Browsing by Author "ATLAS collaboration (2902 authors)"
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- Direct top-quark decay width measurement in the $t\bar{t}$ lepton+jets channel at $\sqrt{s}$=8 TeV with the ATLAS experimentPublication . ATLAS collaboration (2902 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Oleiro Seabra, Luis Filipe; Onofre, António; Pedro, Rute; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThis paper presents a direct measurement of the decay width of the top quark using $t\bar{t}$ events in the lepton+jets final state. The data sample was collected by the ATLAS detector at the LHC in proton–proton collisions at a centre-of-mass energy of 8 TeV and corresponds to an integrated luminosity of 20.2 fb$^{-1}$ . The decay width of the top quark is measured using a template fit to distributions of kinematic observables associated with the hadronically and semileptonically decaying top quarks. The result, $\Gamma _t = 1.76 \pm 0.33~(\text {stat.})~ ^{+0.79-0.68}~(\text {syst.})~\text {GeV}$ for a top-quark mass of 172.5 GeV, is consistent with the prediction of the Standard Model.
- Measurement of longitudinal flow decorrelations in Pb+Pb collisions at $\sqrt{s_{\text {NN}}}=2.76$ and 5.02 TeV with the ATLAS detectorPublication . ATLAS collaboration (2902 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Oleiro Seabra, Luis Filipe; Onofre, António; Pedro, Rute; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutMeasurements of longitudinal flow decorrelations are presented for charged particles in the pseudorapidity range $|\eta|<2.4$ using 7 $\mu$b$^{-1}$ and 470 $\mu$b$^{-1}$ of Pb+Pb collisions at $\sqrt{s_{\textrm{NN}}}=2.76$ and 5.02 TeV, respectively, recorded by the ATLAS detector at the LHC. It is found that the correlation between the harmonic flow coefficients $v_n$ measured in two separated $\eta$ intervals does not factorise into the product of single-particle coefficients, and this breaking of factorisation, or flow decorrelation, increases linearly with the $\eta$ separation between the intervals. The slopes for this flow decorrelation are found to be larger at 2.76 TeV than 5.02 TeV. Higher-order moments of the correlations are also measured, and the corresponding linear coefficients for the $k^{\textrm{th}}$-moment of the $v_n$ are found to be proportional to $k$ for $v_3$, but not for $v_2$. The decorrelation effect is separated into contributions from the magnitude of $v_n$ and the event-plane orientation changing with $\eta$. These two contributions are found to be comparable. The longitudinal flow correlations are also measured between $v_n$ of different order in $n$. The longitudinal fluctuations of $v_2$ and $v_3$ are found to be independent of each other, while the longitudinal fluctuations of $v_4$ and $v_5$ are found to be driven by the nonlinear contribution from $v_2^2$ and $v_2v_3$, respectively.
- Search for a heavy Higgs boson decaying into a $Z$ boson and another heavy Higgs boson in the $\ell\ell bb$ final state in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detectorPublication . ATLAS collaboration (2902 authors); Aguilar Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Araque Espinosa, Juan Pedro; Castro, Nuno Filipe; Conde Muino, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Dias do vale, Tiago; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Goncalo, Ricardo; Jorge, Pedro; Machado Miguens, Joana; Maio, Amelia; Maneira, Jose; Oleiro Seabra, Luis Filipe; Onofre, Antonio; Costa Batalha Pedro, Rute; Pereira Peixoto, Ana Paula; Santos, Helena; Saraiva, Joao; Silva, Jose Manuel; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for a heavy neutral Higgs boson, A , decaying into a Z boson and another heavy Higgs boson, H , is performed using a data sample corresponding to an integrated luminosity of 36.1 fb −1 from proton–proton collisions at s=13 TeV recorded in 2015 and 2016 by the ATLAS detector at the Large Hadron Collider. The search considers the Z boson decaying to electrons or muons and the H boson into a pair of b -quarks. No evidence for the production of an A boson is found. Considering each production process separately, the 95% confidence-level upper limits on the pp→A→ZH production cross-section times the branching ratio H→bb are in the range of 14–830 fb for the gluon–gluon fusion process and 26–570 fb for the b -associated process for the mass ranges 130–700 GeV of the H boson and 230–800 GeV of the A boson. The results are interpreted in the context of two-Higgs-doublet models.
- Search for dark matter candidates and large extra dimensions in events with a jet and missing transverse momentum with the ATLAS detectorPublication . ATLAS collaboration (2902 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Do Valle Wemans, André; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Lopes, Lourenco; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Oliveira, Miguel Alfonso; Onofre, António; Palma, Alberto; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Veloso, Filipe; Wolters, HelmutA 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 TeV with the ATLAS experiment at the Large Hadron 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.
- Search for dark matter produced in association with bottom or top quarks in $\sqrt{s}=13$ TeV pp collisions with the ATLAS detectorPublication . ATLAS collaboration (2902 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Araque, Juan Pedro; Castro, Nuno Filipe; Conde Muiño, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Gonçalo, Ricardo; Jorge, Pedro; Machado Miguens, Joana; Maio, Amélia; Maneira, José; Oleiro Seabra, Luis Filipe; Onofre, António; Pedro, Rute; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for weakly interacting massive dark-matter particles produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and missing transverse momentum are considered. The analysis uses $36.1 \, \mathrm {fb}^{-1}$ of proton–proton collision data recorded by the ATLAS experiment at $\sqrt{s}=13$ TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are interpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour-neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross-section of 300 times the predicted rate for mediators with masses between 10 and $50\, \hbox {GeV}$ and assuming a dark-matter mass of $1 \,\hbox {GeV}$ and unitary coupling. Constraints on colour-charged scalar simplified models are also presented. Assuming a dark-matter particle mass of $35\, \hbox {GeV}$ , mediator particles with mass below $1.1\, \hbox {TeV}$ are excluded for couplings yielding a dark-matter relic density consistent with measurements.
- Search for pair production of up-type vector-like quarks and for four-top-quark events in final states with multiple $b$-jets with the ATLAS detectorPublication . ATLAS collaboration (2902 authors); Aguilar Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Araque Espinosa, Juan Pedro; Castro, Nuno Filipe; Conde Muino, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Dias DO Vale, Tiago; Faisca Rodrigues Pereira, Rui Miguel; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Goncalo, Ricardo; Jorge, Pedro; Machado Miguens, Joana; Maio, Amelia; Maneira, Jose; Oleiro Seabra, Luis Filipe; Onofre, Antonio; Costa Batalha Pedro, Rute; Pereira Peixoto, Ana Paula; Santos, Helena; Saraiva, Joao; Silva, Jose Manuel; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for pair production of up-type vector-like quarks (T ) with a significant branching ratio into a top quark and either a Standard Model Higgs boson or a Z boson is presented. The same analysis is also used to search for four-top-quark production in several new physics scenarios. The search is based on a dataset of pp collisions at $ \sqrt{s}=13 $ TeV recorded in 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider and corresponds to an integrated luminosity of 36.1 fb$^{−1}$. Data are analysed in the lepton+jets final state, characterised by an isolated electron or muon with high transverse momentum, large missing transverse momentum and multiple jets, as well as the jets + E$_{T}^{miss}$ final state, characterised by multiple jets and large missing transverse momentum. The search exploits the high multiplicity of jets identified as originating from b-quarks, and the presence of boosted, hadronically decaying top quarks and Higgs bosons reconstructed as large-radius jets, characteristic of signal events. No significant excess above the Standard Model expectation is observed, and 95% CL upper limits are set on the production cross sections for the different signal processes considered. These cross-section limits are used to derive lower limits on the mass of a vector-like T quark under several branching ratio hypotheses assuming contributions from T → W b, Zt, Ht decays. The 95% CL observed lower limits on the T quark mass range between 0.99 TeV and 1.43 TeV for all possible values of the branching ratios into the three decay modes considered, significantly extending the reach beyond that of previous searches. Additionally, upper limits on anomalous four-top-quark production are set in the context of an effective field theory model, as well as in an universal extra dimensions model.
- Search for resonant $WZ$ production in the fully leptonic final state in proton-proton collisions at $\sqrt{s} = 13$ TeV with the ATLAS detectorPublication . ATLAS collaboration (2902 authors); Aguilar Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Araque Espinosa, Juan Pedro; Castro, Nuno Filipe; Conde Muino, Patricia; Da Cunha Sargedas De Sousa, Mario Jose; Dias DO Vale, Tiago; Faisca Rodrigues Pereira, Rui Miguel; Fiolhais, Miguel; Galhardo, Bruno; Gomes, Agostinho; Goncalo, Ricardo; Jorge, Pedro; Machado Miguens, Joana; Maio, Amelia; Maneira, Jose; Oleiro Seabra, Luis Filipe; Onofre, Antonio; Costa Batalha Pedro, Rute; Pereira Peixoto, Ana Paula; Santos, Helena; Saraiva, Joao; Silva, Jose Manuel; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search for a heavy resonance decaying into WZ in the fully leptonic channel (electrons and muons) is performed. It is based on proton–proton collision data collected by the ATLAS experiment at the Large Hadron Collider at a centre-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 36.1 fb −1 . No significant excess is observed over the Standard Model predictions and limits are set on the production cross section times branching ratio of a heavy vector particle produced either in quark–antiquark fusion or through vector-boson fusion. Constraints are also obtained on the mass and couplings of a singly charged Higgs boson, in the Georgi–Machacek model, produced through vector-boson fusion.