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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Search for a Heavy Neutral Particle Decaying to e mu, e tau, or μtau in pp Collisions at root s=8 TeV with the ATLAS Detector. Phys. Rev. Lett., 115(3), 031801–18pp.
Abstract: This Letter presents a search for a heavy neutral particle decaying into an opposite-sign different-flavor dilepton pair, e(+/-) mu(-/+), e(+/-) tau(-/+), or mu(+/-) tau(-/+) using 20.3 fb(-1) of pp collision data at root s = 8 TeV collected by the ATLAS detector at the LHC. The numbers of observed candidate events are compatible with the standard model expectations. Limits are set on the cross section of new phenomena in two scenarios: the production of (nu) over tilde (tau) in R-parity-violating supersymmetric models and the production of a lepton-flavor-violating Z' vector boson.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Observation and measurement of Higgs boson decays to WW* with the ATLAS detector. Phys. Rev. D, 92(1), 012006–84pp.
Abstract: We report the observation of Higgs boson decays to WW* based on an excess over background of 6.1 standard deviations in the dilepton final state, where the Standard Model expectation is 5.8 standard deviations. Evidence for the vector-boson fusion (VBF) production process is obtained with a significance of 3.2 standard deviations. The results are obtained from a data sample corresponding to an integrated luminosity of 25 fb(-1) from root s = 7 and 8 TeV pp collisions recorded by the ATLAS detector at the LHC. For a Higgs boson mass of 125.36 GeV, the ratio of the measured value to the expected value of the total production cross section times branching fraction is 1.09(-0.15)(+0.16)(stat)(-0.14)(+0.17)(syst). The corresponding ratios for the gluon fusion and vector-boson fusion production mechanisms are 1.02 +/- 0.19(stat)(-0.18)(+0.22)(syst) and 1.27(-0.40)(+0.44)(stat)(-0.21)(+0.30)(syst), respectively. At root s = 8 TeV, the total production cross sections are measured to be sigma(gg -> H -> WW*) = 4.6 +/- 0.9(stat)(-0.7)(+0.8)(syst) pb and sigma(VBF H -> WW*) = 0.51(-0.15)(+0.17)(stat)(-0.08)(+0.13)(syst) pb. The fiducial cross section is determined for the gluon-fusion process in exclusive final states with zero or one associated jet.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., Ferrer, A., et al. (2015). Evidence of W gamma gamma Production in pp Collisions at root s=8 TeV and Limits on Anomalous Quartic Gauge Couplings with the ATLAS Detector. Phys. Rev. Lett., 115(3), 031802–18pp.
Abstract: This Letter reports evidence of triple gauge boson production pp -> W(l nu)gamma gamma + X, which is accessible for the first time with the 8 TeV LHC data set. The fiducial cross section for this process is measured in a data sample corresponding to an integrated luminosity of 20.3 fb(-1), collected by the ATLAS detector in 2012. Events are selected using the W boson decay to e nu or μnu as well as requiring two isolated photons. The measured cross section is used to set limits on anomalous quartic gauge couplings in the high diphoton mass region.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Search for type-III seesaw heavy leptons in pp collisions at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 92(3), 032001–20pp.
Abstract: A search for the pair production of heavy leptons (N-0, L-+/-) predicted by the type-III seesaw theory formulated to explain the origin of small neutrino masses is presented. The decay channels N-0 -> W(+/-)l(+/-) (l = e, mu, tau) and L-+/- -> W(+/-)v (v = v(e), v(mu), v(tau)) are considered. The analysis is performed using the final state that contains two leptons (electrons or muons), two jets from a hadronically decaying W boson and large missing transverse momentum. The data used in the measurement correspond to an integrated luminosity of 20.3 fb(-1) of pp collisions at root s = 8 TeV collected by the ATLAS detector at the LHC. No evidence of heavy lepton pair production is observed. Heavy leptons with masses below 325-540 GeV are excluded at the 95% confidence level, depending on the theoretical scenario considered.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Search for long-lived, weakly interacting particles that decay to displaced hadronic jets in proton-proton collisions at root s=8 TeV with the ATLAS detector. Phys. Rev. D, 92(1), 012010–28pp.
Abstract: A search for the decay of neutral, weakly interacting, long-lived particles using data collected by the ATLAS detector at the LHC is presented. This analysis uses the full data set recorded in 2012: 20.3 fb(-1) of proton-proton collision data at root s = 8 TeV. The search employs techniques for reconstructing decay vertices of long-lived particles decaying to jets in the inner tracking detector and muon spectrometer. Signal events require at least two reconstructed vertices. No significant excess of events over the expected background is found, and limits as a function of proper lifetime are reported for the decay of the Higgs boson and other scalar bosons to long-lived particles and for Hidden Valley Z' and Stealth SUSY benchmark models. The first search results for displaced decays in Z' and Stealth SUSY models are presented. The upper bounds of the excluded proper lifetimes are the most stringent to date.
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