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- Measurement of charged-particle spectra in Pb+Pb collisions at $\sqrt{{s}_\mathsf{{NN}}} = 2.76$ TeV with the ATLAS detector at the LHCPublication . ATLAS collaboration (2829 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, HelmutCharged-particle spectra obtained in Pb+Pb interactions at $ \sqrt{s_{\mathrm{NN}}}=2.76 $ TeV and pp interactions at $ \sqrt{s_{\mathrm{NN}}}=2.76 $ TeV with the ATLAS detector at the LHC are presented, using data with integrated luminosities of 0.15 nb$^{−1}$ and 4.2 pb$^{−1}$, respectively, in a wide transverse momentum (0.5 < p$_{T}$ < 150 GeV) and pseudorapidity (|η| < 2) range. For Pb+Pb collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeters located on both sides of the interaction point. The nuclear modification factors R$_{AA}$ and R$_{CP}$ are presented in detail as a function of centrality, p$_{T}$ and η. They show a distinct p$_{T}$-dependence with a pronounced minimum at about 7 GeV. Above 60 GeV, R$_{AA}$ is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is 0.55 ± 0.01(stat.) ± 0.04(syst.) in the most central collisions. The R$_{AA}$ distribution is consistent with flat |η| dependence over the whole transverse momentum range in all centrality classes.
- Search for Higgs boson pair production in the $b\bar{b}b\bar{b}$ final state from pp collisions at $\sqrt{s} = 8$ TeVwith the ATLAS detectorPublication . ATLAS collaboration (2838 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 pair production $pp \rightarrow hh$ is performed with 19.5 fb$^{-1}$ of proton–proton collision data at $\sqrt{s}=8$ TeV, which were recorded by the ATLAS detector at the Large Hadron Collider in 2012. The decay products of each Higgs boson are reconstructed as a high-momentum $b\bar{b}$ system with either a pair of small-radius jets or a single large-radius jet, the latter exploiting jet substructure techniques and associated b-tagged track-jets. No evidence for resonant or non-resonant Higgs boson pair production is observed. The data are interpreted in the context of the Randall–Sundrum model with a warped extra dimension as well as the two-Higgs-doublet model. An upper limit on the cross-section for $pp\rightarrow G^{*\mathrm {KK}} \rightarrow hh \rightarrow b\bar{b}b\bar{b} $ of 3.2 (2.3) fb is set for a Kaluza–Klein graviton $G^{*\mathrm {KK}}$ mass of 1.0 (1.5) TeV, at the 95 % confidence level. The search for non-resonant Standard Model hh production sets an observed 95 % confidence level upper limit on the production cross-section $\sigma (pp \rightarrow hh \rightarrow b\bar{b}b\bar{b})$ of 202 fb, compared to a Standard Model prediction of $\sigma (pp \rightarrow hh \rightarrow b\bar{b}b\bar{b}) = 3.6 \pm 0.5$ fb.
- Measurement of the forward-backward asymmetry of electron and muon pair-production in $pp$ collisions at $\sqrt{s}$ = 7 TeV with the ATLAS detectorPublication . ATLAS collaboration (2882 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; 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; Onofre, António; Palma, Alberto; Pedro, Rute; Pina, João Antonio; Pinto, Belmiro; Santos, Helena; Saraiva, João; Silva, José; Tavares Delgado, Ademar; Veloso, Filipe; Wolters, HelmutThis paper presents measurements from the ATLAS experiment of the forward-backward asymmetry in the reaction pp → Z/γ$^{*}$ → l$^{+}$ l$^{−}$, with l being electrons or muons, and the extraction of the effective weak mixing angle. The results are based on the full set of data collected in 2011 in pp collisions at the LHC at $ \sqrt{s}=7 $ TeV, corresponding to an integrated luminosity of 4.8 fb$^{−1}$. The measured asymmetry values are found to be in agreement with the corresponding Standard Model predictions. The combination of the muon and electron channels yields a value of the effective weak mixing angle of sin$^{2}$ θ$_{eff}^{lept}$ = 0.2308 ± 0.0005(stat.) ± 0.0006(syst.) ± 0.0009(PDF), where the first uncertainty corresponds to data statistics, the second to systematic effects and the third to knowledge of the parton density functions. This result agrees with the current world average from the Particle Data Group fit.
- Search for Higgs bosons decaying to $aa$ in the $\mu\mu\tau\tau$ final state in $pp$ collisions at $\sqrt{s} = $ 8 TeV with the ATLAS experimentPublication . ATLAS collaboration (2826 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 decay to a pair of new particles of either the 125 GeV Higgs boson (h) or a second charge parity (CP)-even Higgs boson (H) is presented. The data set corresponds to an integrated luminosity of 20.3 fb-1 of pp collisions at s=8 TeV recorded by the ATLAS experiment at the LHC in 2012. The search was done in the context of the next-to-minimal supersymmetric standard model, in which the new particles are the lightest neutral pseudoscalar Higgs bosons (a). One of the two a bosons is required to decay to two muons while the other is required to decay to two τ leptons. No significant excess is observed above the expected backgrounds in the dimuon invariant mass range from 3.7 to 50 GeV. Upper limits are placed on the production of h→aa relative to the standard model gg→h production, assuming no coupling of the a boson to quarks. The most stringent limit is placed at 3.5% for ma=3.75 GeV. Upper limits are also placed on the production cross section of H→aa from 2.33 to 0.72 pb, for fixed ma=5 GeV with mH ranging from 100 to 500 GeV.