⁻ production cross-section in proton-proton collisions at √s = 7 TeV and 8 TeV. The treatise covers the material in great detail, starting with the introduction of the theoretical framework of the Standard Model and follows with an extensive discussion of the methods implemented in recording and reconstructing physics events in an experiment of this magnitude. The associated online and offline software tools are included in the discussion. The relevant experiments are covered, including a very detailed section about the ATLAS detector. The final chapter of this thesis contains a detailed description of the analysis of the W-pair production in the leptonic decay channels using the datasets recorded by the ATLAS experiment during 2011 and 2012 (Run I). The analyses use 4.60 fb⁻¹ recorded at √s = 7 TeV and 20.28 fb⁻¹ recorded at 8 TeV. The experimentally measured cross section for the production of W bosons at the ATLAS experiment is consistently enhanced compared to the predictions of the Standard Model at centre-of-mass energies of 7 TeV and 8 TeV. The thesis concludes with the presentation of differential cross-section measurement results.
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Test du Modèle Standard à basse énergie :<br />- Mesure précise des rapports d'embranchement de 62Ga<br />- Mesure précise de la durée de vie de 38CaBey, Anissa 14 November 2008 (has links) (PDF)
L'étude précise des transitions β de Fermi super-permises 0+ → 0+ offre un outil précieux pour explorer les propriétés de l'interaction faible dans le cadre du Modèle Standard (SM). Collectivement, les forces (ft) mesurées pour ces transitions permettent de vérifier l'hypothèse CVC et contribuent au test le plus rigoureux de l'unitarité de la première ligne de la matrice de mélange des quarks CKM en fournissant l'évaluation la plus précise de l'élément dominant (Vud). Jusqu'à récemment, un apparent défaut d'unitarité a semé le doute sur la validité du SM minimal et a mobilisé un effort considérable afin d'élargir le champ d'étude à d'autres émetteurs de Fermi. Le 62Ga et 38Ca sont parmi des noyaux clés pour mener ces tests de précision au travers de la vérification de la fiabilité des corrections imposées aux valeurs ft expérimentales. La décroissance β de 62Ga a été étudiée auprès du séparateur IGISOL à Jyväskylä, avec un dispositif composé de 3 Clovers EUROBALL pour la détection du rayonnement γ. La mise en évidence de raies γ de très faible intensité (<1‰), au travers de corrélations β-γ et β-γ-γ, a permis de reconstruire partiellement le schéma de niveaux excités dans le 62Zn. Le rapport d'embranchement analogue déduit (B.R.A = 99.893(24) %) est utilisé pour extraire la valeur Ft(62Ga) universelle. Celle-ci s'avère en bon accord avec les 12 valeurs précises connues de la littérature. La compatibilité constatée entre la limite supérieure dressée ici sur le terme (δ1IM) et la prédiction théorique confirme l'importance des corrections de brisure de symétrie d'isospin dans les émetteurs β (A ≥ 62). L'étude de la décroissance de 38Ca a été réalisée auprès de l'installation ISOLDE du CERN. L'utilisation de la fluoration des fragments de réaction, pour isoler chimiquement les isotopes d'intérêt, alliée au piégeage d'ions assisté par REXTRAP et l'analyse TOF, ont permis de s'affranchir totalement du contaminant contraignant 38mK. Pour la première fois, la durée de vie de 38Ca est mesurée avec un échantillon de haute pureté. Le résultat préliminaire établi T1/2(38Ca) = 445.8(10) ms améliore la précision par rapport à l'ancienne valeur d'un facteur proche de 10.
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Randall-Sundrum Model as a Theory of FlavourIyer, Abhishek Muralidhar January 2013 (has links) (PDF)
The discovery of the Higgs boson by the LHC provided the last piece of the puzzle neces- sary for the Standard Model (SM) to be a successful theory of electroweak scale physics. However there exist various phenomenological reasons which serve as pointer towards the existence of physics beyond the Standard Model. For example the explanation for the smallness of the neutrino mass, baryon asymmetry of the universe, the presence of dark matter and dark energy etc. are not within purview of the Standard Model. Con- ceptual issues like the gauge hierarchy problem, weakness of gravity provide some of the theoretical motivation to pursue theories beyond the SM. We consider scenarios with warped extra-dimensions (Randall-Sundrum (RS) Model ) as our preferred candidate to answer some of the questions raised above. RS model gives an elegant geometric solution to address the hierarchy between the two fundamental scales of nature i.e. Planck scale and electroweak scale. In addition to this, the geometry of RS serves as a useful setup wherein the fermion mass hierarchy problem can also be solved. The goal of this thesis is to investigate whether RS model can be an acceptable theory of avour while at the same time serving as a solution to the hierarchy problem.
In Chapter[1] we begin with a brief introduction of the SM, highlighting issues which pro- vides the necessary motivation for constructing new physics models. Various candidates of Beyond Standard Model (BSM) physics are introduced and a few preliminaries es- sential to understand frameworks with additional spatial-dimensions ( at and warped) is provided. In Chapter[2] we specialize to the case of warped extra-dimensions and motivate the need to have the SM elds in the bulk. Mathematical details related to the analysis of various spin elds (0; 12; 1 and 2) in a warped background necessary to understand relevant phenomenology is provided.
The lack of knowledge of Dirac or Majorana nature of the neutrino leads to a wide variety of possibilities as far as neutrino mass generation is concerned. In Chapter[3] we focus on the leptonic sector where three cases of neutrino mass generation are consid- ered: a)Planck Scale lepton number violation (LLHH case) b) Dirac neutrinos c) Bulk Majorana mass terms. We then study the implications of each case on the charged lepton mass tting. The case with Planck scale lepton number violation in normal RS scenario requires large and negative values for the bulk mass parameters for the charged singlets cE. Dirac neutrinos and the case with Bulk Majorana mass terms give good t to data. For completeness, the ts for the hadronic sector is provided in the appendix.
In Chapter[4] avour violation for each of three cases introduced in Chapter[3] is studied. For the case with Planck scale lepton number violation, the non-perturbative Yukawa coupling between the SM singlets and the KK states render the higher order diagrams incalculable. Lepton avour violation (LFV) is particularly large for the Dirac case and the bulk Majorana case for low Kaluza-Klein(KK) mass scales. We then invoke the ansatz of Minimal Flavour violation to suppress LFV with low lying KK scales and examples of avour group is provided for both cases.
In Chapter[5] we present an example with a type II two Higgs doublet model applied to the LLHH case. The setup o ers a solution where LLHH scenario can be consistently realized in RS model, where the masses and mixing angles in the leptonic sector can bet with O(1) choice of bulk parameters.
Assumption of global lepton number conservation (like in Dirac neutrinos) could lead to problems in theories of quantum gravity where it does not hold. This leads us to the question whether Dirac neutrinos can be naturally realized in nature. In Chapter[6] we consider the special case of bulk Majorana mass encountered in Chapter[3] where the bulk Dirac mass terms for the right handed neutrino is set to zero. We nd that this leads to a case where the e ective zero mode neutrino mass is of Dirac type with negligible e ects from the tower of Majorana states.
In Chapter[7] we consider RS at the GUT scale which no longer serves as a solution to the hierarchy problem. SUSY is introduced in the bulk and the low energy SUSY serves as a solution to the hierarchy problem. Such models serve as a useful alternative to SUSY models with family symmetries (e.g. Froggatt-Nielsen Model). However the solutions to the Yukawa hierarchy problem are constrained due to anomaly cancellation conditions. In Chapter[8] supersymmetry breaking due to radion mediation in addition to brane localized sources is considered and detailed analysis of the running of soft masses and the low energy avour observables is considered for both cases separately. In Chapter[9] we conclude and present future directions.
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Elektroslabé procesy v rámci efektivní polní teorie / Elektroslabé procesy v rámci efektivní polní teorieSoukup, Petr January 2009 (has links)
Title: Electroweak processes in the framework of effective field theory Author: Petr Soukup Department: Institute of Particle and Nuclear Physics Supervisor: RNDr. Karol Kampf, Ph.D. Supervisor's e-mail address: kampf@troja.mff.cuni.cz Abstract: In this thesis, we study electroweak processes within the framework of ef- fective field theory employing the approach of effective Lagrangians. We mainly focus on the decay process H → γγ. A complete set of SU(2) × U(1) invariant dimension- six operators is utilized. We present a brief introduction to GWS Standard model and dimension-six effective operators. One-loop Standard model contribution to the process of H → γγ is then evaluated, followed by calculation of tree-level and one-loop level dimension-six operators contribution to the same process. We then present a brief ge- neral summary of renormalization procedure in quantum field theory. Renormalization of performed calculations is implemented, and possible issues that may arise during renormalization of such non-renormalizable theory are also discussed. In the end, we discuss the obtained results, mainly the dependence of H → γγ decay rate on effective theory's free parameters and the scale of the new physics Λ. Focus is made on possible deviations from Standard model results. The results are plotted in charts....
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W boson measurement in the muonic decay channel at forward rapidity with ALICE / Mesure de la production du boson W dans le canal muonique à rapidité à l'avant avec ALICEZhu, Jianhui 01 April 2017 (has links)
La haute densité d’énergie atteinte au Large Hadron Collider (LHC) au CERN permet une production abondante de sondes dures, telles que quarkonia, jets à haute impulsion transverse (p<sub>T</sub>) et bosons vecteurs (W, Z), qui sont produits lors de la collision partonique initiale. Les bosons vecteur se désintègrent avant la formation du Plasma de Quark et de Gluons (PQG), une phase déconfinée de la matière, qui peut être produite lors de collisions d’ions lourds ultra-relativistes. Les leptons issus de la désintégration des bosons électrofaibles ne sont pas sensibles à l’interaction forte avec le PQG. Pour ces raisons les bosons électrofaibles fournissent une référence pour l’étude des modifications induites par le milieu sur les sondes colorées.La production de bosons W en collisions pp à √s=8 TeV et en collisions p-Pb à √s<sub>NN</sub>=5.02 TeV est mesurée dans le canal de désintégration muonique au LHC avec le détecteur ALICE. En collision pp, la gamme de rapidité couverte par la mesure est -4<y<sub>cms</sub><-2.5. En collision p-Pb, la différence d’énergie entre le proton et l’ ion plomb donne lieu à un décalage en rapidité. En inversant la direction des faisceaux, il est possible de couvrir les régions de rapidité -4.46<y<sub>cms</sub><-2.96 et 2.03<y<sub>cms</sub><3.53. Les résultats présentés dans cette thèse consistent dans la mesure de la section efficace de la production de muons avec pT>10GeV/c issus de la désintégration des bosons W+ et W-. La mesure de l’asymétrie de charge, définie comme la différence des taux de production des muons positifs et négatifs divisée par leur somme, est également effectuée. Les résultats sont comparés avec des calculs théoriques obtenus avec ou sans tenir compte des modifications des fonctions de distribution partonique dans les noyaux. La production du boson W est aussi étudiée en fonction de la centralité des collisions : nous observons que, dans les erreurs expérimentales, la section efficace des muons issus de la désintégration du boson W est proportionnelle aux nombre de collisions binaires entre les nucléons. / The high collision energies available at the LHC allow for an abundant production of hard probes, such as quarkonia, high-p<sub>T</sub> jets and vector bosons (W, Z), which are produced in initial hard parton scattering processes. The latter decay before the formation of the Quark-Gluon Plasma (QGP), which is a deconfined phase of QCD matter produced in high-energy heavy-ion collisions. Their leptonic decay products do not interact strongly with the QGP. Thus electroweak bosons introduce a way for benchmarking in-medium modifications to coloured probes. The production of W-boson in pp collisions at √s=8 TeV and p-Pb collisions at √s<sub>NN</sub>=5.02 TeV are measured via the muonic decay channel at the LHC with the ALICE detector. In pp collisions the rapidity covered by the measurement is -4<y<sub>cms</sub><-2.5. In p-Pb collisions, on the other hand, the different energies of the proton and lead ion give rise to a rapidity shift. By exchanging the direction of the beams, it is possible to cover the rapidity ranges -4.46<y<sub>cms</sub><-2.96 and 2.03<y<sub>cms</sub><3.53. The production cross section and charge asymmetry of muons from W-boson decays with p<sup>μ</sup>T>10 GeV/c are determined. The results are compared to theoretical calculations both with and without including the nPDFs. The W-boson production is also studied as a function of the collision centrality: the cross section of muons from W-boson decays is found to scale with the average number of binary nucleon-nucleon collisions with uncertainties.
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Probing Electroweak Gauge Boson Scattering with the ATLAS Detector at the Large Hadron ColliderAnger, Philipp 07 October 2014 (has links) (PDF)
Electroweak gauge bosons as central components of the Standard Model of particle physics are well understood theoretically and have been studied with high precision at past and present collider experiments. The electroweak theory predicts the existence of a scattering process of these particles consisting of contributions from triple and quartic bosonic couplings as well as Higgs boson mediated interactions. These contributions are not separable in a gauge invariant way and are only unitarized in the case of a Higgs boson as it is described by the Standard Model. The process is tied to the electroweak symmetry breaking which introduces the longitudinal modes for the massive electroweak gauge bosons. A study of this interaction is also a direct verification of the local gauge symmetry as one of the fundamental axioms of the Standard Model. With the start of the Large Hadron Collider and after collecting proton-proton collision data with an integrated luminosity of 20.3/fb at a center-of-mass energy of 8 TeV with the ATLAS detector, first-ever evidence for this process could be achieved in the context of this work.
A study of leptonically decaying WWjj, same-electric-charge diboson production in association with two jets resulted in an observation of the electroweak WWjj production with same electric charge of the W bosons, inseparably comprising WW->WW electroweak gauge boson scattering contributions, with a significance of 3.6 standard deviations. The measured production cross section is in agreement with the Standard Model prediction.
In the course of a study for leptonically decaying WZ productions, methods for background estimation, the extraction of systematic uncertainties and cross section measurements were developed. They were extended and applied to the WZjj final state whereof the purely electroweakly mediated contribution is intrinsically tied to the scattering of all Standard Model electroweak gauge bosons: Wγ->WZ and WZ->WZ. Three charged leptons and a neutrino from the decay of the final state bosons allow inferences about the scattering process. A distinct signature is provided by the two accompanying tagging jets as remnants of the incoming quarks radiating the initial electroweak gauge bosons. The cross section of the electroweak WZjj production was measured to σ(fiducial, observed) = (0.63 +0.32 -0.28 (stat.) +0.41 -0.24 (syst.)) fb and was found to be consistent with the Standard Model prediction at next-to-leading order in perturbative quantum chromodynamics, σ(fiducial, theory) = (0.31 +0.03 -0.05) fb. Unfolded differential cross sections of kinematic variables sensitive to models of new physics were derived. Anomalous quartic electroweak gauge couplings are introduced as dimensionless coupling parameters of additional operators within an effective field theory approach. Constraints on the parameters of operators with dimension eight were set employing a unitarization prescription based on form factors.
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