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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. (2016). Search for a high-mass Higgs boson decaying to a W boson pair in pp collisions at root s=8TeV with the ATLAS detector. J. High Energy Phys., 01(1), 032–66pp.
Abstract: A search for a high-mass Higgs boson H is performed in the H -> WW -> l nu l nu and H -> WW -> l nu qq decay channels using pp collision data corresponding to an integrated luminosity of 20.3 fb(-1) collected at root s = 8TeV by the ATLAS detector at the Large Hadron Collider. No evidence of a high-mass Higgs boson is found. Limits on sigma(H) x BR(H -> WW) as a function of the Higgs boson mass m(H) are determined in three different scenarios: one in which the heavy Higgs boson has a narrow width compared to the experimental resolution, one for a width increasing with the boson mass and modeled by the complex-pole scheme following the same behavior as in the Standard Model, and one for intermediate widths. The upper range of the search is m(H) = 1500 GeV for the narrow-width scenario and m(H) = 1000 GeV for the other two scenarios. The lower edge of the search range is 200{300 GeV and depends on the analysis channel and search scenario. For each signal interpretation, individual and combined limits from the two WW decay channels are presented. At m(H) = 1500 GeV, the highest-mass point tested, sigma(H) x BR(H -> WW) for a narrow-width Higgs boson is constrained to be less than 22 fb and 6.6 fb at 95% CL for the gluon fusion and vector-boson fusion production modes, respectively.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2015). Search for flavour-changing neutral current top quark decays t -> Hq in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 12(12), 061–65pp.
Abstract: A search for flavour-changing neutral current decays of a top quark to an up-type quark (q = u; c) and the Standard Model Higgs boson, where the Higgs boson decays to b (b) over bar, is presented. The analysis searches for top quark pair events in which one top quark decays to W b, with the W boson decaying leptonically, and the other top quark decays to Hq. The search is based on pp collisions at root s = 8TeV recorded in 2012 with the ATLAS detector at the CERN Large Hadron Collider and uses an integrated luminosity of 20.3 fb(-1). Data are analysed in the lepton-plus-jets final state, characterised by an isolated electron or muon and at least four jets. The search exploits the high multiplicity of b quark jets characteristic of signal events, and employs a likelihood discriminant that uses the kinematic differences between the signal and the background, which is dominated by t (t) over bar -> WbWb decays. No significant excess of events above the background expectation is found, and observed (expected) 95% CL upper limits of 0.56% (0.42%) and 0.61% (0.64%) are derived for the t -> Hc and t -> Hu branching ratios respectively. The combination of this search with other ATLAS searches in the H -> gamma gamma and H -> WW*, tau tau decay modes significantly improves the sensitivity, yielding observed (expected) 95% CL upper limits on the t -> Hc and t -> Hu branching ratios of 0.46% (0.25%) and 0.45% (0.29%) respectively. The corresponding combined observed (expected) upper limits on the vertical bar lambda(tcH)vertical bar and vertical bar lambda(tuH)vertical bar couplings are 0.13 (0.10) and 0.13 (0.10) respectively. These are the most restrictive direct bounds on t q H interactions measured so far.
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., Costa, M. J., et al. (2016). Search for charged Higgs bosons in the H-+/- -> tb decay channel in pp collisions at root s=8 TeV using the ATLAS detector. J. High Energy Phys., 03(3), 127–48pp.
Abstract: Charged Higgs bosons heavier than the top quark and decaying via H-+/- -> tb are searched for in proton-proton collisions measured with the ATLAS experiment at root s = 8 TeV corresponding to an integrated luminosity of 20.3 fb(-1). The production of a charged Higgs boson in association with a top quark, gb -> tH(+/-), is explored in the mass range 200 to 600 GeV using multi-jet final states with one electron or muon. In order to separate the signal from the Standard Model background, analysis techniques combining several kinematic variables are employed. An excess of events above the background only hypothesis is observed across a wide mass range, amounting to up to 2.4 standard deviations. Upper limits are set on the gb -> tH(+/-) production cross section times the branching fraction BR(H-+/- -> tb). Additionally, the complementary s-channel production, qq' -> H-+/-, is investigated through a reinterpretation of W' -> tb searches in ATLAS. Final states with one electron or muon are relevant for H-+/- masses from 0.4 to 2.0 TeV, whereas the all-hadronic final state covers the range 1.5 to 3.0 TeV. In these search channels, no significant excesses from the predictions of the Standard Model are observed, and upper limits are placed on the qq' -> H-+/- production cross section times the branching fraction BR(H-+/- -> tb).
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ATLAS Collaboration(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., et al. (2020). Search for the HH ->b bbar b bbar process via vector-boson fusion production using proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 108–37pp.
Abstract: A search for Higgs boson pair production via vector-boson fusion (VBF) in the b bbar b bbar final state is carried out with the ATLAS experiment using 126 fb(-1) of proton- proton collision data delivered at root s = 13 TeV by the Large Hadron Collider. This search is sensitive to VBF production of additional heavy bosons that may decay into Higgs boson pairs, and in a non-resonant topology it can constrain the quartic coupling between the Higgs bosons and vector bosons. No significant excess relative to the Standard Model expectation is observed, and limits on the production cross-section are set at the 95% confidence level for a heavy scalar resonance in the context of an extended Higgs sector, and for non-resonant Higgs boson pair production. Interpretation in terms of the coupling between a Higgs boson pair and two vector bosons is also provided: coupling values normalised to the Standard Model expectation of kappa (2V)< -0.76 and <kappa>(2V)> 2.90 are excluded at the 95% confidence level in data.
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ATLAS Collaboration(Aad, G. et al), Akiot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2023). Search for flavour-changing neutral tqH interactions with H → γγ in pp collisions at √s=13 TeV using the ATLAS detector. J. High Energy Phys., 12(12), 195–53pp.
Abstract: A search for flavour-changing neutral interactions involving the top quark, the Higgs boson and an up-type quark q ( q = c, u) is presented. The proton-proton collision data set used, with an integrated luminosity of 139 fb(-1), was collected at root s = 13TeV by the ATLAS experiment at the Large Hadron Collider. Both the decay process t -> qH in tt production and the production process pp. tH, with the Higgs boson decaying into two photons, are investigated. No significant excess is observed and upper limits are set on the t. cH and the t. uH branching ratios of 4.3x10(-4) and 3.8x10(-4), respectively, at the 95% confidence level, while the expected limits in the absence of signal are 4.7x10(-4) and 3.9x10(-4). Combining this search with ATLAS searches in the H. t+ t- and H. b <overline> b final states yields observed (expected) upper limits on the t -> cH branching ratio of 5.8 x 10(-4) (3.0 x 10(-4)) at the 95% confidence level. The corresponding observed (expected) upper limit on the t -> uH branching ratio is 4.0 x 10(-4) (2.4 x 10(-4)).
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ATLAS Collaboration(Aad, G. et al), Aikot, A., Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., et al. (2024). Studies of new Higgs boson interactions through nonresonant HH production in the b(b)over-barγγ final state in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 01(1), 066–48pp.
Abstract: A search for nonresonant Higgs boson pair production in the b (b) over bar gamma gamma final state is performed using 140 fb(-1) of proton-proton collisions at a centre-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. This analysis supersedes and expands upon the previous nonresonant ATLAS results in this final state based on the same data sample. The analysis strategy is optimised to probe anomalous values not only of the Higgs (H) boson self-coupling modifier kappa(lambda) but also of the quartic HHVV (V = W, Z) coupling modifier kappa(2V). No significant excess above the expected background from Standard Model processes is observed. An observed upper limit mu(HH) < 4.0 is set at 95% confidence level on the Higgs boson pair production cross-section normalised to its Standard Model prediction. The 95% confidence intervals for the coupling modifiers are -1.4 < kappa(lambda) < 6.9 and -0.5 < kappa(2V) < 2.7, assuming all other Higgs boson couplings except the one under study are fixed to the Standard Model predictions. The results are interpreted in the Standard Model effective field theory and Higgs effective field theory frameworks in terms of constraints on the couplings of anomalous Higgs boson (self-)interactions.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Search for the Standard Model Higgs boson produced by vector-boson fusion and decaying to bottom quarks in root s=8 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 11(11), 112–37pp.
Abstract: A search with the ATLAS detector is presented for the Standard Model Higgs boson produced by vector-boson fusion and decaying to a pair of bottom quarks, using 20.2 fb(-1) of LHC proton-proton collision data at root s – 8 TeV. The signal is searched for as a resonance in the invariant mass distribution of a pair of jets containing b-hadrons in vector-boson-fusion candidate events. The yield is measured to be -0.8 +/- 2.3 times the Standard Model cross-section for a Higgs boson mass of 125 GeV. The upper limit on the cross-section times the branching ratio is found to be 4.4 times the Standard Model cross-section at the 95% confidence level, consistent with the expected limit value of 5.4 (5.7) in the background-only (Standard Model production) hypothesis.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for direct top squark pair production in events with a Higgs or Z boson, and missing transverse momentum in root s=13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 08(8), 006–46pp.
Abstract: A search for direct top squark pair production resulting in events with either a same-flavour opposite-sign dilepton pair with invariant mass compatible with a Z boson or a pair of jets compatible with a Standard Model (SM) Higgs boson (h) is presented. Requirements on the missing transverse momentum, together with additional selections on leptons, jets, jets identified as originating from b-quarks are imposed to target the other decay products of the top squark pair. The analysis is performed using proton-proton collision data at root s = 13 TeV collected with the ATLAS detector at the LHC in 20152016, corresponding to an integrated luminosity of 36.1 fb(-1). No excess is observed in the data with respect to the SM predictions. The results are interpreted in two sets of models. In the first set, direct production of pairs of lighter top squarks ((t) over tilde (1)) with long decay chains involving Z or Higgs bosons is considered. The second set includes direct pair production of the heavier top squark pairs ((t) over tilde (2)) decaying via (t) over tilde (2) -> Z (t) over tilde (1) or (t) over tilde (2) -> h (t) over tilde (1). The results exclude at 95% confidence level (t) over tilde (2) and (t) over tilde (1) masses up to about 800 GeV, extending the exclusion region of supersymmetric parameter space covered by previous LHC searches.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Searches for the Z gamma decay mode of the Higgs boson and for new high-mass resonances in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 112–51pp.
Abstract: This article presents searches for the Z gamma decay of the Higgs boson and for narrow high-mass resonances decaying to Z gamma, exploiting Z boson decays to pairs of electrons or muons. The data analysis uses 36.1 fb(-1) of pp collisions at root s = 13 recorded by the ATLAS detector at the CERN Large Hadron Collider. The data are found to be consistent with the expected Standard Model background. The observed (expected – assuming Standard Model pp -> H -> Z gamma production and decay) upper limit on the production cross section times the branching ratio for pp -> H -> Z gamma is 6.6. (5.2) times the Standard Model prediction at the 95% confidence level for a Higgs boson mass of 125.09 GeV. In addition, upper limits are set on the production cross section times the branching ratio as a function of the mass of a narrow resonance between 250 GeV and 2.4 TeV, assuming spin-0 resonances produced via gluon-gluon fusion, and spin-2 resonances produced via gluon-gluon or quark-antiquark initial states. For high-mass spin-0 resonances, the observed (expected) limits vary between 88 fb (61 fb) and 2.8 fb (2.7 fb) for the mass range from 250 GeV to 2.4 TeV at the 95% confidence level.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Measurement of inclusive and differential cross sections in the H -> ZZ* -> 4l decay channel in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 132–49pp.
Abstract: Inclusive and differential fiducial cross sections of Higgs boson production in proton-proton collisions are measured in the H -> Z Z* -> 4l 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 36.1 fb(-1). The inclusive fiducial cross section in the H -> Z Z* -> 4l decay channel is measured to be 3.62 +/- 0.50 (stat) (+0.25)(-0.20) (sys) fb, in agreement with the Standard Model prediction of 2.91 +/- 0.13 fb. The cross section is also extrapolated to the total phase space including all Standard Model Higgs boson decays. Several differential fiducial cross sections are measured for observables sensitive to the Higgs boson production and decay, including kinematic distributions of jets produced in association with the Higgs boson. Good agreement is found between data and Standard Model predictions. The results are used to put constraints on anomalous Higgs boson interactions with Standard Model particles, using the pseudo-observable extension to the kappa-framework.
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