Browsing by Author "ATLAS collaboration (2935 authors)"
Now showing 1 - 7 of 7
Results Per Page
Sort Options
- Measurement of the mass difference between top and anti-top quarks in pp collisions at $\sqrt(s) = 7$ TeV using the ATLAS detectorPublication . ATLAS collaboration (2935 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Costa Batalha Pedro, Rute; 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; Marques, Carlos; Oliveira, Miguel Alfonso; Onofre, António; Palma, Alberto; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA measurement of the mass difference between top and anti-top quarks is presented. In a 4.7 fb$^{-1}$ data sample of proton–proton collisions at √s = 7 TeV recorded with the ATLAS detector at the LHC, events consistent with ttbar production and decay into a single charged lepton final state are reconstructed. For each event, the mass difference between the top and anti-top quark candidate is calculated. A two b-tag requirement is used in order to reduce the background contribution. A maximum likelihood fit to these per-event mass differences yields $\Delta m ≣ m_t - m_{\bar{t}}$ = 0.67 ± 0.61(stat) ± 0.41(syst) GeV, consistent with CPT invariance.
- Measurement of the top quark pair production charge asymmetry in proton-proton collisions at $\sqrt{s}$ = 7 TeV using the ATLAS detectorPublication . ATLAS collaboration (2935 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; Marques, Carlos; Oliveira, Miguel Alfonso; Onofre, António; Palma, Alberto; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThis paper presents a measurement of the top quark pair ($t\bar{t}$) production charge asymmetry $A_C$ using 4.7 fb−1 of proton-proton collisions at a centre-of-mass energy √s = 7 TeV collected by the ATLAS detector at the LHC. A $t\bar{t}$ enriched sample of events with a single lepton (electron or muon), missing transverse momentum and at least four high transverse momentum jets, of which at least one is tagged as coming from a b–quark, is selected. A likelihood fit is used to reconstruct the $t\bar{t}$ event kinematics. A Bayesian unfolding procedure is employed to estimate $A_C$ at the parton–level. The measured value of the $t\bar{t}$ production charge asymmetry is $A_C$ = 0.006± 0.010, where the uncertainty includes both the statistical and the systematic components. Differential $A_C$ measurements as a function of the invariant mass, the rapidity and the transverse momentum of the $t\bar{t}$-system are also presented. In addition, $A_C$ is measured for a subset of events with large $t\bar{t}$ velocity, where physics beyond the Standard Model could contribute. All measurements are consistent with the Standard Model predictions.
- Measurements of gluon-gluon fusion and vector-boson fusion Higgs boson production cross-sections in the $H \to WW^{\ast} \to e\nu\mu\nu$ decay channel in $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detectorPublication . ATLAS collaboration (2935 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; Mendes Gouveia, Emanuel Demetrio; 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, HelmutHiggs boson production cross-sections in proton–proton collisions are measured in the H→WW⁎→eνμν decay channel. The proton–proton collision data were produced at the Large Hadron Collider at a centre-of-mass energy of 13 TeV and recorded by the ATLAS detector in 2015 and 2016, corresponding to an integrated luminosity of 36.1fb−1 . The product of the H→WW⁎ branching fraction times the gluon–gluon fusion and vector-boson fusion cross-sections are measured to be 11.4−1.1+1.2(stat.)−1.7+1.8(syst.)pb and 0.50−0.22+0.24(stat.)±0.17(syst.)pb , respectively, in agreement with Standard Model predictions.
- Observation of $H \rightarrow b\bar{b}$ decays and $VH$ production with the ATLAS detectorPublication . ATLAS collaboration (2935 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; Mendes Gouveia, Emanuel Demetrio; 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 the decay of the Standard Model Higgs boson into a bb¯ pair when produced in association with a W or Z boson is performed with the ATLAS detector. The data, corresponding to an integrated luminosity of 79.8fb−1 were collected in proton–proton collisions during Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13TeV . For a Higgs boson mass of 125GeV , an excess of events over the expected background from other Standard Model processes is found with an observed (expected) significance of 4.9 (4.3) standard deviations. A combination with the results from other searches in Run 1 and in Run 2 for the Higgs boson in the bb¯ decay mode is performed, which yields an observed (expected) significance of 5.4 (5.5) standard deviations, thus providing direct observation of the Higgs boson decay into b -quarks. The ratio of the measured event yield for a Higgs boson decaying into bb¯ to the Standard Model expectation is 1.01±0.12(stat.)−0.15+0.16(syst.) . Additionally, a combination of Run 2 results searching for the Higgs boson produced in association with a vector boson yields an observed (expected) significance of 5.3 (4.8) standard deviations.
- Search for a multi-Higgs-boson cascade in $W^+W^−b\bar{b}$ events with the ATLAS detector in pp collisions at $\sqrt{s} = 8$ TeVPublication . ATLAS collaboration (2935 authors); Aguilar-Saavedra, Juan Antonio; Amor Dos Santos, Susana Patricia; Anjos, Nuno; Cantrill, Robert; Carvalho, João; Castro, Nuno Filipe; Conde Muiño, Patricia; Costa Batalha Pedro, Rute; 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; Marques, Carlos; Oliveira, Miguel Alfonso; Onofre, António; Palma, Alberto; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, Helmut
A search is presented for new particles in an extension to the Standard Model that includes a heavy Higgs boson (H0), an intermediate charged Higgs-boson pair (H±), and a light Higgs boson (h0). The analysis searches for events involving the production of a single heavy neutral Higgs boson which decays to the charged Higgs boson and a W boson, where the charged Higgs boson subsequently decays into a W boson and the lightest neutral Higgs boson decaying to a bottom–antibottom-quark pair. Such a cascade results in a W-boson pair and a bottom–antibottom-quark pair in the final state. Events with exactly one lepton, missing transverse momentum, and at least four jets are selected from a data sample corresponding to an integrated luminosity of 20.3 fb−1, collected by the ATLAS detector in proton-proton collisions at s=8 TeV at the LHC. The data are found to be consistent with Standard Model predictions, and 95% confidence-level upper limits are set on the product of cross section and branching ratio. These limits range from 0.065 to 43 pb as a function of H0 and H± masses, with mh0 fixed at 125 GeV.
- Search for charginos nearly mass degenerate with the lightest neutralino based on a disappearing-track signature in pp collisions at $\sqrt(s)$=8 TeV with the ATLAS detectorPublication . ATLAS collaboration (2935 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; Marques, Carlos; Oliveira, Miguel Alfonso; Onofre, António; Palma, Alberto; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutA search is presented for direct chargino production based on a disappearing-track signature using 20.3 fb-1 of proton-proton collisions at sqrt(s) = 8 TeV collected with the ATLAS experiment at the LHC. In anomaly-mediated supersymmetry breaking (AMSB) models, the lightest chargino is nearly mass-degenerate with the lightest neutralino and its lifetime is long enough to be detected in the tracking detectors by identifying decays that result in tracks with no associated hits in the outer region of the tracking system. Some models with supersymmetry also predict charginos with a significant lifetime. This analysis attains sensitivity for charginos with a lifetime between 0.1 ns and 10 ns, and significantly surpasses the reach of the LEP experiments. No significant excess above the background expectation is observed for candidate tracks with large transverse momentum, and constraints on chargino properties are obtained. In the AMSB scenarios, a chargino mass below 270 GeV is excluded at 95% confidence level.
- Search for excited electrons and muons in $\sqrt{s}$=8 TeV proton-proton collisions with the ATLAS detectorPublication . ATLAS collaboration (2935 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; Marques, Carlos; Oliveira, Miguel Alfonso; Onofre, António; Palma, Alberto; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThe ATLAS detector at the Large Hadron Collider is used to search for excited electrons and excited muons in the channel pp → ℓℓ* → ℓℓγ, assuming that excited leptons are produced via contact interactions. The analysis is based on 13 fb−1 of pp collisions at a centre-of-mass energy of 8 TeV. No evidence for excited leptons is found, and a limit is set at the 95% credibility level on the cross section times branching ratio as a function of the excited-lepton mass mℓ*. For mℓ* ⩾ 0.8 TeV, the respective upper limits on σB(ℓ* → ℓγ) are 0.75 and 0.90 fb for the e* and μ* searches. Limits on σB are converted into lower bounds on the compositeness scale Λ. In the special case where Λ = mℓ*, excited-electron and excited-muon masses below 2.2 TeV are excluded.