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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Observation of a New chi(b) State in Radiative Transitions to Y(1S) and Y(2S) at ATLAS. Phys. Rev. Lett., 108(15), 152001–17pp.
Abstract: The chi(b)(np) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider at root s=7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb(-1), these states are reconstructed through their radiative decays to Y(1S,2S) with Y ->mu(+)mu(-). In addition to the mass peaks corresponding to the decay modes chi(b)(1P,2P)-> Y(1S)gamma, a new structure centered at a mass of 10.530 +/- 0.005(stat)+/- 0.009(syst) GeV is also observed, in both the Y(1S)gamma and Y(2S)gamma decay modes. This structure is interpreted as the chi(b)(3P) system.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Rapidity gap cross sections measured with the ATLAS detector in pp collisions at root s=7 TeV. Eur. Phys. J. C, 72(3), 1926–31pp.
Abstract: Pseudorapidity gap distributions in proton-proton collisions at root s = 7 TeV are studied using a minimum bias data sample with an integrated luminosity of 7.1 μb(-1). Cross sections are measured differentially in terms of Delta eta(F), the larger of the pseudorapidity regions extending to the limits of the ATLAS sensitivity, at eta = +/- 4.9, in which no final state particles are produced above a transverse momentum threshold p(T)(cut). The measurements span the region 0 < Delta eta(F) < 8 for 200 MeV < p(T)(cut) < 800 MeV. At small Delta eta(F), the data test the reliability of hadronisation models in describing rapidity and transverse momentum fluctuations in final state particle production. The measurements at larger gap sizes are dominated by contributions from the single diffractive dissociation process (pp -> Xp), enhanced by double dissociation (pp -> XY) where the invariant mass of the lighter of the two dissociation systems satisfies M-Y less than or similar to 7 GeV. The resulting cross section is ds sigma/d Delta eta(F) approximate to 1 mb for Delta eta(F) greater than or similar to 3. The large rapidity gap data are used to constrain the value of the Pomeron intercept appropriate to triple Regge models of soft diffraction. The cross section integrated over all gap sizes is compared with other LHC inelastic cross section measurements.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Study of jets produced in association with a W boson in pp collisions at root s=7 TeV with the ATLAS detector. Phys. Rev. D, 85(9), 092002–40pp.
Abstract: We report a study of final states containing a W boson and hadronic jets, produced in proton-proton collisions at a center-of-mass energy of 7 TeV. The data were collected with the ATLAS detector at the CERN LHC and comprise the full 2010 data sample of 36 pb(-1). Cross sections are determined using both the electron and muon decay modes of the W boson and are presented as a function of inclusive jet multiplicity, N-jet, for up to five jets. At each multiplicity, cross sections are presented as a function of jet transverse momentum, the scalar sum of the transverse momenta of the charged lepton, missing transverse momentum, and all jets, the invariant mass spectra of jets, and the rapidity distributions of various combinations of leptons and final-state jets. The results, corrected for all detector effects and for all backgrounds such as diboson and top quark pair production, are compared with particle-level predictions from perturbative QCD. Leading-order multiparton event generators, normalized to the next-to-next-to-leading-order total cross section for inclusive W-boson production, describe the data reasonably well for all measured inclusive jet multiplicities. Next-to-leading-order calculations from MCFM, studied here for N-jet <= 2, and BLACKHAT-SHERPA, studied here for N-jet <= 4, are found to be mostly in good agreement with the data.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for excited leptons in proton-proton collisions at root s=7 TeV with the ATLAS detector. Phys. Rev. D, 85(7), 072003–23pp.
Abstract: The ATLAS detector is used to search for excited leptons in the electromagnetic radiative decay channel l* -> l gamma. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 2.05 fb(-1). No evidence for excited leptons is found, and limits are set on the compositeness scale Lambda as a function of the excited lepton mass m(l*). In the special case where Lambda = m(l*), excited electron and muon masses below 1.87 TeV and 1.75 TeV are excluded at 95% C.L., respectively.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for extra dimensions using diphoton events in 7 TeV proton-proton collisions with the ATLAS detector ATLAS Collaboration. Phys. Lett. B, 710(4-5), 538–556.
Abstract: Using data recorded in 2011 with the ATLAS detector at the Large Hadron Collider, a search for evidence of extra spatial dimensions has been performed through an analysis of the diphoton final state. The analysis uses data corresponding to an integrated luminosity of 2.12 fb(-1) of root s = 7 TeV proton-proton collisions. The diphoton invariant mass (m(gamma gamma)) spectrum is observed to be in good agreement with the expected Standard Model background. In the large extra dimension scenario of Arkani-Hamed, Dimopoulos and Dvali, the results provide 95% CL lower limits on the fundamental Planck scale between 2.27 and 3.53 TeV, depending on the number of extra dimensions and the theoretical formalism used. The results also set 95% CL lower limits on the lightest Randall-Sundrum graviton mass of between 0.79 and 1.85 TeV, for values of the dimensionless coupling k/(M) over bar (Pl) varying from 0.01 to 0.1. Combining with previously published ATLAS results from the dielectron and dimuon final states, the 95% CL lower limit on the Randall-Sundrum graviton mass for k/(M) over bar (Pl) = 0.01 (0.1) is 0.80 (1.95) TeV.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for Scalar Bottom Quark Pair Production with the ATLAS Detector in pp Collisions at root s=7 TeV. Phys. Rev. Lett., 108(18), 181802–18pp.
Abstract: The results of a search for pair production of the scalar partners of bottom quarks in 2: 05 fb(-1) of pp collisions at root s = 7 TeV using the ATLAS experiment are reported. Scalar bottom quarks are searched for in events with large missing transverse momentum and two jets in the final state, where both jets are identified as originating from a bottom quark. In an R-parity conserving minimal supersymmetric scenario, assuming that the scalar bottom quark decays exclusively into a bottom quark and a neutralino, 95% confidence-level upper limits are obtained in the (b) over tilde (1) – (chi) over tilde (0)(1) mass plane such that for neutralino masses below 60 GeV scalar bottom masses up to 390 GeV are excluded.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for the Standard Model Higgs boson in the decay channel H -> ZZ((*)) -> 4l with 4.8 fb(-1) of pp collision data at root s=7 TeV with ATLAS. Phys. Lett. B, 710(3), 383–402.
Abstract: This Letter presents a search for the Standard Model Higgs boson in the decay channel H -> ZZ((*)) -> l(+)l(-)l(-)+l(')-, where l,l' = e or mu, using proton-proton collisions at root s = 7 TeV recorded with the ATLAS detector and corresponding to an integrated luminosity of 4.8 fb(-1). The four-lepton invariant mass distribution is compared with Standard Model background expectations to derive upper limits on the cross section of a Standard Model Higgs boson with a mass between 110 GeV and 600 GeV. The mass ranges 134-156 GeV, 182-233 GeV, 256-265 GeV and 268-415 GeV are excluded at the 95% confidence level. The largest upward deviations from the background-only hypothesis are observed for Higgs boson masses of 125 GeV, 244 GeV and 500 GeV with local significances of 2.1, 2.2 and 2.1 standard deviations, respectively. Once the look-elsewhere effect is considered, none of these excesses are significant.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Search for anomaly-mediated supersymmetry breaking with the ATLAS detector based on a disappearing-track signature in pp collisions at root s=7 TeV. Eur. Phys. J. C, 72(4), 1993–20pp.
Abstract: In models of anomaly-mediated supersymmetry breaking (AMSB), the lightest chargino is predicted to have a lifetime long enough to be detected in collider experiments. This letter explores AMSB scenarios in pp collisions at root s = 7 TeV by attempting to identify decaying charginos which result in tracks that appear to have few associated hits in the outer region of the tracking system. The search was based on data corresponding to an integrated luminosity of 1.02 fb(-1) collected with the ATLAS detector in 2011. The p(T) spectrum of candidate tracks is found to be consistent with the expectation from Standard Model background processes and constraints on the lifetime and the production cross section were obtained. In the minimal AMSB framework with m(3/2) < 32 TeV, m(0) < 1.5 TeV, tan beta = 5 and μ> 0, a chargino having mass below 92 GeV and a lifetime between 0.5 ns and 2 ns is excluded at 95 % confidence level.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Combined search for the Standard Model Higgs boson using up to 4.9 fb(-1) of pp collision data at root s=7 TeV with the ATLAS detector at the LHC. Phys. Lett. B, 710(1), 49–66.
Abstract: A combined search for the Standard Model Higgs boson with the ATLAS experiment at the LHC using datasets corresponding to integrated luminosities from 1.04 fb(-1) to 4.9 fb(-1) of pp collisions collected at root s = 7 TeV is presented. The Higgs boson mass ranges 112.9-115.5 GeV, 131-238 GeV and 251-466 GeV are excluded at the 95% confidence level (CL), while the range 124-519 GeV is expected to be excluded in the absence of a signal. An excess of events is observed around m(H) similar to 126 GeV with a local significance of 3.5 standard deviations (sigma). The local significances of H -> gamma, H -> ZZ(()*()) -> l(+)l(-)l'(+)l'(-) and H -> WW(*()) l(+)nu l'(-)(nu) over bar, the three most sensitive channels in this mass range, are 2.8 sigma, 2.1 sigma and 1.4 sigma. respectively. The global probability for the background to produce such a fluctuation anywhere in the explored Higgs boson mass range 110-600 GeV is estimated to be similar to 1.4% or, equivalently, 2.2 sigma.
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ATLAS Collaboration(Aad, G. et al), Amoros, G., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., et al. (2012). Observation of Spin Correlation in t(t)over-bar Events from pp Collisions at root s=7 TeV Using the ATLAS Detector. Phys. Rev. Lett., 108(21), 212001–19pp.
Abstract: A measurement of spin correlation in t (t) over bar production is reported using data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 2.1 fb(-1). Candidate events are selected in the dilepton topology with large missing transverse energy and at least two jets. The difference in azimuthal angle between the two charged leptons in the laboratory frame is used to extract the correlation between the top and antitop quark spins. In the helicity basis the measured degree of correlation corresponds to A(helicity) = 0.40(-0.08)(+0.09), in agreement with the next-to-leading-order standard model prediction. The hypothesis of zero spin correlation is excluded at 5.1 standard deviations.
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