ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fernandez Martinez, P., et al. (2015). Constraints on the off-shell Higgs boson signal strength in the high-mass ZZ and WW final states with the ATLAS detector. Eur. Phys. J. C, 75(7), 335–34pp.
Abstract: Measurements of the ZZ and WW final states in the mass range above the 2m(Z) and 2m(W) thresholds provide a unique opportunity to measure the off-shell coupling strength of the Higgs boson. This paper presents constraints on the off-shell Higgs boson event yields normalised to the Standard Model prediction (signal strength) in the ZZ -> 4l, ZZ -> 2l2 nu and WW -> e nu μnu final states. The result is based on pp collision data collected by the ATLAS experiment at the LHC, corresponding to an integrated luminosity of 20.3 fb(-1) at a collision energy of root s = 8 TeV. Using the CLs method, the observed 95 % confidence level (CL) upper limit on the off-shell signal strength is in the range 5.1-8.6, with an expected range of 6.7-11.0. In each case the range is determined by varying the unknown gg -> ZZ and gg -> WW background K-factor from higher-order quantum chromodynamics corrections between half and twice the value of the known signal K-factor. Assuming the relevant Higgs boson couplings are independent of the energy scale of the Higgs boson production, a combination with the on-shell measurements yields an observed (expected) 95 % CL upper limit on Gamma(H)/Gamma(SM)(H) in the range 4.5-7.5 (6.5-11.2) using the same variations of the background K-factor. Assuming that the unknown gg -> VV background K-factor is equal to the signal K-factor, this translates into an observed (expected) 95 % CL upper limit on the Higgs boson total width of 22.7 (33.0) MeV.
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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 supersymmetry in events containing a same-flavour opposite-sign dilepton pair, jets, and large missing transverse momentum in root s=8 TeV pp collisions with the ATLAS detector. Eur. Phys. J. C, 75(7), 318–40pp.
Abstract: Two searches for supersymmetric particles in final states containing a same-flavour opposite-sign lepton pair, jets and large missing transverse momentum are presented. The proton-proton collision data used in these searches were collected at a centre-of-mass energy root s = 8TeV by the ATLAS detector at the Large Hadron Collider and corresponds to an integrated luminosity of 20.3 fb(-1). Two leptonic production mechanisms are considered: decays of squarks and gluinos with Z bosons in the final state, resulting in a peak in the dilepton invariant mass distribution around the Z-boson mass; and decays of neutralinos (e.g.. (chi) over tilde (0)(2) -> l(+)l(-) (chi) over tilde (0)(1)), resulting in a kinematic endpoint in the dilepton invariant mass distribution. For the former, an excess of events above the expected Standard Model background is observed, with a significance of three standard deviations. In the latter case, the data are well-described by the expected Standard Model background. The results from each channel are interpreted in the context of several supersymmetric models involving the production of squarks and gluinos.
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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). Determination of spin and parity of the Higgs boson in the WW* -> ev μv decay channel with the ATLAS detector. Eur. Phys. J. C, 75(5), 231–40pp.
Abstract: Studies of the spin and parity quantum numbers of the Higgs boson in the WW* -> ev μv final state are presented, based on proton-proton collision data collected by the ATLAS detector at the Large Hadron Collider, corresponding to an integrated luminosity of 20.3 fb(-1) at a centre-of-mass energy of root s = 8 TeV. The Standard Model spin-parity J(CP) = 0(++) hypothesis is compared with alternative hypotheses for both spin andCP. The case where the observed resonance is a mixture of the Standard-Model-like Higgs boson and CP-even (J(CP) = 0(++)) or CP-odd (J(CP) = 0(+-)) Higgs boson in scenarios beyond the Standard Model is also studied. The data are found to be consistent with the Standard Model prediction and limits are placed on alternative spin and CP hypotheses, including CP mixing in different scenarios.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2015). Measurement of the forward-backward asymmetry of electron and muon pair-production in pp collisions at root s=7 TeV with the ATLAS detector. J. High Energy Phys., 09(9), 049–43pp.
Abstract: This paper presents measurements from the ATLAS experiment of the forward-backward asymmetry in the reaction pp -> Z/gamma* -> 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 root 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) theta(lept)(eff) = 0.2308 +/- 0.0005(stat.)+/- 0.0006(syst.)+/- 0.0009(PDF), where the first uncertainty corresponds to data statistics, the second to s ystematic 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.
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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 a Charged Higgs Boson Produced in the Vector-Boson Fusion Mode with Decay H-+/- -> W(+/-)Z using pp Collisions at root S=8 TeV with the ATLAS Experiment. Phys. Rev. Lett., 114(23), 231801–18pp.
Abstract: A search for a charged Higgs boson, H-+/-, decaying to a W-+/- boson and a Z boson is presented. The search is based on 20.3 fb(-1) of proton-proton collision data at a center-of-mass energy of 8 TeV recorded with the ATLAS detector at the LHC. The H-+/- boson is assumed to be produced via vector-boson fusion and the decays W-+/- -> q (q') over bar and Z -> e(+)e(-)/mu(+)mu(-) are considered. The search is performed in a range of charged Higgs boson masses from 200 to 1000 GeV. No evidence for the production of an H+ boson is observed. Upper limits of 31-1020 fb at 95% C.L. are placed on the cross section for vector-boson fusion production of an H-+/- boson times its branching fraction to W(+/-)Z. The limits are compared with predictions from the Georgi-Machacek Higgs triplet model.
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