No description available.

This thesis presents the measurement and results of the Z/γ* → μ⁺μ⁻ Drell-Yan triple-differential cross-section, using 20.24 fb⁻¹ of ATLAS data recorded in 2012 at a centre-of-mass energy of √s = 8TeV. The triple-differential cross-section is measured as a function of dimuon invariant mass, m_μμ, dimuon rapidity, γ_μμ , and lepton decay angle in the Collins-Soper frame, cosθ*. These dimensions provide sensitivity to the parton composition of the proton through the parton density functions, PDFs, and the weak effective mixing angle, sin²θeff./W, via the forward-backward asymmetry, A_FB. The measurement is performed on and around the Z-boson's invariant mass peak, 46 < m_μμ < 200 GeV, in a kinematic ducial volume of muon transverse momentum, pT > 20 GeV, and muon pseudo-rapidity |η|≤2.4. The results are unfolded from the detector level to the Born, bare and dressed levels, where a precision of < 0:6% is reported in the central bins. The data is combined with an electron channel measurement resulting in a combined result with reduced total uncertainty. The combined result is shown to constrain PDF uncertainties and achieve the most constrained sin²θeff./W uncertainty yet reported at the LHC.

Després de 20 anys de preparació, el LHC s'encendrà a finals del 2009, col.lisionant protons a energies de 14TeV, per recrear els primers moments del Big Bang. Les partícules s'acceleraran al llarg del túnel circular de 27 km de circumferència fins a velocitats properes a la de la llum. El túnel, situat prop de Ginebra, i els seus experiments formen un dels més grans esforços de la història per estudiar l'estructura fonamental de la matèria. S'espera que a aquestes altes energies de col.lisió nous fenòmens físics puguin esdevenir i siguin descoberts. Entre les restes resultants de les col.lisions entre protons, evidències de dimensions extres, de la misteriosa matèria fosca que inunda el nostre univers o de la partícula de Higgs que dona massa a les partícules elementals, podran ser observades. ATLAS és un dels experiments del LHC. A més a més del descobriment de nova Física en el seu programa, també està inclosa la investigació de la Física ja coneguda. Un millor coneixement de la teoria pertorbativa de Quantum Chromodynamics és un dels objectius d'ATLAS. QCD és la teoria que descriu les interaccions fortes entre quarks i gluons. Aquesta teoria encara roman sense resoldre, ja que no exiteix un mètode d'aproximació vàlid per qualsevol escala d'energies. La teoria pertorbativa de QCD pot descriure un gran nombre d'interaccions a altes energies. El seu formalisme ha donat una eina molt valuosa dins l'estudi de les interaccions fortes. La signatura més notable dels processos QCD en col.lisionadors d'hadrons és la producció de jets de partícules col.limats. La mesura d'aquests jets en associació amb un bosó vector, W o Z, proporciona un test estricte dels càlculs de la teoria pertorbativa de QCD. A més, alguns dels processos de la nova Física, com la producció del bosó Higgs o de partícules supersimètriques, poden ser imitats per la producció dels bosons vectros en associació amb jets, que constitueixen un background irreduïble a l'investigació d'aquests nous processos. Aquesta tesi presenta la mesura de la "inclusive jet cross section" en events amb un bosó Z(e,e,) o Z(mu,mu), comparant les prediccions teòriques amb les dades "real", i.e. events Monte Carlo "fully-reconstructed", pel primer 1fb-1 de dades obtingudes amb el detector ATLAS. Les dades reconstruïdes i corregides es comparen amb les prediccions de la teoria pertorbativa de QCD a nivell NLO i LO. Aquestes prediccions pertorbatives es corregeixen amb les contribucions de processos no pertorbatius, com l'underlying event o la fragmentació dels partons dins dels jets d'hadrons. Aquests processos no poden ser descrits per la teoria pertorbativa, i s'estimen amb models fenomenològics. S'han utilitzat dos tipus diferents de dades reconstruides Monte Carlo, ALPGEN i PYHTIA. S'han estudiat les comparacions entre les prediccions d'ambdós generadors. Els processos background han estat estimats utilitzant diferents mètodes "data-driven". Per la recerca dels jets, s'ha utilitzat l'algoritme ATLAS Cone 0.4, després d'identificar la presència d'un bosó Z mitjançant la reconstrucció del seu decaiment (electrons o muons). Les dades reconstruides han estat corregides pels efectes del detector, utilitzant factors independents. Aquest anàlisi ha estat realitzat abans que ATLAS recollís les primeres dades reals, abans de l'inici del LHC, per aquest motiu aquesta tesi es basa en dades simulades amb generadors Monte Carlo. Mentres dura la preparació d'un experiment de Física d'altes energies com ATLAS, aquestes simulacions són molt importants pel desenvolupament d'estratègies eficaces d'anàlisi de dades i reconstrucció d'objectes físics mesurats pel detector. / Después de 20 años de preparación, el LHC va a encenderse a finales del 2009, colisionando protones a una energia de 14 TeV, para recrear los primeros momentos después del Big Bang. Las partículas serán aceleradas a lo largo del tunel circular de 27 km de circumferencia hasta velocidades próximas a la de la luz. El túnel, situado cerca de Ginebra, y sus experimentos forman uno de los más grandes esfuerzos de la história para estudiar la estructura fundamental de la materia. Se espera que a estas elevadas energías de colisión, nuevos fenómenos físicos puedan ocurrir y sean descubiertos. Entre los resultantes escombros de las colisiones entre protones, evidencias de dimensiones extras, de la misteriosa materia oscura que inunda nuestro universo o de la partícula de Higgs que da masa a las partículas elementales serán observadas. ATLAS es uno de los experimentos del LHC. Además del descubrimiento de nueva Fisica en su programa, también está la investigación de la Fisica ya descubierta. Un mejor conocimiento de la teoría perturbativa de Quantum Chromodynamics es uno de los objetivos de ATLAS. Quantum Chromodynamics es la teoria de campos que describe las interacciones fuertes entre quarks y gluones. Esta teoría todavia permanece sin resolver, ya que no existe un método de aproximación válido para cualquier escalada de energía. La teoría perturbativa de QCD puede describir un gran numero de interacciones a altas energías. Su formulismo ha dado una herramienta muy valiosa en el estudio de las interacciones fuertes. La signatura más notable de procesos QCD en collisionadores de hadrones es la producción de jets colimados de hadrones. La medición de estos jets en asociación con un bosón vector, W o Z, proporciona un test severo de los cálculos de la teoría perturbativa de QCD. Además, algunos de los procesos de nueva Física en los colisionadores de hadrones, como la production del boson de Higgs o de partículas supersimétricas, pueden ser imitados por la producción de bosones vectores en asociación con jets, que constituyen un background irreducible a la investigación de estos nuevos procesos. Esta tesis presenta la medida de la "inclusive jet cross section" en sucesos con un boson Z (ee) o Z(mu,mu), comparando las prediciones teoricas con los datos "reales", i.e. sucesos Monte Carlo "fully-reconstructed", para el primer 1fb-1 de datos obtenidos con el detector ATLAS. Los datos reconstruidos y corregidos son comparados a las predicciones de la teoria perturbativa de QCD a nivel NLO y LO. Estas predicciones perturbativas se corrigen con las contribuciones de procesos no perturbativos, como el underlying event o la fragmentación de los partones en jets de hadrones. Estos procesos no pueden ser descritos por la teoria perturbativa, y son estimados con modelos fenomenologicos. Se han ultilizado dos tipos distintos de datos reconstruidos Monte Carlo, ALPGEN y PYTHIA. Comparaciones entre las predicciones de ambos generadores Monte Carlo han sido estudiadas. Los procesos background han sido estimados usando distintos métodos "data-driven". Para la búsqueda de jets se ha usado el algoritmo ATLAS Cone 0.4 jet, después de identificar la presencia de un bosón Z a través de la reconstrucción de su decaimiento (electrones o muones). Los datos reconstruidos son corregidos por los efectors del detector, usando factores independientes. Este analisis ha sido realizado antes que ATLAS recogiera los primeros datos reales, antes del inicio del LHC, por este motivo esta tesis esta basada en datos simulados con generadores Monte Carlo. Mientras dura la preparación de un experimento de fisica de altas energías como ATLAS, estas simulaciones son de vital importancia para el desarrollo de estrategias eficaces de analisis de datos y reconstrucción de objetos físicos medidos por el detector. / After 20 years of preparation, the Large Hadron Collider is going to be switched on in late 2009, smashing protons at an energy of 14 (10) TeV, to recreate the first moments after the Big Bang. Particles will whizz around a circular tunnel of 27 km in circumference at near light speed. The tunnel, built near Geneva, and its experiments constitute one of the largest coordinated efforts ever made to study the fundamental structure of nature. It is expected that at the energies reached in the proton-proton collisions at the LHC, unknown physical phenomena will have to occur and will be observable. Among the particle debris may lie evidence for extra dimensions, mysterious dark matter that pervades the universe or the Higgs boson, which gives mass to elementary particles. ATLAS is one of the LHC experiments. Besides the new phenomena goals in its physics program, there is the understanding of the already known physics. The better understanding of perturbative Quantum Chromodynamics theory is one of the aims of ATLAS. Quantum Chromodynamics is the field theory which describes the strong interaction between quarks and gluons. It remains an "unsolved" theory, since no single approximation method can be used to all length scales. Perturbative QCD naturally describes a large set of high-energy, large-momentum-transfer cross sections and its formalism has provided an invaluable tool in the study of the strong interactions. The most prominent signature of QCD at hadron colliders is the production of collimated jets of hadrons. The measurement of the production of such jets in association with a vector boson, W or Z, provides a stringent test of perturbative QCD (pQCD) calculations. Furthermore, some of new physics processes at hadron colliders, such as the production of Higgs bosons and supersymmetric particles, can be mimicked by the production of vector bosons in association with jets that constitute irreducible backgrounds to these searches. This PhD. thesis presents the measurement of the inclusive jet cross section in Z &#8594; e+e&#8722; and in Z &#8594; &#956;+&#956;&#8722; events, comparing theory predictions with "real data", i.e. Monte Carlo fully-reconstructed events, for the first 1fb&#8722;1 of data at the ATLAS detector. Reconstructed corrected data is compared to next-to-LO (NLO) and LO pQCD predictions. Perturbative predictions are corrected for the contributions of the non-perturbative processes, like the underlying event and the fragmentation of the partons into jets of hadrons. These processes are not described by perturbation theory and must be estimated using phenomenological models. Two different reconstructed data are used, PYTHIA and ALPGEN Monte Carlo data. Comparisons of both Monte Carlo predictions are studied. Background processes are estimated proposing different data-driven methods to be applied to real data. ATLAS Cone 0.4 algorithm is used to look for jets in the events after identifying the presence of a Z boson through the reconstruction of its decay (electrons or muons). Reconstructed data is corrected for detector effects, using independent factors. As this work was carried out before the "physics-data" start of the LHC, the presented studies are based on Monte Carlo simulations. During the preparation of a high-energy collider experiment, such simulations are important to develop efficient strategies for data analysis and for the reconstruction of the physics objects observed with the detectors.

Este trabalho apresenta os resultados da primeira medida da secao de choque de producao W± e± utilizando dados de colisoes proton-proton com energia de centro de massa s = 13 TeV pelo detector ATLAS em dois regimes diferentes de operacao do Large Hadron Collider (LHC): um perodo de baixa luminosidade em que foram acumulados 81 pb1 de dados e que permitiram medir a secao de choque fiducial e total da producao de bosons W+ e W na regiao inclusiva (regiao de massa transversa acima de 50 GeV, abrangendo o pico de massa transversa do boson W) e um perodo de alta luminosidade em que foi obtido 76,6 fb1 de dados tornando acessvel a medida da secao de choque fiducial de producao de bosons W+ e W na regiao de alta massa transversa (regiao de massa transversa entre 130 GeV e 2,5 TeV). As secoes de choque medidas e as distribuicoes cinematicas referentes a energia transversa faltante (ETmiss ), massa transversa do boson W (mTW), pseudorapidez (), momento transverso (pTe) do eletron decorrente do decaimento do W e a diferenca entre o angulo azimutal () do eletron e da ETmiss sao consistentes com previsoes teoricas obtidas em NLO e normalizadas em NNLO, considerando-se as incertezas sistematicas, estatsticas e na luminosidade. / This work presents the results of the first W± e± production cross section measurements in the proton-proton collisions at the center of mass energy of s = 13 TeV obtained with the ATLAS detector in two different operation regimes: a low luminosity period when 81pb1 of data was accumulated allowing the measurement of the fiducial and total production cross section of W+ and W bosons in the inclusive region (region of transverse mass above 50 GeV, covering the transverse mass peak of the W boson) and a high luminosity period when 76,6 fb1 made accessible the high W boson transverse mass region (between 130 GeV and 2.5 TeV), allowing the measurement of the fiducial cross section of W+ and W boson production. The measured cross sections and the kinematic distributions for the missing transverse energy, (ETmiss), W boson transverse mass (mTW), the pseudorapidity (), the transverse momentum (pTe) of the electron originated from the W boson decay as well as the azimuthal angle difference () between the electron and the ETmiss are consistent with the theoretical predictions obtained at NLO and normalized to NNLO, considering the systematic, statistical and luminosity uncertainties.

Data collected at the LHC are analyzed by the ATLAS collaboration for evidence of dark matter. In this thesis, a fast simulation of the ATLAS detector response using the Delphes software is assessed for dark matter models with a leptonically decaying Z boson and a pair of dark matter particles in the final state. Limits for the Two Higgs Doublet plus pseudoscalar dark matter model are obtained using simplified systematics, and found to be nearly indistinguishable to limits obtained using the more complex standard ATLAS analysis. / Graduate

The performance of the ATLAS missing transverse momentum (ETmiss) high-level trigger during 2015 operation is presented. In 2015, the Large Hadron Collider operated at a higher centre-of-mass energy and shorter bunch spacing (sqrt(s) = 13 TeV and 25 ns, respectively) than in previous operation. In future operation, the Large Hadron Collider will operate at even higher instantaneous luminosity (O(10^34 cm^−2 s^−1)) and produce a higher average number of interactions per bunch crossing, <mu>. These operating conditions will pose significant challenges to the ETmiss trigger efficiency and rate. An overview of the new algorithms implemented to address these challenges, and of the existing algorithms is given. An integrated luminosity of 1.4 fb^−1 with <mu> = 14 was collected from pp collisions of the Large Hadron Collider by the ATLAS detector during October and November 2015 and was used to study the efficiency, correlation with offline reconstruction, and rates of the trigger algorithms. The performance was found to be satisfactory. From these studies, recommendations for future operating specifications of the trigger were made. / Graduate / 0798, / jchiu@uvic.ca

No description available.

The impressive progress that elementary particle physics made in the second half of the last century led to the formulation of a comprehensive theory, known as the Standard Model (SM), which correctly describes all fundamental interactions in nature, except for the gravitational one. Indirect discoveries have always played an important role in high energy physics scenario and indirect research can be considered to all intents and purposes complementary to the direct one, since allows to test much higher energy scales than those the current colliders are able to reach. This is very important now that electroweak precision tests and measurements on Flavour Changing Neutral Currents (FCNC) processes put very stringent constraints on physics beyond the SM, requiring it to appear at scales O(10 TeV). On the other hand, New Physics (NP) is expected already at scales O(1 TeV) in order to offer a natural explanation to the smallness of the Higgs mass. This scale is also confirmed by recent constraints on thermal dark matter [1] which show how new physics should manifest not far above the electroweak scale. Rare B decays have always played a crucial role in shaping the flavour structure of the SM and particle physics in general. Since the first measurement of rare radiative B æ Kú“ decays by the CLEO Collaboration [2] this area of particle physics has received much experimental and theoretical attention. In particular, FCNC B decays, involving the b-quark transition b æ (s, d) + “ and b æ (s, d) + ¸+¸≠(¸ = e, μ, ·, ‹), provided crucial tests for the SM at the quantum level since they proceed through loop or box diagrams, and they are highly suppressed in the SM (also by helicity). Hence, these rare B decays are characterised by their high sensitivity to NP. The B0 s æ μ+μ≠ channel is the most direct example of the b æ s ¸¸ transitions. The SM predicted branching ratio [3] can be enhanced by coupling to non-SM heavy particles, such as those predicted by the Minimal Supersymmetric Standard Model (MSSM) and other extensions. Updated measurements on the B0 s æ μ+μ≠ branching ratio have been presented by ATLAS [4], LHCb [5] and CMS [6] collaborations. In this thesis I will report all the studies I performed within the rare B decays ATLAS group, measuring the branching ratio of the B0 s æ μ+μ≠ channel on data collected during LHC Run 1. The first chapter provides a general introduction to the SM, focusing in particular on the flavour sector and the possible new physics scenarios. Chapter 2 briefly introduces the LHC collider and the ATLAS detector, detailing the muon and trigger systems, fundamental for the rare B decays measurements. In chapters 3 and 4, I will summarise the work done, during my presence at CERN, on the ATLAS semiconductor strip detector, monitoring the Lorentz angle during Run 1 and measuring the backplane resistance of the silicon modules installed in the ATLAS cavern. In chapter 5, I will review the strategy adopted to measure the B0 s æ μ+μ≠ branching ratio, reporting all the studies I performed on the combinatorial background, and the results obtained on 4.9 fb≠1 of data collected in 2011. Chapters 6 and 7 detail respectively the additional studies I performed on the 2011 datasets and all the tests I made in preparation for the analysis on 20 fb≠1 of data collected in 2012. I will show the studies on the discriminating variables for the rejection of the background, the tests on the multivariate analysis and on the possible strategies for the invariant mass fit for the extraction of the signal yield. All these studies proved to be fundamental for the 2012 measurement detailed in chapter 8.

Title: Study of jet fragmentation and inclusive jet production in heavy-ion colli- sions with the ATLAS experiment Author: Mgr. Radim Slovák Institute: Institute of Particle and Nuclear Physics Supervisor: Mgr. Martin Spousta, Ph.D., Institute of Particle and Nuclear Physics, Charles University Abstract: This thesis presents two measurements - measurement of the jet frag- mentation functions and inclusive jet production in heavy-ion collisions. The frag- mentation functions are measured in Pb+Pb and pp collisions with the ATLAS detector at the LHC. The distributions are measured as a function of jet trans- verse momentum and rapidity. The analysis utilises an integrated luminosity of 0.14 nb−1 of Pb+Pb data and 4.0 pb−1 of pp data collected in 2011 and 2013, respectively, at the same centre-of-mass energy of 2.76 TeV. Modest but signif- icant centrality-dependent modifications of fragmentation functions in Pb+Pb collisions with respect to those in pp collisions are seen. Measurements of the yield and nuclear modification factor, RAA, for inclusive jets are performed us- ing 25 pb−1 of pp data at √ s = 5.02 TeV and 0.49 nb−1 of Pb+Pb data at √ sNN = 5.02 TeV. The jets are measured over the transverse momentum range of 100-1000 GeV in six rapidity intervals covering |y| < 2.8. The magnitude of the RAA...

The Standard Model of particle physics is currently the most complete theory of subatomic particles. The discovery of the Higgs boson with a mass of 125 GeV in 2012 further validated the Standard Model, providing evidence for the theory that vector bosons obtain non-zero masses through the Higgs mechanism. Studies are ongoing to determine the exact nature and properties of the Higgs boson. A Higgs boson of this mass is predicted to decay to a pair of b-b quarks with a branching ratio of 58%, however this decay mode has not yet been observed. This thesis presents a search for the associated production of a Higgs boson with a leptonically decaying W boson, WH → ℓvb-b, using 20.3 fb-1 of Run 1 data collected by ATLAS at the LHC from pp collisions at a centre-of-mass energy of ps = 8 TeV. The observed (expected) significance of a Higgs boson with a mass of 125 GeV for the WH → ℓvb-b process is found to be 2:7σ (1:3σ). The measured cross section in units of the expected Standard Model cross section has a best-fit value of μ = μ/μSM = 2:2+0:67-0:64(stat:)+0:7-0:59(syst:) = 2:2+0:97-0:87. The results are combined with the search for ZH → v-vb-b and ZH → ℓ+ℓ-b-b to provide a best-fit value of μ = μ/μSM = 1:1+0:61-0:56. The start of Run 2 of the LHC in 2015 saw the collision energy being raised to √s = 13 TeV, increasing the probability of particles being produced with a large momentum boost. At these high energies there is also a possibility to discover new particles and interactions. An extension of the Standard Model, the Heavy Vector Triplet (HVT) model, describes new heavy vector bosons W¹ and Z¹, which can decay to pairs of heavy bosons (W, Z or Higgs bosons). If the W0 and Z0 bosons are sufficiently heavy, the hadronic decays of the diboson final states produce boosted jets. In this thesis, methods for identifying hadronically decaying boosted bosons are developed, based on techniques that examine the internal substructure of the jet. Multiple substructure variables are combined into a single discriminant using two machine learning techniques: boosted decision trees and deep neural networks. Simulated events of W¹→WZ → q-qq-q are used to develop these boosted W boson taggers. An improvement in the background rejection power, whilst keeping 50% of the signal, over previous boosted W boson taggers of up to 13%-when using deep neural networks-and 36%-when using boosted decision trees-is obtained. The performance of the new boosted W boson taggers are evaluated in a search for a narrow WW resonances from the decay of a Z¹ with boson-tagged jets in 3.2 fb-1 of pp collisions at √s = 13 TeV collected with the ATLAS detector.