Search for new physics in single-top-quark production with the ATLAS detector at the LHC

Esta tesis presenta dos análisis dedicados a la búsqueda de nueva física en eventos de producción de quarks top únicos con datos recopilados por el detector ATLAS en el LHC. A pesar de su gran éxito, se cree que debe haber una teoría más general que subyace al Modelo Estándar de física de partículas que arroge luz a preguntas sin respuesta, como la asimetría de materia-antimateria del universo o la existencia de la Materia Oscura. Esta tesis cubre dos enfoques diferentes para abordar los dos temas anteriores. El objetivo del primer análisis es explorar la estructura del vértice Wtb a través de la medición de asimetrías angulares relacionadas con los observables de polarización del bosón W que utilizan eventos de quark top producidos en el canal t. El conjunto de datos corresponde a eventos de colisión protón-protón producidos con una energía de centro de masa de 8 TeV con una luminosidad total integrada de 20.2 / fb. Los eventos seleccionados contienen un leptón aislado (electrón o muón), gran momento transverso faltante y exactamente dos jets, con uno de ellos identificado como probable que contenga un B-hadrón. Se aplican requisitos de selección adicionales para separar los eventos de señal del canal t del fondo. Los observables de polarización del bosón W se extraen de asimetrías en distribuciones angulares del leptón cargado medidos con respecto a un eje de cuantización de espín definido. Las mediciones de las asimetrías se realizan a nivel de partón corrigiendo las distribuciones angulares observadas por los efectos del detector (resolución y eficiencia de selección) y la hadronización, después de restar las contribuciones de fondo. Los valores de polarización de bosón W medidos están de acuerdo con las predicciones del modelo estándar. Los resultados se utilizan para establecer límites en la parte imaginaria del acoplamiento anómalo gR de una manera independiente del modelo. El segundo análisis presenta una búsqueda de eventos con un quark top y una gran energía transversa faltante en el estado final. Tales estados finales no son posibles dentro del Modelo Estándar de física de partículas. El conjunto de datos corresponde a los datos de colisión protón-protón a una energía de centro de masa de 13 TeV recopilado por el detector ATLAS durante 2015 y 2016, que corresponde a una luminosidad integrada de 36.1 / fb. El canal con decaimiento leptonico del bosón W del quark top está cubierto en esta tesis. Los eventos seleccionados contienen exactamente un leptón aislado (electrón o muón), gran momento transverso faltante y exactamente un jet, identificado como probable que contenga un B-hadrón. Además, se aplican requisitos de selección adicionales para discriminar los eventos de señal del fondo. La forma de la distribución de energía transversa faltante se utiliza para ajustar los fondos esperados a los datos observados. Los resultados obtenidos se combinan con el análisis de la desintegración hadrónica del bosón W y se interpretan en el contexto de modelos genéricos para la producción de materia oscura. En ausencia de evidencia para cualquiera de estas señales, se obtienen límites superiores con un nivel de confianza del 95% en las secciones eficaces de producción correspondientes y estos límites se traducen en restricciones en el espacio de parámetros de los modelos considerados. Además de los dos análisis mencionados, en este manuscrito se presentan algunas contribuciones al alineamiento del detector interno ATLAS.This thesis presents two analyses devoted to the search of new physics in single-top-quark events with data collected by the ATLAS detector at the LHC. In spite of its great success, it is believed that there must be a more general theory underlying the Standard Model of particle physics which sheds light to unanswered questions such as the matter-antimatter asymmetry of the Universe or the existence of Dark Matter. This thesis covers two different approaches to address these two topics. The aim of the first analysis is to probe the structure of the Wtb vertex through the measurement of angular asymmetries related to W-boson polarization observables using single-top-quark events produced in the t-channel. The dataset corresponds to proton-proton collision events produced at a center-of-mass energy of 8 TeV with a total integrated luminosity of 20.2/fb. Selected events contain one isolated lepton (electron or muon), large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Further selection requirements are applied to further separate signal t-channel single-top-quark events from background. The W boson polarization observables are extracted from asymmetries in angular distributions of the charged lepton measured with respect to a defined spin quantization axes. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronization after subtracting the background contributions. The measured W-boson polarization values are in agreement with the Standard Model predictions. The results are used to set limits on the imaginary part of the anomalous coupling gR in a model-independent way. The second analysis presents a search for events with one top quark and large missing transverse energy at the final state. Such final states are not possible within the Standard Model of particle physics. The dataset corresponds to proton-proton collision data at a center-of-mass energy of 13 TeV collected by the ATLAS detector during 2015 and 2016, corresponding to an integrated luminosity of 36.1/fb. The channel with leptonic decay of the W boson from the top quark is covered in this thesis. Selected events contain exactly one isolated lepton (electron or muon), large missing transverse momentum and exactly one jet, identified as likely to contain a b-hadron. In addition, further selection requirements are applied to discriminate signal events from background. The shape of the missing transverse energy distribution is used to fit the expected backgrounds to the observed data. The obtained results are combined with the analysis of the hadronic W boson decay and interpreted in the context of generic models for Dark Matter production. In the absence of evidence for any of these signals, 95% confidence level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the considered models. In addition to the two mentioned analyses, some contributions to the alignment of the ATLAS inner detector are presented in this manuscript

A solvatofluorochromic silicon-substituted xanthene dye useful in bioimaging

In this work, we have performed an in-depth study of the photophysics and solvatofluorochromism of a red-emitting Si-xanthene dye, an analog of Tokyo Magenta (TM) historically developed by Egawa et al. (Chem. Commun. 2011, 47, 4162–4164). The results show a strong dependency of the emission properties of 2-Me-4-OMe-TM on the polarity of the solvent. For instance, the dye exhibited an increase in its fluorescence lifetime with the decrease in solvent polarity. Therefore, in this work, this spectral behavior has been used as a new approach for determining the intracellular microenvironment polarity through the measurement of its fluorescence lifetime by Fluorescence Lifetime Imaging Microscopy (FLIM). Our experiments confirmed the ability of the dye to detect changes in polarity between different intracellular compartments.This work was funded by grants CTQ2017-85658-R, CTQ2014-55474-C2-2-R (Spanish Ministry of Economy and Competitiveness; Agencia Estatal de Investigacion, AEI; and European Regional Development Fund, ERDF), QM2012-790 (Junta de Andalucía), and a grant from the Fundación Botín

Search for vector-boson resonances decaying to a top quark and bottom quark in the lepton plus jets final state in pp collisions at s=13 TeV with the ATLAS detector

A search for new charged massive gauge bosons, W, is performed with the ATLAS detector at the LHC. Data were collected in proton–proton collisions at a center-of-mass energy of s=13 TeV and correspond to an integrated luminosity of 36.1 fb. This analysis searches for W bosons in the W→tb¯ decay channel in final states with an electron or muon plus jets. The search covers resonance masses between 0.5 and 5.0 TeV and considers right-handed W bosons. No significant deviation from the Standard Model (SM) expectation is observed and upper limits are set on the W→tb¯ cross section times branching ratio and the W boson effective couplings as a function of the W boson mass. For right-handed W bosons with coupling to the SM particles equal to the SM weak coupling constant, masses below 3.15 TeV are excluded at the 95% confidence level. This search is also combined with a previously published ATLAS result for W→tb¯ in the fully hadronic final state. Using the combined searches, right-handed W bosons with masses below 3.25 TeV are excluded at the 95% confidence level.Peer Reviewe

Search for heavy particles decaying into a top-quark pair in the fully hadronic final state in pp collisions at s =13 TeV with the ATLAS detector

A search for new particles decaying into a pair of top quarks is performed using proton-proton collision data recorded with the ATLAS detector at the Large Hadron Collider at a center-of-mass energy of s=13 TeV corresponding to an integrated luminosity of 36.1 fb-1. Events consistent with top-quark pair production and the fully hadronic decay mode of the top quarks are selected by requiring multiple high transverse momentum jets including those containing b-hadrons. Two analysis techniques, exploiting dedicated top-quark pair reconstruction in different kinematic regimes, are used to optimize the search sensitivity to new hypothetical particles over a wide mass range. The invariant mass distribution of the two reconstructed top-quark candidates is examined for resonant production of new particles with various spins and decay widths. No significant deviation from the Standard Model prediction is observed and limits are set on the production cross-section times branching fraction for new hypothetical Z′ bosons, dark-matter mediators, Kaluza-Klein gravitons and Kaluza-Klein gluons. By comparing with the predicted production cross sections, the Z′ boson in the topcolor-assisted-technicolor model is excluded for masses up to 3.1-3.6 TeV, the dark-matter mediators in a simplified framework are excluded in the mass ranges from 0.8 to 0.9 TeV and from 2.0 to 2.2 TeV, and the Kaluza-Klein gluon is excluded for masses up to 3.4 TeV, depending on the decay widths of the particles.Peer Reviewe

Search for pairs of highly collimated photon-jets in pp collisions at s =13 TeV with the ATLAS detector

Results of a search for the pair production of photon-jets - collimated groupings of photons - in the ATLAS detector at the Large Hadron Collider are reported. Highly collimated photon-jets can arise from the decay of new, highly boosted particles that can decay to multiple photons collimated enough to be identified in the electromagnetic calorimeter as a single, photonlike energy cluster. Data from proton-proton collisions at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.7 fb-1, were collected in 2015 and 2016. Candidate photon-jet pair production events are selected from those containing two reconstructed photons using a set of identification criteria much less stringent than that typically used for the selection of photons, with additional criteria applied to provide improved sensitivity to photon-jets. Narrow excesses in the reconstructed diphoton mass spectra are searched for. The observed mass spectra are consistent with the Standard Model background expectation. The results are interpreted in the context of a model containing a new, high-mass scalar particle with narrow width, X, that decays into pairs of photon-jets via new, light particles, a. Upper limits are placed on the cross section times the product of branching ratios σ×B(X→aa)×B(a→γγ)2 for 200 GeV<mX<2 TeV and for ranges of ma from a lower mass of 100 MeV up to between 2 and 10 GeV, depending upon mX. Upper limits are also placed on σ×B(X→aa)×B(a→3π0)2 for the same range of mX and for ranges of ma from a lower mass of 500 MeV up to between 2 and 10 GeV.Peer Reviewe

Measurement of the t t Z and t t W cross sections in proton-proton collisions at s =13 TeV with the ATLAS detector

A measurement of the associated production of a top-quark pair (tt) with a vector boson (W, Z) in proton-proton collisions at a center-of-mass energy of 13 TeV is presented, using 36.1 fb-1 of integrated luminosity collected by the ATLAS detector at the Large Hadron Collider. Events are selected in channels with two same- or opposite-sign leptons (electrons or muons), three leptons or four leptons, and each channel is further divided into multiple regions to maximize the sensitivity of the measurement. The ttZ and ttW production cross sections are simultaneously measured using a combined fit to all regions. The best-fit values of the production cross sections are σttZ=0.95±0.08stat±0.10syst pb and σttW=0.87±0.13stat±0.14syst pb in agreement with the Standard Model predictions. The measurement of the ttZ cross section is used to set constraints on effective field theory operators which modify the ttZ vertex.Peer Reviewe

Search for invisible Higgs boson decays in vector boson fusion at s=13TeV with the ATLAS detector

We report a search for Higgs bosons that are produced via vector boson fusion and subsequently decay into invisible particles. The experimental signature is an energetic jet pair with invariant mass of O(1)TeV and O(100)GeV missing transverse momentum. The analysis uses 36.1 fb of pp collision data at s=13TeV recorded by the ATLAS detector at the LHC. In the signal region the 2252 observed events are consistent with the background estimation. Assuming a 125GeV scalar particle with Standard Model cross sections, the upper limit on the branching fraction of the Higgs boson decay into invisible particles is 0.37 at 95% confidence level where 0.28 was expected. This limit is interpreted in Higgs portal models to set bounds on the WIMP–nucleon scattering cross section. We also consider invisible decays of additional scalar bosons with masses up to 3TeV for which the upper limits on the cross section times branching fraction are in the range of 0.3–1.7pb.Peer Reviewe

Search for the Higgs boson decays H → ee and H → eμ in pp collisions at s=13TeV with the ATLAS detector

Searches for the Higgs boson decays H→ee and H→eμ are performed using data corresponding to an integrated luminosity of 139fb collected with the ATLAS detector in pp collisions at s=13 TeV at the LHC. No significant signals are observed, in agreement with the Standard Model expectation. For a Higgs boson mass of 125 GeV, the observed (expected) upper limit at the 95% confidence level on the branching fraction B(H→ee) is 3.6×10 (3.5×10) and on B(H→eμ) is 6.2×10 (5.9×10). These results represent improvements by factors of about five and six on the previous best limits on B(H→ee) and B(H→eμ) respectively

Measurement of W±Z production cross sections and gauge boson polarisation in pp collisions at √s=13TeV with the ATLAS detector

This paper presents measurements of WZ production cross sections in pp collisions at a centre-of-mass energy of 13 TeV. The data were collected in 2015 and 2016 by the ATLAS experiment at the Large Hadron Collider, and correspond to an integrated luminosity of 36.1fb-1. The WZ candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The measured inclusive cross section in the detector fiducial region for a single leptonic decay mode is σW±Z→ℓ′νℓℓfid.=63.7±1.0(stat.)±2.3(syst.)±1.4(lumi.) fb, reproduced by the next-to-next-to-leading-order Standard Model prediction of 61.5-1.3+1.4 fb. Cross sections for WZ and WZ production and their ratio are presented as well as differential cross sections for several kinematic observables. An analysis of angular distributions of leptons from decays of W and Z bosons is performed for the first time in pair-produced events in hadronic collisions, and integrated helicity fractions in the detector fiducial region are measured for the W and Z bosons separately. Of particular interest, the longitudinal helicity fraction of pair-produced vector bosons is also measured.We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWFW and FWF, Austria; ANAS, Azerbaijan; SSTC, Belarus; CNPq and FAPESP, Brazil; NSERC, NRC and CFI, Canada; CERN; CONICYT, Chile; CAS, MOST and NSFC, China; COLCIENCIAS, Colombia; MSMT CR, MPO CR and VSC CR, Czech Republic; DNRF and DNSRC, Denmark; IN2P3-CNRS, CEA-DRF/IRFU, France; SRNSFG, Georgia; BMBF, HGF, and MPG, Germany; GSRT, Greece; RGC, Hong Kong SAR, China; ISF and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; NWO, Netherlands; RCN, Norway; MNiSW and NCN, Poland; FCT, Portugal; MNE/IFA, Romania; MES of Russia and NRC KI, Russian Federation; JINR; MESTD, Serbia; MSSR, Slovakia; ARRS and MIZŠ, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden; SERI, SNSF and Cantons of Bern and Geneva, Switzerland; MOST, Taiwan; TAEK, Turkey; STFC, UK; DOE and NSF, USA. In addition, individual groups and members have received support from BCKDF, CANARIE, CRC and Compute Canada, Canada; COST, ERC, ERDF, Horizon 2020, and Marie Skłodowska-Curie Actions, European Union; Investissements d’ Avenir Labex and Idex, ANR, France; DFG and AvH Foundation, Germany; Herakleitos, Thales and Aristeia programmes co-financed by EU-ESF and the Greek NSRF, Greece; BSF-NSF and GIF, Israel; CERCA Programme Generalitat de Catalunya, Spain; The Royal Society and Leverhulme Trust, UK. The crucial computing support from all WLCG partners is acknowledged gratefully, in particular from CERN, the ATLAS Tier-1 facilities at TRIUMF (Canada), NDGF (Denmark, Norway, Sweden), CC-IN2P3 (France), KIT/GridKA (Germany), INFN-CNAF (Italy), NL-T1 (Netherlands), PIC (Spain), ASGC (Taiwan), RAL (UK) and BNL (USA), the Tier-2 facilities worldwide and large non-WLCG resource providers. Major contributors of computing resources are listed in Ref. [106].Peer Reviewe

Search for the Production of a Long-Lived Neutral Particle Decaying within the ATLAS Hadronic Calorimeter in Association with a Z Boson from pp Collisions at s =13 TeV

This Letter presents a search for the production of a long-lived neutral particle (Zd) decaying within the ATLAS hadronic calorimeter, in association with a standard model (SM) Z boson produced via an intermediate scalar boson, where Z→+ (=e, μ). The data used were collected by the ATLAS detector during 2015 and 2016 pp collisions with a center-of-mass energy of s=13 TeV at the Large Hadron Collider and correspond to an integrated luminosity of 36.1±0.8 fb-1. No significant excess of events is observed above the expected background. Limits on the production cross section of the scalar boson times its decay branching fraction into the long-lived neutral particle are derived as a function of the mass of the intermediate scalar boson, the mass of the long-lived neutral particle, and its cτ from a few centimeters to one hundred meters. In the case that the intermediate scalar boson is the SM Higgs boson, its decay branching fraction to a long-lived neutral particle with a cτ approximately between 0.1 and 7 m is excluded with a 95% confidence level up to 10% for mZd between 5 and 15 GeV