In this thesis, we review the basics of phenomenology in particle physics at neutrino beam dump experiments and hadron colliders. We then consider the phenomenology of various new particles, with masses between 1 MeV and 1 TeV, at the intensity and energy frontiers. We perform sensitivity analyses for physics beyond the standard model at particle colliders (LEP and LHC) and a number of past and future neutrino beam dump experiments (SHiP, DUNE, LSND, MicroBooNE, MiniBooNE and SBND). In particular, we motivate searches for new heavy neutral leptons in single photon events at neutrino and collider experiments (and also via supernova cooling), millicharged particles in single electron events at neutrino experiments, lepton flavor violating scalars via standard model induced mixed flavor neutrino trident production at neutrino experiments, and colored scalar doublets at colliders in events with many jets, soft leptons and low missing energy. In the process, we set novel new bounds on the parameters of these theories and propose powerful new searches that can be performed. We also motivate the construction of a new detector at the LHC called milliQan, and perform a full Geant4 simulation to calculate its projected sensitivity for millicharged particles. The milliQan experiment has since been approved and is currently undergoing construction. / Thesis / Doctor of Philosophy (PhD) / In this thesis, we review the basics of particle physics at neutrino experiments and particle colliders. We then motivate and develop key searches that can be performed to look for new particles at a series of existing and future experiments. We focus on new particles with masses between the electron mass and 1000 times the proton mass. The many searches we consider involve looking for processes that produce a single ray of light, a single electron, a pair of oppositely charged "electron-like" events, new collider signatures, and/or modifications of star explosions. In the process, we set novel new bounds on many theories. We also motivate the construction of a new detector at the Large Hadron Collider called milliQan, and perform a full simulation to assess its future performance. The milliQan experiment has since been approved and is currently undergoing construction.

The Standard Model of particle physics is a successful theory, yet it is incomplete. Supersymmetry is one of the favoured extensions of the Standard Model, elegantly addressing several unresolved issues. This thesis presents a search for the pair production of supersymmetric particles chargino one and neutralino two, where the neutralino two decays to the lightest neutralino and the 125 GeV Higgs boson. The final states considered for the search have large missing transverse momentum, an isolated lepton and two jets identified as originating from bottom quarks. The analysis is based on 20.3 inverse femtobarns of 8 TeV proton-proton collision data delivered by the Large Hadron Collider and recorded with the ATLAS detector. No excess over Standard Model predictions is observed. The analysis has been combined with three independent searches that probe other decay modes of the Standard Model Higgs boson. Limits are set at 95% confidence level in the context of a simplified supersymmetric model. Common masses of chargino one and neutralino two are excluded up to 250 GeV for a massless neutralino one. The analysis of this dissertation has been reinterpreted in the context of a large scan of the phenomenological Minimal Supersymmetric Standard Model, along with 22 other ATLAS Run 1 searches. The resulting summary paper represents the most comprehensive assessment of the ATLAS constraints on Supersymmetry models to date. / Graduate / 0798 / cdavid@uvic.ca

Coloured final states are ubiquitous at hadron colliders such as the Large Hadron Collider (LHC). Therefore understanding high energy perturbative quantum chromodynamics (QCD) at these experiments is essential not only as a test of the Standard Model, but also because these processes form the dominant background to many searches for new physics. One such `standard candle' is the production of a dilepton pair in association with dijets. Here we present a new description of this final state (through the production of a Z⁰ boson and γ*). This calculation adds to the fixed-order accuracy the dominant logarithms in the limit of large partonic centre-of-mass energy to all orders in the strong coupling αs. This is achieved within the framework of High Energy Jets. This calculation is made possible by extending the high energy treatment to take into account the multiple t-channel exchanges arising from Z⁰ and gamma* -emissions off several quark lines. The correct description of the interference effects from the various t-channel exchanges requires an extension of the subtraction terms in the all-order calculation. We describe this construction and compare the resulting predictions to a number of recent analyses of LHC data. The description of a wide range of observables is good, and, as expected, stands out from other approaches in particular in the regions of large dijet invariant mass and large dijet rapidity spans. In addition we also present the application of the High Energy Jets framework to two new experimental scenarios. Firstly, we show a comparison of High Energy Jets matched to the ARIADNE parton shower to an ATLAS study of gap activity in dijet events. We see that our description agrees well with the data throughout and in many distributions gives the best theoretical description. This shows the extra logarithmic corrections are essential to describe data already in LHC Run I. Secondly, we present a study of Z⁰/γ* plus dijets at 100 TeV. We compare the behaviour of the high energy logarithmic enhancements to the QCD perturbative series at 7 TeV and 100 Tev and see that at any high energy hadronic Future Circular Collider (FCC) the effects described by our resummation become significantly more important.

Orientador: Pedro Galli Mercadante / Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, 2012

Ce travail présente la mesure de la section efficace différentielle du boson Z en impulsion transverse (ptz), dans le canal de désintégration electron-positron, avec le détecteur ATLAS au LHC. La mesure utilise 4.64 inverse fb de données de collisions proton-proton, prises en 2011 avec une énergie du centre de masse de 7 TeV. Le résultat est combiné avec une mesure indépendante faite dans le canal muon-antimuon. La mesure est faite jusqu'à ptz = 800 GeV, et a une incertitude typique de 0.5 % pour ptz < 60 GeV, atteignant jusqu'à 5 % vers la fin du spectre. La mesure est comparée avec modèles théoriques et prédictions des générateurs Monte Carlo.

Orientador: Jun Takahashi / Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Física Gleb Wataghin / Made available in DSpace on 2018-08-19T21:22:39Z (GMT). No. of bitstreams: 1 Chinellato_DavidDobrigkeit_D.pdf: 12088648 bytes, checksum: c79560f4593e813870fc0e788aadf075 (MD5) Previous issue date: 2012 / Resumo: Neste trabalho, estudamos a produção de hádrons estranhos em colisões próton-próton (pp) a energias de (s)^(1/2)= 7 TeV medidas pelo experimento ALICE no acelerador LHC. São resentadas medidas inéditas das taxas de produção em rapidez central do méson (K) também dos bárions ?,? e O- e suas antipartículas correspondentes. As taxas totais de produção excedem significativamente as previsões de modelos que utilizam a Cromodinâmica Quântica Perturbativa (pQCD). Em particular, comparamos as medidas com previsões do simulador de eventos PYTHIA, onde é observado que as previsões concordam com as medidas apenas para momentos transversais (Pt) acima de 6- 7 GcV / c. Este resultado indica que os processos de primeira ordem não-nula implementados no PYTHIA estão bem ajustados, mas a implementação de um cálculo que descreva os dados em baixo Pt ainda é um problema em aberto. Os resultados aqui apresentados devem contribuir para melhorar o entendimento dos mecanismos de produção de partículas na região de baixo Pt¿ Colisões pp são também utilizadas como referência para colisões nucleares de diferentes multiplicidades no estudo da formação do Quark-Gluon Plasma (QGP). Em particular, o aumento da produção de estranheza é considerado um importante observável do QGP. Neste contexto, estudamos a dependência da produção de partículas estranhas com a multiplicidade de partículas carregadas e comprovamos que não há indício de aumento da produção relativa de estranheza cm eventos pp. Isto é um importante resultado, pois complementa os estudos que consideram a possibilidade de formação de QGP em colisões pp / Abstract: In this work, we study the production of strange hadrons in proton-proton (pp) collisions at energies of (s)^(1/2)= 7 TeV measured by the ALICE experiment at the LHC. We present the first measurements of particle yields at central rapidities for the (K)meson as well as for the ?, ? e O- baryon and their antiparticles. Total particle production rates exceed predictions by models that use Perturbative Quantum Chromodynamics (pQCD). In particular, we compare our measurements to predictions by the PYTHIA event generator and find that predictions agree with data only at transverse momenta (Pt) higher than 6- 7 GeV / c. This result indicates that the leading order processes implemented in PYTHIA are well adjusted, but the implementation of a calculation that describes the data at low Pt is still an open issue. The results presented here should contribute to improve our understanding of particle production mechanisms at low Pt ¿ Proton-proton collisions are also used as a reference for nuclear collisions of different multiplicities in the study of Quark-Gluon Plasma (QGP) formation. The increase of strangeness production is considered an important observable of the QGP. In this context, we study the dependence of strange particle production with charged particle multiplicity and find that there is no indication of an increase strangeness production rate in pp collisions. This is an important result that contributes to the studies that consider the possibility of QGP formation in pp collisions / Doutorado / Física / Doutor em Ciências

Cette thèse de doctorat a pour cadre l’expérience CMS auprès du grand collisionneur de protons du CERN, le LHC. Le LHC, qui a permis la découverte en 2012 du boson de Brout-Englert-Higgs, est destiné à fonctionner pour encore 20 ans, avec une luminosité qui croîtra progressivement pour atteindre d’ici 2025 la valeur de 7.5 x 10^34 cm^-2 s^-1, c'est à dire environ cinq fois la valeur initialement prévue. Ceci a pour conséquence que les expériences doivent s’adapter et mettre à niveau une série de leurs composants et détecteurs. Une des prochaines mises à niveau de l’expérience CMS concerne les détecteurs Triple Gas Electron Multiplier (GEM) qui sont actuellement en développement pour la partie avant du spectromètre à muons de l’expérience. Ces détecteurs seront installés dans CMS durant le deuxième long arrêt du LHC, en 2018-2019, appelé LS2. Cette mise à niveau a pour but de contrôler les taux de déclenchement d’événements pour la détection de muons, grâce à la haute performance de ces détecteurs Triple GEM en présence de taux de particules extrêmement élevés (>1 kHz/cm^2). De plus, grâce à sa très bonne résolution spatiale (~250 um), la technologie GEM peut améliorer la reconstruction des traces de muons et la capacité d’identification du détecteur avant.Le but de mon travail de recherche est d’estimer la sensitivité des Triple GEMs à l’environnement de radiation hostile dans CMS, essentiellement composé de neutrons et de photons produits lors des interactions entre les particules et les détecteurs constituant l’expérience CMS. L’estimation précise de cette sensitivité est très importante, car une sous-estimation pourrait avoir des effets désastreux pour l’efficacité des Triple GEMs une fois installés dans CMS. Pour valider mes simulations, j’ai également reproduit des résultats expérimentaux obtenus avec d’autres détecteurs similaires déjà installés dans CMS, tels que les Resistive Plate Chambers (RPC).La deuxième partie de mon travail concerne l’étude de la capacité de l’expérience CMS à discerner différents modèles de nouvelle physique prédisant l’existence de bosons vecteurs, appelés Z'. Ces modèles font partie des extensions plausibles du Modèle Standard. En particulier, l’analyse se concentre sur des simulations dans lesquelles le Z' se désintègre en deux muons, et sur l’impact que les mises à niveau avec les détecteurs Triple GEM apporteront à ces mesures tout le long de la phase de haute intensité du LHC. Mes simulations montrent que plus de 20% des événements simulés comptent au moins un muon dans la région en pseudo-rapidité (eta) de CMS couverte par les détecteurs Triple GEM. Les résultats préliminaires démontrent que, dans le case de modèles à 3 TeV/c^2, il sera possible dès la fin de la Phase I de distinguer un Z'I d'un Z'SSM avec un niveau de signification alpha > 3 sigma. / This PhD thesis takes place in the CMS experiment at CERN's Large Hadron Collider (LHC). The LHC allowed the discovery of the Brout-Englert-Higgs boson in 2012, and is designed to run for at least 20 years, with an increasing luminosity that will reach by 2025 a value of 7.5 x 10^34 cm^-2 s^-1, that is a yield five times greater than the one initially intended. As a consequence, the experiments must adapt and upgrade many of their components and particle detectors. One of the foreseen upgrades of the CMS experiment concerns the Triple Gas Electron Multiplier (GEM) detectors, currently in development for the forward muon spectrometer. These detectors will be installed in CMS during the second long LHC shutdown (LS2), in 2018-2019. The aim of this upgrade is to better control the event trigger rate at Level 1 for muon detection, thanks to the high performance of these Triple GEM detectors, in presence of very high particle rates (>1 kHz/cm^2). Moreover, thanks to its excellent spatial resolution (~250 um), the GEM technology can improve the muon track reconstruction and the identification capability of the forward detector.The goal of my research is to estimate the sensitivity of Triple GEMs to the hostile background radiation in CMS, essentially made of neutron and photons generated by the interaction between the particles and CMS detectors. The accurate evaluation of this sensitivity is very important, as an underestimation could have ruinous effects of the Triple GEMs efficiency, once they are installed in CMS. To validate my simulations, I have reproduced experimental results obtained with similar detectors already installed in CMS, such as the Resistive Plate Chambers (RPC).The second part of my work regards the study of the CMS experiment capability to discriminate between different models of new physics predicting the existence of neutral vector bosons called Z'. These models belong to plausible extensions of the Standard Model. In particular, the analysis is focused on simulated samples in which the Z' decays in two muons, and on the impact that the Triple GEM detectors upgrades will bring to these measurements during the high luminosity phase of the LHC, called Phase II. My simulations prove that more than 20% of the simulated events see at least one muon in the CMS pseudo-rapidity (eta) region covered by Triple GEM detectors. Preliminary results show that, in the case of 3 TeV/c^2 models, it will be possible already at the end of Phase I to discriminate a Z'I from a Z'SSM with a significance level alpha > 3 sigma. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

Nous abordons le problème de la recherche indépendante du modèle pour la Nouvelle Physique (NP), au Grand Collisionneur de Hadrons (LHC) en utilisant le détecteur ATLAS. Une attention particulière est accordée au développement et à la mise à l'essai de nouvelles techniques d'apprentissage automatique à cette fin. Le présent ouvrage présente trois résultats principaux. Tout d'abord, nous avons mis en place un système de surveillance automatique des signatures génériques au sein de TADA, un outil logiciel d'ATLAS. Nous avons exploré plus de 30 signatures au cours de la période de collecte des données de 2017 et aucune anomalie particulière n'a été observée par rapport aux simulations des processus du modèle standard. Deuxièmement, nous proposons une méthode collective de détection des anomalies pour les recherches de NP indépendantes du modèle au LHC. Nous proposons l'approche paramétrique qui utilise un algorithme d'apprentissage semi-supervisé. Cette approche utilise une probabilité pénalisée et est capable d'effectuer simultanément une sélection appropriée des variables et de détecter un comportement anormal collectif possible dans les données par rapport à un échantillon de fond donné. Troisièmement, nous présentons des études préliminaires sur la modélisation du bruit de fond et la détection de signaux génériques dans des spectres de masse invariants à l'aide de processus gaussiens (GPs) sans information préalable moyenne. Deux méthodes ont été testées dans deux ensembles de données : une procédure en deux étapes dans un ensemble de données tiré des simulations du modèle standard utilisé pour ATLAS General Search, dans le canal contenant deux jets à l'état final, et une procédure en trois étapes dans un ensemble de données simulées pour le signal (Z′) et le fond (modèle standard) dans la recherche de résonances dans le cas du spectre de masse invariant de paire supérieure. Notre étude est une première étape vers une méthode qui utilise les GPs comme outil de modélisation qui peut être appliqué à plusieurs signatures dans une configuration plus indépendante du modèle. / We address the problem of model-independent searches for New Physics (NP), at the Large Hadron Collider (LHC) using the ATLAS detector. Particular attention is paid to the development and testing of novel Machine Learning techniques for that purpose. The present work presents three main results. Firstly, we put in place a system for automatic generic signature monitoring within TADA, a software tool from ATLAS. We explored over 30 signatures in the data taking period of 2017 and no particular discrepancy was observed with respect to the Standard Model processes simulations. Secondly, we propose a collective anomaly detection method for model-independent searches for NP at the LHC. We propose the parametric approach that uses a semi-supervised learning algorithm. This approach uses penalized likelihood and is able to simultaneously perform appropriate variable selection and detect possible collective anomalous behavior in data with respect to a given background sample. Thirdly, we present preliminary studies on modeling background and detecting generic signals in invariant mass spectra using Gaussian processes (GPs) with no mean prior information. Two methods were tested in two datasets: a two-step procedure in a dataset taken from Standard Model simulations used for ATLAS General Search, in the channel containing two jets in the final state, and a three-step procedure from a simulated dataset for signal (Z′) and background (Standard Model) in the search for resonances in the top pair invariant mass spectrum case. Our study is a first step towards a method that takes advantage of GPs as a modeling tool that can be applied to several signatures in a more model independent setup.

This dissertation presents a search for dark matter particles produced in association with a Z boson in proton-proton collisions. The dataset consists of 139 fb^{-1} of collision events with centre-of-mass energy of 13 TeV, and was collected by the ATLAS detector from 2015-2018 at the Large Hadron Collider. Signal region events are required to contain a Z boson that decays leptonically to either e^+e^- or μ^+μ^-, and a significant amount of missing transverse momentum, which indicates the presence of undetected particles. Two types of dark matter models are studied: (1) simplified models with an s-channel axial-vector or vector mediator that couples to dark matter Dirac fermions, and (2) two-Higgs-doublet models with an additional pseudo-scalar that couples to dark matter Dirac fermions. The main Standard Model background sources are ZZ, WZ, non-resonant l^+l^-, and Z+jets processes, which are estimated using a combination of data and/or simulation. A new reweighting technique is developed for estimating the Z+jets background using γ+jets events in data; the resulting estimate significantly improves on the statistical and systematic errors compared to the estimate obtained from simulation. The observed data in the signal region are compared to Standard Model prediction using a transverse mass discriminant distribution. No significant excess in data is observed for the simplified models and two-Higgs-doublet models studied. A statistical analysis is performed and several exclusion limits are set on the parameters of the dark matter models. Results are compared to direct detection experiments, the CMS experiment, and other ATLAS searches. Prospects and improvements for future iterations of the search are also presented. / Graduate

[pt] Na última década, desde o início do funcionamento do LHC, o Modelo Padrão de física de partículas vem sendo posto à prova com precisão sem precedentes, com enorme êxito. Um de seus experimentos é o LHCb, dedicado à física dos hádrons contendo os quarks beauty e charm. Um dos importantes temas de pesquisa do LHCb é o estudo de efeitos de assimetria partícula-antipartícula em processos de decaimento, devido à chamada violação de Carga-Paridade (CP). A violação de CP é prevista pelo Modelo Padrão e, em decaimentos envolvendo o quark charm, pode ocorrer em certos processos chamados de suprimidos por Cabibbo. No entanto, este efeito é muito pequeno, da ordem de 0.1 porcento. Esta pequenez faz com que o ambiente de decaimentos charmosos seja atraente para busca por física além do Modelo Padrão. O objetivo deste trabalho é o estudo de sensibilidade para violação de CP nos canais D+ -> K- K+π+ e D+ -> π- π+π+: no run II do LHCb. Através de uma representação do espaço de fase desses decaimentos, chamado de Dalitz Plot, e a utilização do método de Mirandizing, que se baseia em procurar significâncias locais na diferença da distribuição de eventos no Dalitz Plot de partícula e antipartícula, pode-se buscar por assimetrias de carga que indicariam efeitos de violação de CP nestes decaimentos. Baseando-se nas estatísticas dos dados tomados entre 2016 e 2018 no LHCb, foram desenvolvidos pseudoexperimentos, via método de Monte Carlo, visando reproduzir a dinâmica dos dados reais, e inserindo pequenos efeitos de violação de CP. Verificamos que há sensibilidade para a violação de CP com efeitos da ordem de 10-3 em algumas situações, que condiz com as expectativas do Modelo Padrão, o que indica a possibilidade de observação de violação de CP nos dados reais do run II. / [en] In the last decade, since the beginning of the operation of the LHC, the Standard Model of particle physics has been tested with unprecedented precision, with enormous success. One of its experiment is LHCb, dedicated to the physics of hadrons containing the beauty and charm quarks. One of the important research topics of the LHCb is the study of the effects of particle-antiparticle asymmetry in decay processes, due to the so-called charge-parity violation (CP). CP violation is predicted by the Standard Model and, in decays involving the charm quark, it can occur in certain processes called Cabibbo-suppressed. However, this effect is very small, of the order of 0.1 percent. This smallness makes the environment of charm decays attractive to search for physics beyond the Standard Model. The aim of this work is the study of sensitivity for CP violation in the channels D+ -> K-K+π+ and D+ -> π-π+π+ in run II of the LHCb. Through the phase space of these decays, called Dalitz Plot, and the use of the Mirandizing method, which is based on looking for local significance in the difference in the distribution of events for particle and antiparticle Dalitz Plots, one can seek for local asymmetries that would indicate effects of CP violation in these decays. Based on the data statistics taken between 2016 and 2018 at the LHCb, pseudo experiments were performed, using the Monte Carlo method, aiming to reproduce the dynamics of real data, and inserting small effects of CP violation. We found that there is sensitivity for CP violation with effects of the order of 10−3 in some situations, which is the expected level predicted by the Standard Model, indicating a possibility for observing CP violation in the real data from run II.