Browsing by Author "ATLAS collaboration (2845 authors)"
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- Measurement of the angular coefficients in $Z$-boson events using electron and muon pairs from data taken at $\sqrt{s}=8$ TeV with the ATLAS detectorPublication . ATLAS collaboration (2845 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; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThe angular distributions of Drell-Yan charged lepton pairs in the vicinity of the Z-boson mass peak probe the underlying QCD dynamics of Z-boson production. This paper presents a measurement of the complete set of angular coefficients A$_{0−7}$ describing these distributions in the Z-boson Collins-Soper frame. The data analysed correspond to 20.3 fb$^{−1}$ of pp collisions at $ \sqrt{s}=8 $ TeV, collected by the ATLAS detector at the CERN LHC. The measurements are compared to the most precise fixed-order calculations currently available $ \left(\mathcal{O}\left({\alpha}_{\mathrm{s}}^2\right)\right) $ and with theoretical predictions embedded in Monte Carlo generators. The measurements are precise enough to probe QCD corrections beyond the formal accuracy of these calculations and to provide discrimination between different parton-shower models. A significant deviation from the $ \left(\mathcal{O}\left({\alpha}_{\mathrm{s}}^2\right)\right) $ predictions is observed for A$_{0}$ − A$_{2}$. Evidence is found for non-zero A$_{5,6,7}$, consistent with expectations.
- Muon reconstruction performance of the ATLAS detector in proton–proton collision data at $\sqrt{s}$ =13 TeVPublication . ATLAS collaboration (2845 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; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThis article documents the performance of the ATLAS muon identification and reconstruction using the LHC dataset recorded at $\sqrt{s} = 13$ TeV in 2015. Using a large sample of $J/\psi \rightarrow \mu \mu $ and $Z\rightarrow \mu \mu $ decays from 3.2 fb$^{-1}$ of pp collision data, measurements of the reconstruction efficiency, as well as of the momentum scale and resolution, are presented and compared to Monte Carlo simulations. The reconstruction efficiency is measured to be close to $99~\%$ over most of the covered phase space ( $|\eta |<2.5$ and $5 < p_{\mathrm {T}} < 100$ GeV). The isolation efficiency varies between 93 and $100~\%$ depending on the selection applied and on the momentum of the muon. Both efficiencies are well reproduced in simulation. In the central region of the detector, the momentum resolution is measured to be $1.7~\%$ ( $2.3~\%$ ) for muons from $J/\psi \rightarrow \mu \mu $ ( $Z\rightarrow \mu \mu $ ) decays, and the momentum scale is known with an uncertainty of $0.05~\%$ . In the region $|\eta |>2.2$ , the $p_{\mathrm {T}} $ resolution for muons from $Z\rightarrow \mu \mu $ decays is $2.9~\%$ while the precision of the momentum scale for low- $p_{\mathrm {T}} $ muons from $J/\psi \rightarrow \mu \mu $ decays is about $0.2~\%$ .
- Search for single production of a vector-like quark via a heavy gluon in the $4b$ final state with the ATLAS detector in $pp$ collisions at $\sqrt{s} = 8$ TeVPublication . ATLAS collaboration (2845 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, HelmutA search is performed for the process pp→G⁎→BHb¯/B¯Hb→Hbb¯→bb¯bb¯ , predicted in composite Higgs scenarios, where G⁎ is a heavy colour octet vector resonance and BH a vector-like quark of charge −1/3 . The data were obtained from pp collisions at a centre-of-mass energy of 8 TeV corresponding to an integrated luminosity of 19.5 fb−1 , recorded by the ATLAS detector at the LHC. The largest background, multijet production, is estimated using a data-driven method. No significant excess of events with respect to Standard Model predictions is observed, and upper limits on the production cross section times branching ratio are set. Comparisons to the predictions from a specific benchmark model are made, resulting in lower mass limits in the two-dimensional mass plane of mG⁎ vs. mBH .