Measurement of the top quark pair production cross section in proton-proton collisions at center-of-mass energies of 7 TeV in final states with a tau lepton with the ATLAS detector

El Modelo Estándar (SM) de la Física de Partículas es una teoría cuántica de campos desarrollada en los años 60 para explicar el comportamiento de las partículas elementales y las fuerzas fundamentales que gobiernan sus interacciones. El Modelo Estándar ha sido comprobado experimentalmente, ha pronosticado con gran precisión una amplia variedad de fenómenos y ha explicado con éxito numerosos resultados experimentales. Para la exploración de fenómenos de nueva Física, la Organización Europea para la Investigación Nuclear (CERN) ha construido un acelerador y colisionador de partículas, el Gran Colisionador de Hadrones, LHC. El acelerador, los detectores y la infraestructura informática necesarias para producir y estudiar colisiones de protones convierten al LHC en una de las instalaciones científicas más grandes y complejas jamás construidas. El programa de física del quark top es de gran importancia en el LHC ya que el quark top posee el mayor acoplamiento al bosón de Higgs Estándar, responsable de proporcionar masa a las partículas fundamentales. Las altas energías obtenidas y la gran cantidad de datos recogidos hacen posible medir con gran precisión las propiedades del quark top en el LHC, mejorando nuestro entendimiento de la Física de Partículas y abriendo la puerta a nuevos descubrimientos. En particular, los estudios presentados en esta tesis se focalizan en la producción de pares de quark top en estados finales con dos leptones, uno de los cuales es un tau que se desintegra hadrónicamente. Los leptones tau juegan un papel importante en las búsquedas de bosones de Higgs Estándar y más alla del Modelo Estándar. Un bosón de Higgs Estándar se puede producir en asociación con un par de quarks top y desintegrarse a dos leptones tau aproximadamente un 10% de las veces para bajos valores de masa del bosón de Higgs. Este canal de desintegración proporciona una medida directa del acoplamiento del bosón de Higgs a fermiones, aportando una sólida evidencia de que los fermiones adquieren su masa a través del mecanismo de Higgs. Si existe un bosón de Higgs cargado, tal y como predice la teoría MSSM, y su masa es menor que la del quark top menos la del quark bottom, entones el quark top se desintegra predominantemente en un Higgs cargado y un quark b. Un bosón de Higgs cargado más pesado que el quark top puede producirse en asociación con él. En algunos escenarios el Higgs cargado se desintegra predominantemente en un leptón tau y un neutrino, produciendo un exceso en el canal leptón más tau sobre otros canales dileptónicos que, de ser observado, constituiría una evidencia experimental de la existencia de un bosón de Higgs cargado. La medida de la sección eficaz de producción de pares de top quark en el canal leptón más tau presentada en esta tesis se ha obtenido analizando 2.05 fb-1 de datos de colisiones protón-protón a energía del centro de masa de 7 TeV recogidos con el detector ATLAS en 2011. Los eventos de pares de quark top con un leptón y un tau en el estado final son seleccionados atendiendo a la topología de dichos estados finales en el detector: un electrón o muón de alto momento transverso, 2 jets de alta energía, al menos uno de ellos identificado como un b-jet, alta energía transversa perdida debida a los neutrinos y un leptón tau que se desintegra hadrónicamente. El principal fondo tras la selección procede de eventos de pares de quark top con un leptón y un jet en el estado final en los que un jet ha sido falsamente identificado como un tau hadrónico. El único discriminador, pues, entre señal y fondo es la identificación de leptones tau. En este análisis explotamos una variable discriminante, BDTj, obtenida mediante técnicas multivariadas de árboles de decisión y especialmente entrenada para separar taus hadrónicos de otro tipo de jets. Las contribuciones de señal y fondo en la muestra de datos se estiman mediante un ajuste chi2 de las distribuciones BDTj de los tau reconstruidos en los datos con un patrón de señal obtenido mediante simulación y patrones para los fondos obtenidos a partir de datos para minimizar las incertidumbres sistemáticas. Se emplean dos patrones de fondo diferentes, uno para jets producidos por la hadronización de quarks ligeros y otro para los jets producidos por la hadronización de gluones, que constituyen la mayor fuente de falsos taus para la señal tras los cortes de selección. Los parámetros del ajuste son la cantidad de fondo y de señal, manteniendo fija la forma de las distribuciones. La sección eficaz medida para la producción de pares de quark top en el canal leptón más tau excede la precisión de medidas anteriores en el mismo canal realizadas por los experimentos CDF, D0 y CMS. También se mejoran los resultados previos publicados por ATLAS, reduciendo la incertidumbre estadística en la medida de la sección eficaz y mejorando la descripción de los fondos mediante la construcción de un modelo que refleja adecuadamente la composición de jets para estimar correctamente la contaminación de falsos taus en la región de señal. Las incertidumbres sistemáticas dominan la medida y se han mantenido al mismo nivel de anteriores medidas. La sección eficaz medida está en buen acuerdo, dentro de las incertidumbres, con la predicción teórica del Modelo Estándar. Asimismo, los resultados obtenidos son consistentes con las secciones eficaces medidas por ATLAS en otros canales de desintegración. Hemos desarrollado una técnica novedosa que tiene un gran potencial para alcanzar precisiones elevadas en la medida de la sección eficaz cuando se aplique a muestras de datos con mucha estadística y se puede utilizar para comprobaciones del Modelo Estándar y búsquedas de nueva física en las futuras colisiones de protones que se esperan en el LHC a partir de 2015, con una energía de centro de masas de 13-14 TeV

Charged-particle multiplicities in pp interactions at root s=900 GeV measured with the ATLAS detector at the LHC

22 páginas, 4 figuras, 1 tabla.-- et al.(ATLAS Collaboration).-- arXiv:1003.3124v2The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity. and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range vertical bar eta vertical bar 500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity eta = 0 is measured to be 1.333 +/- 0.003(stat.) +/- 0.040(syst.), which is 5-15% higher than the Monte Carlo models predict.We are greatly indebted to all CERN’s departments and to the LHC project for their immense efforts not only in building the LHC, but also for their direct contributions to the construction and installation of the ATLAS detector and its infrastructure. All our congratulations go to the LHC operation team for the superb performance during this initial data-taking period. We acknowledge equally warmly all our technical colleagues in the collaborating Institutions without whom the ATLAS detector could not have been built. Furthermore we are grateful to all the funding agencies which supported generously the construction and the commissioning of the ATLAS detector and also provided the computing infrastructure. The ATLAS detector design and construction has taken about fifteen years, and our thoughts are with all our colleagues who sadly could not see its final realisation. We acknowledge the support of ANPCyT, Argentina; Yerevan Physics Institute, Armenia; ARC and DEST, Australia; Bundesministerium für Wissenschaft und Forschung, Austria; National Academy of Sciences of Azerbaijan; State Committee on Science & Technologies of the Republic of Belarus; CNPq and FINEP, Brazil; NSERC, NRC, and CFI, Canada; CERN; CONICYT, Chile; NSFC, China; COLCIENCIAS, Colombia; Ministry of Education, Youth and Sports of the Czech Republic, Ministry of Industry and Trade of the Czech Republic, and Committee for Collaboration of the Czech Republic with CERN; Danish Natural Science Research Council and the Lundbeck Foundation; European Commission, through the ARTEMIS Research Training Network; IN2P3-CNRS and Dapnia-CEA, France; Georgian Academy of Sciences; BMBF, HGF, DFG and MPG, Germany; Ministry of Education and Religion, through the EPEAEK program PYTHAGORAS II and GSRT, Greece; ISF, MINERVA, GIF, DIP, and Benoziyo Center, Israel; INFN, Italy; MEXT, Japan; CNRST, Morocco; FOM and NWO, Netherlands; The Research Council of Norway; Ministry of Science and Higher Education, Poland; GRICES and FCT, Portugal; Ministry of Education and Research, Romania; Ministry of Education and Science of the Russian Federation and State Atomic Energy Corporation “Rosatom”; JINR; Ministry of Science, Serbia; Department of International Science and Technology Cooperation, Ministry of Education of the Slovak Republic; Slovenian Research Agency, Ministry of Higher Education, Science and Technology, Slovenia; Ministerio de Educación y Ciencia, Spain; The Swedish Research Council, The Knut and Alice Wallenberg Foundation, Sweden; State Secretariat for Education and Science, Swiss National Science Foundation, and Cantons of Bern and Geneva, Switzerland; National Science Council, Taiwan; TAEK, Turkey; The Science and Technology Facilities Council and The Leverhulme Trust, United Kingdom; DOE and NSF, United States of America.Peer reviewe

Jet energy measurement with the ATLAS detector in proton-proton collisions at sqrt(s) = 7 TeV

The jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector at the LHC in proton-proton collision data at a centre-of-mass energy of sqrt(s) = 7 TeV corresponding to an integrated luminosity of 38 inverse pb. Jets are reconstructed with the anti-kt algorithm with distance parameters R=0.4 or R=0.6. Jet energy and angle corrections are determined from Monte Carlo simulations to calibrate jets with transverse momenta pt > 20 GeV and pseudorapidities eta 50 GeV after a dedicated correction for this effect. The JES is validated for jet transverse momenta up to 1 TeV to the level of a few percent using several in situ techniques by comparing a well-known reference such as the recoiling photon pt, the sum of the transverse momenta of tracks associated to the jet, or a system of low-pt jets recoiling against a high-pt jet. More sophisticated jet calibration schemes are presented based on calorimeter cell energy density weighting or hadronic properties of jets, providing an improved jet energy resolution and a reduced flavour dependence of the jet response. The JES systematic uncertainty determined from a combination of in situ techniques are consistent with the one derived from single hadron response measurements over a wide kinematic range. The nominal corrections and uncertainties are derived for isolated jets in an inclusive sample of high-pt jets

Measurement of the production cross section of prompt J/ψ mesons in association with a W ± boson in pp collisions at √s = 7 TeV with the ATLAS detector

Aad, G. et al.The process pp → W ± J/ψ provides a powerful probe of the production mechanism of charmonium in hadronic collisions, and is also sensitive to multiple parton interactions in the colliding protons. Using the 2011 ATLAS dataset of 4.5 fb−1 of s√ = 7 TeV pp collisions at the LHC, the first observation is made of the production of W ± + prompt J/ψ events in hadronic collisions, using W ± → μν μ and J/ψ → μ + μ −. A yield of 27.4+7.5−6.5 W ± + prompt J/ψ events is observed, with a statistical significance of 5.1σ. The production rate as a ratio to the inclusive W ± boson production rate is measured, and the double parton scattering contribution to the cross section is estimated.We acknowledge the support of ANPCyT, Argentina; YerPhI, Armenia; ARC, Australia; BMWF 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, DNSRC and Lundbeck Foundation, Denmark; EPLANET, ERC and NSRF, European Union; IN2P3-CNRS, CEA-DSM/IRFU, France; GNSF, Georgia; BMBF, DFG, HGF, MPG and AvH Foundation, Germany; GSRT and NSRF, Greece; ISF, MINERVA, GIF, DIP and Benoziyo Center, Israel; INFN, Italy; MEXT and JSPS, Japan; CNRST, Morocco; FOM and NWO, Netherlands; BRF and RCN, Norway; MNiSW and NCN, Poland; GRICES and FCT, Portugal; MNE/IFA, Romania; MES of Russia and ROSATOM, Russian Federation; JINR; MSTD, Serbia; MSSR, Slovakia; ARRS and MIZˇS, Slovenia; DST/NRF, South Africa; MINECO, Spain; SRC and Wallenberg Foundation, Sweden; SER, SNSF and Cantons of Bern and Geneva, Switzerland; NSC, Taiwan; TAEK, Turkey; STFC, the Royal Society and Leverhulme Trust, United Kingdom; DOE and NSF, United States of America.Peer reviewe

ATLAS search for a heavy gauge boson decaying to a charged lepton and a neutrino in pp collisions at root s=7 TeV

The ATLAS detector at the LHC is used to search for high-mass states, such as heavy charged gauge bosons (W'), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.7 fb(-1). No excess beyond Standard Model expectations is observed. A W' with Sequential Standard Model couplings is excluded at the 95 % credibility level for masses up to 2.55 TeV. Excited chiral bosons (W*) with equivalent coupling strength are excluded for masses up to 2.42 TeV

Dynamics of isolated-photon plus jet production in pp collisions at root s=7 TeV with the ATLAS detector

The dynamics of isolated-photon plus jet production in pp collisions at a centre-of-mass energy of 7 TeV has been studied with the ATLAS detector at the LHC using an integrated luminosity of 37 pb(-1). Measurements of isolated-photon plus jet bin-averaged cross sections are presented as functions of photon transverse energy, jet transverse momentum and jet rapidity. In addition, the bin-averaged cross sections as functions of the difference between the azimuthal angles of the photon and the jet, the photon jet invariant mass and the scattering angle in the photon jet centre-of-mass frame have been measured. Next-to-leading-order QCD calculations are compared to the measurements and provide a good description of the data, except for the case of the azimuthal opening angle

Measurement of the cross-section for W boson production in association with b-jets in pp collisions at root s=7 TeV with the ATLAS detector

This paper reports a measurement of the W+b-jets (W+b+X and W+b (b) over bar +X) production cross-section in proton-proton collisions at a centre-of-mass energy of 7 TeV at the LHC. These results are based on data corresponding to an integrated luminosity of 4.6 fb(-1), collected with the ATLAS detector. Cross-sections are presented as a function of jet multiplicity and of the transverse momentum of the leading b-jet for both the muon and electron decay modes of the W boson. The W+b-jets cross-section, corrected for all known detector effects, is quoted in a limited kinematic range. Combining the muon and electron channels, the fiducial cross-section for W+b-jets is measured to be 7.1 +/- 0.5 (stat) +/- 1.4 (syst) pb, consistent with the next-to-leading order QCD prediction, corrected for non-perturbative and double-parton interactions (DPI) contributions, of 4.70 +/- 0.09 (stat) (+0.60)(-0.49) (scale) +/- 0.06 (PDF) +/- 0.16 (non-pert) (+0.52)(-0.38) (DPI) pb

Measurement of the flavour composition of dijet events in pp collisions at √s =7 TeV with the ATLAS detector

This paper describes a measurement of the flavour composition of dijet events produced in pp collisions at root s = 7 TeV using the ATLAS detector. The measurement uses the full 2010 data sample, corresponding to an integrated luminosity of 39 pb(-1). Six possible combinations of light, charm and bottom jets are identified in the dijet events, where the jet flavour is defined by the presence of bottom, charm or solely light flavour hadrons in the jet. Kinematic variables, based on the properties of displaced decay vertices and optimised for jet flavour identification, are used in a multidimensional template fit to measure the fractions of these dijet flavour states as functions of the leading jet transverse momentum in the range 40 GeV to 500 GeV and jet rapidity vertical bar y vertical bar < 2.1. The fit results agree with the predictions of leading-and next-to-leading-order calculations, with the exception of the dijet fraction composed of bottom and light flavour jets, which is underestimated by all models at large transverse jet momenta. The ability to identify jets containing two b-hadrons, originating from e. g. gluon splitting, is demonstrated. The difference between bottom jet production rates in leading and subleading jets is consistent with the next-to-leading-order predictions

Search for dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detector

A search for new phenomena in events with a high-energy jet and large missing transverse momentum is performed using data from proton-proton collisions at root s = 7 TeV with the ATLAS experiment at the Large flatiron Collider. Four kinematic regions are explored using a dataset corresponding to an integrated luminosity of 4.7 fb(-1). No excess of events beyond expectations from Standard Model processes is observed, and limits are set on large extra dimensions and the pair production of dark matter particles

ATLAS search for new phenomena in dijet mass and angular distributions using pp collisions at root s=7 TeV

Mass and angular distributions of dijets produced in LHC proton-proton collisions at a centre-of-mass energy root s = 7TeV have been studied with the ATLAS detector using the full 2011 data set with an integrated luminosity of 4.8 fb(-1). Dijet masses up to similar to 4.0TeV have been probed. No resonance-like features have been observed in the dijet mass spectrum, and all angular distributions are consistent with the predictions of QCD. Exclusion limits on six hypotheses of new phenomena have been set at 95% CL in terms of mass or energy scale, as appropriate. These hypotheses include excited quarks below 2.83 TeV, colour octet scalars below 1.86TeV, heavy W bosons below 1.68 TeV, string resonances below 3.61 TeV, quantum black holes with six extra space-time dimensions for quantum gravity scales below 4.11 TeV, and quark contact interactions below a compositeness scale of 7.6 TeV in a destructive interference scenario