Measurement of the top-quark pair production cross-section involving a tau & muon studies with tilecal in atlas

La presente tesis se ha desarrollado en el marco del experimento ATLAS, un detector de propósito general y que al igual que los detectores CMS, ALICE y LHCb, se halla ubicado en el Gran Colisionador de Hadrones (LHC), en el Centro Europeo para la Investigación Nuclear (CERN). El trabajo se compone de varias partes diferenciadas y comienza con una descripción básica de la física que se estudia en el LHC centrada en el quark top, cuya producción de pares es uno de los procesos dominantes en las interacciones protón-protón, después de los jets de QCD y la producción de los bosones W y Z. También se describe la partícula tau, uno de los principales ingredientes del análisis de física presentado. A continuación se describen las partes fundamentales del acelerador LHC y del detector ATLAS. Lo siguiente se dedica al sub-detector TileCal, centrándose en la electrónica y el proceso de validación de datos utilizado durante la fase de puesta en marcha del detector. También se describe el algoritmo TileMuID, implementado en el nivel-2 del Trigger de Muones de ATLAS y basado en TileCal. Estos estudios contribuyeron a la puesta en marcha de TileCal utilizando datos de muones cósmicos durante el periodo de preparación para colisiones, permitiendo confirmar el correcto funcionamiento del detector. La parte principal de este trabajo corresponde a análisis de física, con investigaciones relacionadas con la física del quark top. Comienza con la selección de objetos comunes a todo el análisis y la descripción de las muestras utilizadas. Continua con estudios sobre la identificación de la partícula tau, en donde se calcula la probabilidad de que un tau sea erróneamente identificado por jets, para lo que se usan muestras de γ +jets, que tienen una gran cantidad de jets provenientes de light-quark. El objetivo de este trabajo es elegir el mejor algoritmo de identificación para la medida de la sección eficaz. A esto le sigue el cálculo de los factores de escala del fondo Z → ll, para conseguir un mayor acuerdo entre muestras de datos y Monte Carlo (MC). Estos números son necesarios para el medida de la sección eficaz. A continuación se presenta la descripción de la selección de eventos y estudio de la tabla de cortes correspondientes a esa selección, con el fin de estudiar el fondo MC y poder maximizar el número de eventos de señal frente al fondo. Por último se presenta la medida de la sección eficaz del par top en el canal leptón +tau, aplicando el método ‘fit’ a muestras de 4,73 fb-1. Este método usa la técnica TauID, para separar procesos con leptones tau (señal) de aquellos con jets que pueden ser confundidos con taus (falsos taus), mediante un ajuste χ2 a muestras de señal y fondo, que permiten obtener el números de eventos en muestras y poder calcular la sección eficaz. También se realiza un estudio de las incertidumbres sistemáticas para poder medir con más precisión la sección eficaz en el canal leptón + tau, obteniendo un resultado acorde con el Modelo Estándar y con las medidas previas en el mismo canal.This thesis has been development in the framework of the ATLAS experiment, one of the main general-purpose detectors of the Large Hadron Collider (LHC) located at CERN. This work has to differentiated parts, starting with the basic description of the physic that involve LHC studies, centred in the top quark. The pair production of the top quark is one of the dominant processes in the proton-proton interactions, after QCD jets and the production of W and Z bosons. Also, the tau particle is described, one of the main ingredients of the physic analysis. After this, the fundamental parts of the LHC accelerator and the ATLAS detector are showed. The next is dedicated to TileCal sub-detector, describing mainly the electronic and the data quality assessment performed during commissioning. This is completed with the description of the TileMUID algorithm, based on TileCal and implemented in the second muon level trigger of ATLAS. These studies contributed to the commissioning of TileCal using cosmic muons taken during the previous period of collisions, showing the correct operation of the detector. The main part of this work is the physic analysis, with research concerned to the top quark physic. It starts with the selection of common objects for the whole analysis and the description of the samples used. This is followed by tau identification studies, with the calculation of tau-misidentification probabilities using γ +jets samples. The purpose of this study is to choice the best tau identification algorithm for the measurement of the cross-section. In the next, the calculation of the scale factors of Z background is performed, in order to obtain a better agreement between data and Monte Carlo (MC). These numbers are necessaries for the measurement of the cross-section. The description of the event selections and the cutflow study is also shown, with the goal of study the background and maximize the number of events in the signal against the background. Finally, it is presented the measurement of the top quark pair production cross-section in the lepton + tau channel, applying the fitting method to 4,73 fb-1 samples. This method use the TauID technique to separate process from tau leptons (signal) from those with jets that can be misidentified as taus (fake taus), applying a χ2 fit to signal and background samples, allowing to obtain the number of events from the samples and calculate the cross-section. The study of the main systematic uncertainties is also presented, in order to measure with more precision the cross-section in the lepton + tau channel. The result of the measurement agrees with the Standard Model predictions and the previous measurements in the same channel

Development of a detector (ALFA) to measure the absolute LHC luminosity at ATLAS

The ATLAS collaboration plans to determine the absolute luminosity of the CERN LHC at Interaction Point 1 by measuring the trajectory of protons elastically scattered at very small angles ($\mu rad$). A scintillating fibre tracker system called ALFA (Absolute Luminosity For ATLAS) is proposed for this measurement. Detector modules will be placed above and below the LHC beam axis in roman pot units at a distance of 240 m on each side of the ATLAS interaction point. They allow the detectors to approach the beam axis to millimeter distance. Overlap detectors also based on the scintillating fibre technology, will measure the precise relative position of the two detector modules. Results obtained during beam tests at DESY and at CERN validate the detectors design and demonstrate the achievable resolution. We also report about radiation hardness studies of the scintillating fibres to estimate the lifetime of the ALFA system at different operating conditions of the LHC

Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC

The uncertainty on the calorimeter energy response to jets of particles is derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the calorimeter response to single isolated charged hadrons is measured and compared to the Monte Carlo simulation using proton-proton collisions at centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009 and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter response to specific types of particles (positively and negatively charged pions, protons, and anti-protons) is measured and compared to the Monte Carlo predictions. Finally, the jet energy scale uncertainty is determined by propagating the response uncertainty for single charged and neutral particles to jets. The response uncertainty is 2-5% for central isolated hadrons and 1-3% for the final calorimeter jet energy scale.Comment: 24 pages plus author list (36 pages total), 23 figures, 1 table, submitted to European Physical Journal

Measurement of χ c1 and χ c2 production with s√ = 7 TeV pp collisions at ATLAS

The prompt and non-prompt production cross-sections for the χ c1 and χ c2 charmonium states are measured in pp collisions at s√ = 7 TeV with the ATLAS detector at the LHC using 4.5 fb−1 of integrated luminosity. The χ c states are reconstructed through the radiative decay χ c → J/ψγ (with J/ψ → μ + μ −) where photons are reconstructed from γ → e + e − conversions. The production rate of the χ c2 state relative to the χ c1 state is measured for prompt and non-prompt χ c as a function of J/ψ transverse momentum. The prompt χ c cross-sections are combined with existing measurements of prompt J/ψ production to derive the fraction of prompt J/ψ produced in feed-down from χ c decays. The fractions of χ c1 and χ c2 produced in b-hadron decays are also measured

Search for squarks and gluinos in events with isolated leptons, jets and missing transverse momentum at s√=8 TeV with the ATLAS detector

The results of a search for supersymmetry in final states containing at least one isolated lepton (electron or muon), jets and large missing transverse momentum with the ATLAS detector at the Large Hadron Collider are reported. The search is based on proton-proton collision data at a centre-of-mass energy s√=8 TeV collected in 2012, corresponding to an integrated luminosity of 20 fb−1. No significant excess above the Standard Model expectation is observed. Limits are set on supersymmetric particle masses for various supersymmetric models. Depending on the model, the search excludes gluino masses up to 1.32 TeV and squark masses up to 840 GeV. Limits are also set on the parameters of a minimal universal extra dimension model, excluding a compactification radius of 1/R c = 950 GeV for a cut-off scale times radius (ΛR c) of approximately 30

Standalone vertex finding in the ATLAS muon spectrometer

A dedicated reconstruction algorithm to find decay vertices in the ATLAS muon spectrometer is presented. The algorithm searches the region just upstream of or inside the muon spectrometer volume for multi-particle vertices that originate from the decay of particles with long decay paths. The performance of the algorithm is evaluated using both a sample of simulated Higgs boson events, in which the Higgs boson decays to long-lived neutral particles that in turn decay to bbar b final states, and pp collision data at √s = 7 TeV collected with the ATLAS detector at the LHC during 2011

Measurements of Higgs boson production and couplings in diboson final states with the ATLAS detector at the LHC

Measurements are presented of production properties and couplings of the recently discovered Higgs boson using the decays into boson pairs, H →γ γ, H → Z Z∗ →4l and H →W W∗ →lνlν. The results are based on the complete pp collision data sample recorded by the ATLAS experiment at the CERN Large Hadron Collider at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV, corresponding to an integrated luminosity of about 25 fb−1. Evidence for Higgs boson production through vector-boson fusion is reported. Results of combined fits probing Higgs boson couplings to fermions and bosons, as well as anomalous contributions to loop-induced production and decay modes, are presented. All measurements are consistent with expectations for the Standard Model Higgs boson

Measurements of fiducial and differential cross sections for Higgs boson production in the diphoton decay channel at s√=8 TeV with ATLAS

Measurements of fiducial and differential cross sections are presented for Higgs boson production in proton-proton collisions at a centre-of-mass energy of s√=8 TeV. The analysis is performed in the H → γγ decay channel using 20.3 fb−1 of data recorded by the ATLAS experiment at the CERN Large Hadron Collider. The signal is extracted using a fit to the diphoton invariant mass spectrum assuming that the width of the resonance is much smaller than the experimental resolution. The signal yields are corrected for the effects of detector inefficiency and resolution. The pp → H → γγ fiducial cross section is measured to be 43.2 ±9.4(stat.) − 2.9 + 3.2 (syst.) ±1.2(lumi)fb for a Higgs boson of mass 125.4GeV decaying to two isolated photons that have transverse momentum greater than 35% and 25% of the diphoton invariant mass and each with absolute pseudorapidity less than 2.37. Four additional fiducial cross sections and two cross-section limits are presented in phase space regions that test the theoretical modelling of different Higgs boson production mechanisms, or are sensitive to physics beyond the Standard Model. Differential cross sections are also presented, as a function of variables related to the diphoton kinematics and the jet activity produced in the Higgs boson events. The observed spectra are statistically limited but broadly in line with the theoretical expectations

Evidence for the Higgs-boson Yukawa coupling to tau leptons with the ATLAS detector

Results of a search for H → τ τ decays are presented, based on the full set of proton-proton collision data recorded by the ATLAS experiment at the LHC during 2011 and 2012. The data correspond to integrated luminosities of 4.5 fb−1 and 20.3 fb−1 at centre-of-mass energies of √s = 7 TeV and √s = 8 TeV respectively. All combinations of leptonic (τ → `νν¯ with ` = e, µ) and hadronic (τ → hadrons ν) tau decays are considered. An excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.5 (3.4) standard deviations. This excess provides evidence for the direct coupling of the recently discovered Higgs boson to fermions. The measured signal strength, normalised to the Standard Model expectation, of µ = 1.43 +0.43 −0.37 is consistent with the predicted Yukawa coupling strength in the Standard Model

Measurement of the production of a W boson in association with a charm quark in pp collisions at √s = 7 TeV with the ATLAS detector

The production of a W boson in association with a single charm quark is studied using 4.6 fb−1 of pp collision data at s√ = 7 TeV collected with the ATLAS detector at the Large Hadron Collider. In events in which a W boson decays to an electron or muon, the charm quark is tagged either by its semileptonic decay to a muon or by the presence of a charmed meson. The integrated and differential cross sections as a function of the pseudorapidity of the lepton from the W-boson decay are measured. Results are compared to the predictions of next-to-leading-order QCD calculations obtained from various parton distribution function parameterisations. The ratio of the strange-to-down sea-quark distributions is determined to be 0.96+0.26−0.30 at Q 2 = 1.9 GeV2, which supports the hypothesis of an SU(3)-symmetric composition of the light-quark sea. Additionally, the cross-section ratio σ(W + +c¯¯)/σ(W − + c) is compared to the predictions obtained using parton distribution function parameterisations with different assumptions about the s−s¯¯¯ quark asymmetry