Browsing by Author "ATLAS collaboration (2824 authors)"
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- Analysis of events with $b$-jets and a pair of leptons of the same charge in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detectorPublication . ATLAS collaboration (2824 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Lopes, Lourenco; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Onofre, António; Palma, Alberto; Pedro, Rute; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutAn analysis is presented of events containing jets including at least one b-tagged jet, sizeable missing transverse momentum, and at least two leptons including a pair of the same electric charge, with the scalar sum of the jet and lepton transverse momenta being large. A data sample with an integrated luminosity of 20.3 fb$^{−1}$ of pp collisions at $ \sqrt{s}=8 $ TeV recorded by the ATLAS detector at the Large Hadron Collider is used. Standard Model processes rarely produce these final states, but there are several models of physics beyond the Standard Model that predict an enhanced rate of production of such events, the ones considered here are production of vector-like quarks, enhanced four-top-quark production, pair production of chiral b′-quarks, and production of two positively charged top quarks. Eleven signal regions are defined, subsets of these regions are combined when searching for each class of models. In the three signal regions primarily sensitive to positively charged top quark pair production, the data yield is consistent with the background expectation. There are more data events than expected from background in the set of eight signal regions defined for searching for vector-like quarks and chiral b′-quarks, but the significance of the discrepancy is less than two standard deviations. The discrepancy reaches 2.5 standard deviations in the set of five signal regions defined for searching for four-top-quark production. The results are used to set 95% CL limits on various models.
- Performance of algorithms that reconstruct missing transverse momentum in $\sqrt{s}=$ 8 TeV proton-proton collisions in the ATLAS detectorPublication . ATLAS collaboration (2824 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Lopes, Lourenco; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Oleiro Seabra, Luis Filipe; Onofre, António; Palma, Alberto; Pedro, Rute; Pina, João Antonio; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThe reconstruction and calibration algorithms used to calculate missing transverse momentum ( $E_{\text {T}}^{\text {miss}}$ ) with the ATLAS detector exploit energy deposits in the calorimeter and tracks reconstructed in the inner detector as well as the muon spectrometer. Various strategies are used to suppress effects arising from additional proton–proton interactions, called pileup, concurrent with the hard-scatter processes. Tracking information is used to distinguish contributions from the pileup interactions using their vertex separation along the beam axis. The performance of the $E_{\text {T}}^{\text {miss}}$ reconstruction algorithms, especially with respect to the amount of pileup, is evaluated using data collected in proton–proton collisions at a centre-of-mass energy of 8 $\text {TeV}$ during 2012, and results are shown for a data sample corresponding to an integrated luminosity of $20.3\, \mathrm{fb}^{-1}$ . The simulation and modelling of $E_{\text {T}}^{\text {miss}}$ in events containing a Z boson decaying to two charged leptons (electrons or muons) or a W boson decaying to a charged lepton and a neutrino are compared to data. The acceptance for different event topologies, with and without high transverse momentum neutrinos, is shown for a range of threshold criteria for $E_{\text {T}}^{\text {miss}}$ , and estimates of the systematic uncertainties in the $E_{\text {T}}^{\text {miss}}$ measurements are presented.
- Search for New Phenomena in Dijet Angular Distributions in Proton-Proton Collisions at $\sqrt{s} = 8$ TeV Measured with the ATLAS DetectorPublication . ATLAS collaboration (2824 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Lopes, Lourenco; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Onofre, António; Palma, Alberto; Pedro, Rute; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for new phenomena in LHC proton-proton collisions at a center-of-mass energy of s=8 TeV was performed with the ATLAS detector using an integrated luminosity of 17.3 fb−1. The angular distributions are studied in events with at least two jets; the highest dijet mass observed is 5.5 TeV. All angular distributions are consistent with the predictions of the standard model. In a benchmark model of quark contact interactions, a compositeness scale below 8.1 TeV in a destructive interference scenario and 12.0 TeV in a constructive interference scenario is excluded at 95% C.L.; median expected limits are 8.9 TeV for the destructive interference scenario and 14.1 TeV for the constructive interference scenario.
- Search for the Standard Model Higgs boson produced in association with top quarks and decaying into $b\bar{b}$ in pp collisions at $\sqrt{s}$ = 8 TeV with the ATLAS detectorPublication . ATLAS collaboration (2824 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Lopes, Lourenco; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Onofre, António; Palma, Alberto; Pedro, Rute; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for the Standard Model Higgs boson produced in association with a top-quark pair, $t\bar{t}H$ , is presented. The analysis uses 20.3 fb$^{−1}$ of pp collision data at $\sqrt{s}=8{{\,\mathrm TeV}}$ , collected with the ATLAS detector at the Large Hadron Collider during 2012. The search is designed for the $H \rightarrow b\bar{b}$ decay mode and uses events containing one or two electrons or muons. In order to improve the sensitivity of the search, events are categorised according to their jet and b-tagged jet multiplicities. A neural network is used to discriminate between signal and background events, the latter being dominated by $t\bar{t}$ +jets production. In the single-lepton channel, variables calculated using a matrix element method are included as inputs to the neural network to improve discrimination of the irreducible $t\bar{t}{+}b\bar{b}$ background. No significant excess of events above the background expectation is found and an observed (expected) limit of 3.4 (2.2) times the Standard Model cross section is obtained at 95 % confidence level. The ratio of the measured ${t\bar{t}H}$ signal cross section to the Standard Model expectation is found to be $\mu = 1.5 \pm 1.1$ assuming a Higgs boson mass of 125 ${{\,\mathrm GeV\,}}$ .
- Study of (W/Z)H production and Higgs boson couplings using $H \rightarrow WW^{\ast}$ decays with the ATLAS detectorPublication . ATLAS collaboration (2824 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Lopes, Lourenco; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Onofre, António; Palma, Alberto; Pedro, Rute; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for Higgs boson production in association with a W or Z boson, in the H→ W W$^{∗}$ decay channel, is performed with a data sample collected with the ATLAS detector at the LHC in proton-proton collisions at centre-of-mass energies $ \sqrt{s}=7 $ TeV and 8 TeV, corresponding to integrated luminosities of 4.5 fb$^{−1}$ and 20.3 fb$^{−1}$, respectively. The WH production mode is studied in two-lepton and three-lepton final states, while two- lepton and four-lepton final states are used to search for the ZH production mode. The observed significance, for the combined W H and ZH production, is 2.5 standard deviations while a significance of 0.9 standard deviations is expected in the Standard Model Higgs boson hypothesis. The ratio of the combined W H and ZH signal yield to the Standard Model expectation, μ$_{V H}$ , is found to be μ$_{V H}$ = 3.0$_{− 1.1}^{+ 1.3}$ (stat.)$_{− 0.7}^{+ 1.0}$ (sys.) for the Higgs boson mass of 125.36 GeV. The W H and ZH production modes are also combined with the gluon fusion and vector boson fusion production modes studied in the H → W W$^{∗}$ → ℓνℓν decay channel, resulting in an overall observed significance of 6.5 standard deviations and μ$_{ggF + VBF + VH}$ = 1. 16$_{− 0.15}^{+ 0.16}$ (stat.)$_{− 0.15}^{+ 0.18}$ (sys.). The results are interpreted in terms of scaling factors of the Higgs boson couplings to vector bosons (κ$_{V}$ ) and fermions (κ$_{F}$ ), the combined results are: |κ$_{V}$ | = 1.06$_{− 0.10}^{+ 0.10}$ , |κ$_{F}$ | = 0. 85$_{− 0.20}^{+ 0.26}$ .