19 research outputs found

    Search for supersymmetric partners of the top quark in events with two leptons in the final state with the ATLAS experiment at the LHC

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    The search for Supersymmetric extensions of the Standard Model (SUSY) remains a hot topic in high energy physics in the light of the discovery of the Higgs boson with mass of 125 GeV. Supersymmetric particles can cancel out the quadratically-divergent loop corrections to the Higgs boson mass and can explain presence of Dark Matter in the Universe. Moreover, SUSY can unify the gauge couplings of the Standard Model (SM) at high energy scales. Under certain theoretical assumptions, the supersymmetric partner of the top quark, i.e. the top squark, is preferred to be lighter than one TeV and its discovery can thus be accessible at the LHC. This thesis describes a search for pair production of top squarks in final states with two leptons with the ATLAS experiment at the LHC, using 4.7 inverse fb and 20.3 inverse fb of proton-proton collisions at a center of mass energy of 7 TeV and 8 TeV, respectively. No excess over the SM predictions has been observed and results of the searches are used to place exclusion limits on the parameters of the models.openDottorato di ricerca in Matematica e fisicaopenBrazzale, Simone Federic

    Search for supersymmetric partners of the top quark in events with two leptons in the final state with the ATLAS experiment at the LHC

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    The search for Supersymmetric extensions of the Standard Model (SUSY) remains a hot topic in high energy physics in the light of the discovery of the Higgs boson with mass of 125 GeV. Supersymmetric particles can cancel out the quadratically-divergent loop corrections to the Higgs boson mass and can explain presence of Dark Matter in the Universe. Moreover, SUSY can unify the gauge couplings of the Standard Model (SM) at high energy scales. Under certain theoretical assumptions, the supersymmetric partner of the top quark, i.e. the top squark, is preferred to be lighter than one TeV and its discovery can thus be accessible at the LHC. This thesis describes a search for pair production of top squarks in final states with two leptons with the ATLAS experiment at the LHC, using 4.7 inverse fb and 20.3 inverse fb of proton-proton collisions at a center of mass energy of 7 TeV and 8 TeV, respectively. No excess over the SM predictions has been observed and results of the searches are used to place exclusion limits on the parameters of the models

    Study of the B-c(+) -> J/psi D-s(+) and Bc(+) -> J/psi D-s*(+) decays with the ATLAS detector

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    The decays B-c(+) -> J/psi D-s(+) and B-c(+) -> J/psi D-s*(+) are studied with the ATLAS detector at the LHC using a dataset corresponding to integrated luminosities of 4.9 and 20.6 fb(-1) of pp collisions collected at centre-of-mass energies root s = 7 TeV and 8 TeV, respectively. Signal candidates are identified through J/psi -> mu(+)mu(-) and D-s(()*()+) -> phi pi(+)(gamma/pi(0)) decays. With a two-dimensional likelihood fit involving the B-c(+) reconstructed invariant mass and an angle between the mu(+) and D-s(+) candidate momenta in the muon pair rest frame, the yields of B-c(+) -> J/psi D-s(+) and B-c(+) -> J/psi D-s*(+), and the transverse polarisation fraction in B-c(+) -> J/psi D-s*(+) decay are measured. The transverse polarisation fraction is determined to be Gamma +/-+/-(B-c(+) -> J/psi D-s*(+))/Gamma(B-c(+) -> J/psi D-s*(+)) = 0.38 +/- 0.23 +/- 0.07, and the derived ratio of the branching fractions of the two modes is B-Bc+ -> J/psi D-s*+/B-Bc+ -> J/psi D-s(+) = 2.8(-0.8)(+1.2) +/- 0.3, where the first error is statistical and the second is systematic. Finally, a sample of B-c(+) -> J/psi pi(+) decays is used to derive the ratios of branching fractions B-Bc+ -> J/psi D-s*+/B-Bc+ -> J/psi pi(+) = 3.8 +/- 1.1 +/- 0.4 +/- 0.2 and B-Bc+ -> J/psi D-s*+/B-Bc+ -> J/psi pi(+) = 10.4 +/- 3.1 +/- 1.5 +/- 0.6, where the third error corresponds to the uncertainty of the branching fraction of D-s(+) -> phi(K+ K-)pi(+) decay. The available theoretical predictions are generally consistent with the measurement

    ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider

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    Searching for pair production of top squarks in s\sqrt s = 7 TeV and s\sqrt s = 8 TeV proton-proton collisions and final state swith two leptons at the ATLAS experiment

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    The search for Supersymmetric extensions of the Standard Model (SUSY) remains a hot topic in high energy physycs in the light of the discovery of the Higgs boson with mass of 125 GeV. Supersymmetric particles can cancel out the quadratically-divergent loop corrections to the Higgs boson mass and can explain presence of Dark Matter in the Universe. Moreover, SUSY can unify the gauge couplings of the Standard Model at high energy scales. Under certain theoretical assumptions, the supersymmetric partner of the top quark, i.e. the top squark, is preferred to be lighter than one TeV and its discovery can thus be accessible at the LHC

    Overview of SUSY results from the ATLAS experiment

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    The search for Supersymmetric extensions of the Standard Model (SUSY) remains a hot topic in high energy phisycs in the light of the discovery of the Higgs boson with mass of 125 GeV. Supersymmetric particles can cancel out the quadratically-divergent loop corrections to the Higgs boson mass and can explain presence of Dark Matter in the Universe. Moreover, SUSY can unify the gauge couplings of the Standard Model at high energy scales. Under certain theoretical assumptions, some of the super-symmetric particles are preferred to be lighter than one TeV and their discovery can thus be accessible at the LHC. The recent results from searches for Supersymmetry with the ATLAS experiment which utilized up to 21 fb−1 of proton-proton collisions at a center of mass energy of 8 TeV are presented. These searches are focused on inclusive production of squarks and gluinos, on production of third generations squarks, and on electroweak production of charginos and neutralinos. Searches for long-lived particles and R-parity violation are also summarized in the document

    SARS-CoV-2 antibody dynamics and transmission from community-wide serological testing in the Italian municipality of Vo'

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    none36In February and March 2020, two mass swab testing campaigns were conducted in Vo', Italy. In May 2020, we tested 86% of the Vo' population with three immuno-assays detecting antibodies against the spike and nucleocapsid antigens, a neutralisation assay and Polymerase Chain Reaction (PCR). Subjects testing positive to PCR in February/March or a serological assay in May were tested again in November. Here we report on the results of the analysis of the May and November surveys. We estimate a seroprevalence of 3.5% (95% Credible Interval (CrI): 2.8-4.3%) in May. In November, 98.8% (95% Confidence Interval (CI): 93.7-100.0%) of sera which tested positive in May still reacted against at least one antigen; 18.6% (95% CI: 11.0-28.5%) showed an increase of antibody or neutralisation reactivity from May. Analysis of the serostatus of the members of 1,118 households indicates a 26.0% (95% CrI: 17.2-36.9%) Susceptible-Infectious Transmission Probability. Contact tracing had limited impact on epidemic suppression.restrictedDorigatti, Ilaria; Lavezzo, Enrico; Manuto, Laura; Ciavarella, Constanze; Pacenti, Monia; Boldrin, Caterina; Cattai, Margherita; Saluzzo, Francesca; Franchin, Elisa; Del Vecchio, Claudia; Caldart, Federico; Castelli, Gioele; Nicoletti, Michele; Nieddu, Eleonora; Salvadoretti, Elisa; Labella, Beatrice; Fava, Ludovico; Guglielmo, Simone; Fascina, Mariateresa; Grazioli, Marco; Alvisi, Gualtiero; Vanuzzo, Maria Cristina; Zupo, Tiziano; Calandrin, Reginetta; Lisi, Vittoria; Rossi, Lucia; Castagliuolo, Ignazio; Merigliano, Stefano; Unwin, H Juliette T; Plebani, Mario; Padoan, Andrea; Brazzale, Alessandra R; Toppo, Stefano; Ferguson, Neil M; Donnelly, Christl A; Crisanti, AndreaDorigatti, Ilaria; Lavezzo, Enrico; Manuto, Laura; Ciavarella, Constanze; Pacenti, Monia; Boldrin, Caterina; Cattai, Margherita; Saluzzo, Francesca; Franchin, Elisa; Del Vecchio, Claudia; Caldart, Federico; Castelli, Gioele; Nicoletti, Michele; Nieddu, Eleonora; Salvadoretti, Elisa; Labella, Beatrice; Fava, Ludovico; Guglielmo, Simone; Fascina, Mariateresa; Grazioli, Marco; Alvisi, Gualtiero; Vanuzzo, Maria Cristina; Zupo, Tiziano; Calandrin, Reginetta; Lisi, Vittoria; Rossi, Lucia; Castagliuolo, Ignazio; Merigliano, Stefano; Unwin, H Juliette T; Plebani, Mario; Padoan, Andrea; Brazzale, Alessandra R; Toppo, Stefano; Ferguson, Neil M; Donnelly, Christl A; Crisanti, Andre

    Search for Scalar Diphoton Resonances in the Mass Range 6560065-600 GeV with the ATLAS Detector in pppp Collision Data at s\sqrt{s} = 8 TeVTeV

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    A search for scalar particles decaying via narrow resonances into two photons in the mass range 65–600 GeV is performed using 20.3fb120.3\text{}\text{}{\mathrm{fb}}^{-1} of s=8TeV\sqrt{s}=8\text{}\text{}\mathrm{TeV} pppp collision data collected with the ATLAS detector at the Large Hadron Collider. The recently discovered Higgs boson is treated as a background. No significant evidence for an additional signal is observed. The results are presented as limits at the 95% confidence level on the production cross section of a scalar boson times branching ratio into two photons, in a fiducial volume where the reconstruction efficiency is approximately independent of the event topology. The upper limits set extend over a considerably wider mass range than previous searches

    Measurements of the Total and Differential Higgs Boson Production Cross Sections Combining the H??????? and H???ZZ*???4??? Decay Channels at s\sqrt{s}=8??????TeV with the ATLAS Detector

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    Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3~fb1^{-1} of pppp collisions produced by the Large Hadron Collider at a center-of-mass energy of s=8\sqrt{s} = 8 TeV and recorded by the ATLAS detector. Cross sections are obtained from measured HγγH \rightarrow \gamma \gamma and HZZ4H \rightarrow ZZ ^{*}\rightarrow 4\ell event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σppH=33.0±5.3(stat)±1.6(sys)pb\sigma_{pp \to H} = 33.0 \pm 5.3 \, ({\rm stat}) \pm 1.6 \, ({\rm sys}) \mathrm{pb}. The measurements are compared to state-of-the-art predictions.Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3  fb-1 of pp collisions produced by the Large Hadron Collider at a center-of-mass energy of s=8  TeV and recorded by the ATLAS detector. Cross sections are obtained from measured H→γγ and H→ZZ*→4ℓ event yields, which are combined accounting for detector efficiencies, fiducial acceptances, and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σpp→H=33.0±5.3 (stat)±1.6 (syst)  pb. The measurements are compared to state-of-the-art predictions.Measurements of the total and differential cross sections of Higgs boson production are performed using 20.3 fb1^{-1} of pppp collisions produced by the Large Hadron Collider at a center-of-mass energy of s=8\sqrt{s} = 8 TeV and recorded by the ATLAS detector. Cross sections are obtained from measured HγγH \rightarrow \gamma \gamma and HZZ4H \rightarrow ZZ ^{*}\rightarrow 4\ell event yields, which are combined accounting for detector efficiencies, fiducial acceptances and branching fractions. Differential cross sections are reported as a function of Higgs boson transverse momentum, Higgs boson rapidity, number of jets in the event, and transverse momentum of the leading jet. The total production cross section is determined to be σppH=33.0±5.3(stat)±1.6(sys)pb\sigma_{pp \to H} = 33.0 \pm 5.3 \, ({\rm stat}) \pm 1.6 \, ({\rm sys}) \mathrm{pb}. The measurements are compared to state-of-the-art predictions

    Search for Higgs and ZZ Boson Decays to J/ψγJ/\psi\gamma and Υ(nS)γ\Upsilon(nS)\gamma with the ATLAS Detector

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    A search for the decays of the Higgs and ZZ bosons to J/ψγJ/\psi\gamma and Υ(nS)γ\Upsilon(nS)\gamma (n=1,2,3n=1,2,3) is performed with pppp collision data samples corresponding to integrated luminosities of up to 20.3fb120.3\mathrm{fb}^{-1} collected at s=8TeV\sqrt{s}=8\mathrm{TeV} with the ATLAS detector at the CERN Large Hadron Collider. No significant excess of events is observed above expected backgrounds and 95% CL upper limits are placed on the branching fractions. In the J/ψγJ/\psi\gamma final state the limits are 1.5×1031.5\times10^{-3} and 2.6×1062.6\times10^{-6} for the Higgs and ZZ bosons, respectively, while in the Υ(1S,2S,3S)γ\Upsilon(1S,2S,3S)\,\gamma final states the limits are (1.3,1.9,1.3)×103(1.3,1.9,1.3)\times10^{-3} and (3.4,6.5,5.4)×106(3.4,6.5,5.4)\times10^{-6}, respectively
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