196 research outputs found

    Searches for lepton-flavour-violating decays of the Higgs boson in s=13\sqrt{s}=13 TeV pp\mathit{pp} collisions with the ATLAS detector

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    This Letter presents direct searches for lepton flavour violation in Higgs boson decays, H → eτ and H → μτ , performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton–proton collisions at a centre-of-mass energy √s = 13 TeV, corresponding to an integrated luminosity of 36.1 fb−1. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the leptonflavour-violating branching ratios are 0.47% (0.34+0.13−0.10%) and 0.28% (0.37+0.14−0.10%) for H → eτ and H → μτ , respectively.publishedVersio

    Combination of searches for Higgs boson pairs in pp collisions at \sqrts = 13 TeV with the ATLAS detector

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    This letter presents a combination of searches for Higgs boson pair production using up to 36.1 fb(-1) of proton-proton collision data at a centre-of-mass energy root s = 13 TeV recorded with the ATLAS detector at the LHC. The combination is performed using six analyses searching for Higgs boson pairs decaying into the b (b) over barb (b) over bar, b (b) over barW(+)W(-), b (b) over bar tau(+)tau(-), W+W-W+W-, b (b) over bar gamma gamma and W+W-gamma gamma final states. Results are presented for non-resonant and resonant Higgs boson pair production modes. No statistically significant excess in data above the Standard Model predictions is found. The combined observed (expected) limit at 95% confidence level on the non-resonant Higgs boson pair production cross-section is 6.9 (10) times the predicted Standard Model cross-section. Limits are also set on the ratio (kappa(lambda)) of the Higgs boson self-coupling to its Standard Model value. This ratio is constrained at 95% confidence level in observation (expectation) to -5.0 &lt; kappa(lambda) &lt; 12.0 (-5.8 &lt; kappa(lambda) &lt; 12.0). In addition, limits are set on the production of narrow scalar resonances and spin-2 Kaluza-Klein Randall-Sundrum gravitons. Exclusion regions are also provided in the parameter space of the habemus Minimal Supersymmetric Standard Model and the Electroweak Singlet Model. For complete list of authors see http://dx.doi.org/10.1016/j.physletb.2019.135103</p

    Search for flavour-changing neutral currents in processes with one top quark and a photon using 81 fb⁻¹ of pp collisions at \sqrts = 13 TeV with the ATLAS experiment

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    A search for flavour-changing neutral current (FCNC) events via the coupling of a top quark, a photon, and an up or charm quark is presented using 81 fb−1 of proton–proton collision data taken at a centre-of-mass energy of 13 TeV with the ATLAS detector at the LHC. Events with a photon, an electron or muon, a b-tagged jet, and missing transverse momentum are selected. A neural network based on kinematic variables differentiates between events from signal and background processes. The data are consistent with the background-only hypothesis, and limits are set on the strength of the tqγ coupling in an effective field theory. These are also interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tuγ coupling of 36 fb (78 fb) and on the branching ratio for t→γu of 2.8×10−5 (6.1×10−5). In addition, they are interpreted as 95% CL upper limits on the cross section for FCNC tγ production via a left-handed (right-handed) tcγ coupling of 40 fb (33 fb) and on the branching ratio for t→γc of 22×10−5 (18×10−5). © 2019 The Author(s

    Mar Menor: una laguna singular y sensible. Evaluación científica de su estado.

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    Este libro recopila las aportaciones que equipos de investigación de la Universidad de Murcia, Universidad Politécnica de Cartagena, Instituto Geológico-Minero de España, Universidad de Alicante, el Instituto Español de Oceanografía y otros organismos hicieron en las Jornadas Científicas del Mar Menor, celebradas en diciembre de 2014.La información recogida en este libro se estructura en dos grandes bloques, uno de Biología y Ecología del Mar Menor (capítulos 1 al 8) y otro de Condiciones fisicoquímicas e impacto de actividades humanas en la laguna (capítulos 9 al 14). El primer bloque resume buena parte de los estudios ecológicos realizados en el Mar Menor, que han servido para mejorar su conocimiento y también para cambiar antiguas asunciones sobre la naturaleza y el funcionamiento de estos ecosistemas lagunares (Capítulo 1). El segundo capítulo muestra que esta laguna alberga en zonas someras de su perímetro hábitats fundamentales para mantener y conservar tanto especies migratorias como residentes, que es necesario conocer para paliar el impacto de las actividades humanas que les afectan. En este sentido la reducción de la carga de nutrientes y contaminantes orgánicos e inorgánicos que fluyen hacia el Mar Menor puede ayudar a preservar la laguna en mejores condiciones, bien sea tratando las escorrentías (plantas de tratamiento, humedales artificiales u otras técnicas) y recuperar este agua para uso agrícola o evitar su descarga en la laguna (Capítulo 3). Estas actuaciones serán clave para la conservación de especies emblemáticas como el caballito de mar (Capítulo 4) y reducir el impacto de las proliferaciones masivas de medusas que se producen en la laguna desde 1993 (Capítulo 5). En este mismo sentido los cambios acaecidos en la laguna han favorecido la incursión de invertebrados marinos alóctonos (Capítulo 6) y han afectado a la respuesta de la dinámica poblacional de las aves acuáticas a distintas escalas (Capítulo 7). Para completar este bloque se ofrece una perspectiva histórica de la importancia que ha tenido la investigación sobre acuicultura realizada en esta laguna, que ha servido de base para su gran desarrollo actual (Capítulo 8). El segundo bloque se inicia con una evaluación del origen y evolución del Mar Menor desde el punto de vista geológico, y evidencia su vulnerabilidad ante el deterioro que puede sufrir la desaparición de la barrera de cierre y/o su colmatación (Capítulo 9). En el Capítulo 10 se describe la relevancia que tiene la interacción de los acuíferos del Campo de Cartagena con la laguna, que se produce no sólo a nivel superficial sino también subterráneo. Esta interacción permite el acceso de nutrientes a la laguna, a pesar de la cierta capacidad de depuración de los humedales que le circundan, y también de metales traza por los aportes de residuos mineros (Capítulo 11). De hecho los metales traza están presentes en los sedimentos de la laguna, y su distribución se ha caracterizado en la columna sedimentaria relacionándola con la granulometría y el contenido de materia orgánica del sedimento (Capítulo 12). Posteriormente se describe la entrada de diversos contaminantes orgánicos, incluyendo pesticidas y fármacos a través de la rambla del Albujón, y su distribución estacional en agua y sedimento de la laguna (Capítulo 13). Este segundo bloque finaliza con el Capítulo 14 en el que se describe la bioacumulación de hidrocarburos aromáticos policíclicos, pesticidas y fármacos en moluscos y peces del Mar Menor, así como los efectos biológicos que la carga contaminante que accede a través de la rambla del Albujón produce en los organismos que allí habitan. El libro concluye con un breve epílogo redactado por los editores de este libro.Versión del edito

    Search for long-lived neutral particles in pp collisions at s√=13 TeV that decay into displaced hadronic jets in the ATLAS calorimeter

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    This paper describes a search for pairs of neutral, long-lived particles decaying in the ATLAS calorimeter. Long-lived particles occur in many extensions to the Standard Model and may elude searches for new promptly decaying particles. The analysis considers neutral, long-lived scalars with masses between 5 and 400 GeV, produced from decays of heavy bosons with masses between 125 and 1000 GeV, where the long-lived scalars decay into Standard Model fermions. The analysis uses either 10.8 fb−1 or 33.0 fb−1 of data (depending on the trigger) recorded in 2016 at the LHC with the ATLAS detector in proton–proton collisions at a centre-of-mass energy of 13 TeV. No significant excess is observed, and limits are reported on the production cross section times branching ratio as a function of the proper decay length of the long-lived particles

    Evidence for the production of three massive vector bosons with the ATLAS detector

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    A search for the production of three massive vector bosons in proton-proton collisions is performed using data at s=13\sqrt{s} = 13 TeV recorded with the ATLAS detector at the Large Hadron Collider in the years 2015-2017, corresponding to an integrated luminosity of 79.879.8 fb1^{-1}. Events with two same-sign leptons \ell (electrons or muons) and at least two reconstructed jets are selected to search for WWWννqqWWW \to \ell \nu \ell \nu qq. Events with three leptons without any same-flavour opposite-sign lepton pairs are used to search for WWWνννWWW \to \ell \nu \ell\nu \ell \nu, while events with three leptons and at least one same-flavour opposite-sign lepton pair and one or more reconstructed jets are used to search for WWZνqqWWZ \to \ell \nu qq \ell \ell. Finally, events with four leptons are analysed to search for WWZννWWZ \to \ell \nu \ell \nu \ell \ell and WZZqqWZZ \to qq \ell \ell \ell \ell. Evidence for the joint production of three massive vector bosons is observed with a significance of 4.1 standard deviations, where the expectation is 3.1 standard deviations.Comment: 38 pages in total, author list starting page 22, 6 figures, 5 tables, matching published paper in Phys. Lett. B. All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2017-2

    Measurements of the production cross-section for a Z boson in association with b- or c-jets in proton–proton collisions at √s = 13 TeV with the ATLAS detector

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    This paper presents a measurement of the production cross-section of a Z boson in association with bor c-jets, in proton–proton collisions at √s = 13 TeV with the ATLAS experiment at the Large Hadron Collider using data corresponding to an integrated luminosity of 140 fb−1. Inclusive and differential cross-sections are measured for events containing a Z boson decaying into electrons or muons and produced in association with at least one b-jet, at least one c-jet, or at least two b-jets with transverse momentum pT > 20 GeV and rapidity |y| < 2.5. Predictions from several Monte Carlo generators based on next-to-leading-order matrix elements interfaced with a parton-shower simulation, with different choices of flavour schemes for initial-state partons, are compared with the measured cross-sections. The results are also compared with novel predictions, based on infrared and collinear safe jet flavour dressing algorithms. Selected Z+ ≥ 1 c-jet observables, optimized for sensitivity to intrinsic-charm, are compared with benchmark models with different intrinsic-charm fractions

    Search for non-resonant Higgs boson pair production in final states with leptons, taus, and photons in pp collisions at √s = 13 TeV with the ATLAS detector

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    A search is presented for non-resonant Higgs boson pair production, targeting the bbZZ, 4V (V = W or Z), V V τ τ , 4τ , γγV V and γγτ τ decay channels. Events are categorised based on the multiplicity of light charged leptons (electrons or muons), hadronically decaying tau leptons, and photons. The search is based on a data sample of proton-proton collisions at √s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 140 fb−1. No evidence of the signal is found and the observed (expected) upper limit on the cross-section for non-resonant Higgs boson pair production is determined to be 17 (11) times the Standard Model predicted cross-section at 95% confidence level under the background-only hypothesis. The observed (expected) constraints on the HHH coupling modifier, κλ, are determined to be −6.2 < κλ < 11.6 (−4.5 < κλ < 9.6) at 95% confidence level, assuming the Standard Model for the expected limits and that new physics would only affect κλ

    Study of Z → llγ decays at √s = 8 TeV with the ATLAS detector

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    This paper presents a study of Z → llγ decays with the ATLAS detector at the Large Hadron Collider. The analysis uses a proton–proton data sample corresponding to an integrated luminosity of 20.2 fb−1 collected at a centre-ofmass energy √s = 8 TeV. Integrated fiducial cross-sections together with normalised differential fiducial cross-sections, sensitive to the kinematics of final-state QED radiation, are obtained. The results are found to be in agreement with stateof-the-art predictions for final-state QED radiation. First measurements of Z → llγ γ decays are also reported

    Search for leptoquark pair production decaying into te−te¯ + or tμ−t¯μ+ in multi-lepton final states in pp collisions at √s = 13 TeV with the ATLAS detector

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    A search for leptoquark pair production decaying into te−te¯ + or tμ−t¯μ+ in final states with multiple leptons is presented. The search is based on a dataset of pp collisions at √s = 13 TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider, corresponding to an integrated luminosity of 139 fb−1. Four signal regions, with the requirement of at least three light leptons (electron or muon) and at least two jets out of which at least one jet is identified as coming from a b-hadron, are considered based on the number of leptons of a given flavour. The main background processes are estimated using dedicated control regions in a simultaneous fit with the signal regions to data. No excess above the Standard Model background prediction is observed and 95% confidence level limits on the production cross section times branching ratio are derived as a function of the leptoquark mass. Under the assumption of exclusive decays into te− (tμ−), the corresponding lower limit on the scalar mixed-generation leptoquark mass mLQd mix is at 1.58 (1.59) TeV and on the vector leptoquark mass mU˜1 at 1.67 (1.67) TeV in the minimal coupling scenario and at 1.95 (1.95) TeV in the Yang–Mills scenario
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