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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for direct top-squark pair production in final states with two leptons in pp collisions at root s=8 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 124–66pp.
Abstract: A search is presented for direct top-squark pair production in final states with two leptons (electrons or muons) of opposite charge using 20.3 fb(-1) of pp collision data at root s = 8 TeV, collected by the ATLAS experiment at the Large Hadron Collider in 2012. No excess over the Standard Model expectation is found. The results are interpreted under the separate assumptions (i) that the top squark decays to a b-quark in addition to an on-shell chargino whose decay occurs via a real or virtual W boson, or (ii) that the top squark decays to a t-quark and the lightest neutralino. A top squark with a mass between 150 GeV and 445 GeV decaying to a b-quark and an on-shell chargino is excluded at 95% confidence level for a top squark mass equal to the chargino mass plus 10 GeV, in the case of a 1 GeV lightest neutralino. Top squarks with masses between 215 (90) GeV and 530 (170) GeV decaying to an on-shell (off-shell) t-quark and a neutralino are excluded at 95% confidence level for a 1 GeV neutralino.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo, F. L., Castillo Gimenez, V., et al. (2023). Search in diphoton and dielectron final states for displaced production of Higgs or Z bosons with the ATLAS detector in root s=13 TeV pp collisions. Phys. Rev. D, 108(1), 012012–32pp.
Abstract: A search is presented for displaced production of Higgs bosons or Z bosons, originating from the decay of a neutral long-lived particle (LLP) and reconstructed in the decay modes H -& gamma;& gamma; and Z -ee. The analysis uses the full Run 2 dataset of proton-proton collisions delivered by the LHC at an energy of p1/4 13 TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an ffiffi s integrated luminosity of 139 fb-1. Exploiting the capabilities of the ATLAS liquid argon calorimeter to precisely measure the arrival times and trajectories of electromagnetic objects, the analysis searches for the signature of pairs of photons or electrons which arise from a common displaced vertex and which arrive after some delay at the calorimeter. The results are interpreted in a gauge-mediated supersymmetry breaking model with pair-produced Higgsinos that decay to LLPs, and each LLP subsequently decays into either a Higgs boson or a Z boson. The final state includes at least two particles that escape direct detection, giving rise to missing transverse momentum. No significant excess is observed above the background expectation. The results are used to set upper limits on the cross section for Higgsino pair production, up to a & chi;& SIM;01 mass of 369 (704) GeV for decays with 100% branching ratio of & chi; & SIM;01 to Higgs (Z) bosons for a & chi;& SIM;01 lifetime of 2 ns. A model-independent limit is also set on the production of pairs of photons or electrons with a significant delay in arrival at the calorimeter.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2020). Evidence for ttbar ttbar production in the multilepton final state in proton-proton collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(11), 1085–32pp.
Abstract: A search is presented for four-top-quark production using an integrated luminosity of 139 fb-1 of proton-proton collision data at a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the LHC. Events are selected if they contain a same-sign lepton pair or at least three leptons (electrons or muons). Jet multiplicity, jet flavour and event kinematics are used to separate signal from the background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The four-top-quark production cross section is measured to be 24-6+7 fb. This corresponds to an observed (expected) significance with respect to the background-only hypothesis of 4.3 (2.4) standard deviations and provides evidence for this process.
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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 Gluinos in Events with Two Same-Sign Leptons, Jets, and Missing Transverse Momentum with the ATLAS Detector in pp Collisions at root s=7 TeV. Phys. Rev. Lett., 108(24), 241802–19pp.
Abstract: A search is presented for gluinos decaying via the supersymmetric partner of the top quark using events with two same-sign leptons, jets, and missing transverse momentum. The analysis is performed with 2: 05 fb(-1) of integrated luminosity from pp collisions at root s = 7 TeV collected by the ATLAS detector at the LHC. No excess beyond the standard model expectation is observed, and exclusion limits are derived for simplified models where the gluino decays via the supersymmetric partner of the top quark and in the minimal supergravity and constrained minimal supersymmetric standard model framework. In those scenarios, gluino masses below 550 GeV are excluded at 95% C.L. within the parameter space considered, significantly extending the coverage with respect to existing limits. Depending on the model parameters, gluino masses up to 750 GeV can also be excluded at 95% C.L.
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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 pair-produced heavy quarks decaying to Wq in the two-lepton channel at root(s)=7 TeV with the ATLAS detector. Phys. Rev. D, 86(1), 012007–24pp.
Abstract: A search is presented for heavy-quark pair production (Q (Q) over bar) under the decay hypothesis Q (Q) over bar -> W(+)qW(-)(q) over bar with q = d, s, b for up-type Q or q = u, c for down-type Q. The search is performed with 1.04 fb(-1) of integrated luminosity from pp collisions at root s = 7 TeV collected by the ATLAS detector at the CERN LHC. Dilepton final states are selected, requiring large missing transverse momentum and at least two jets. Mass reconstruction of heavy-quark candidates is performed by assuming that the W boson decay products are nearly collinear. The data are in agreement with standard model expectations; a heavy quark with mass less than 350 GeV is excluded at 95% confidence level.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for microscopic black holes in a like-sign dimuon final state using large track multiplicity with the ATLAS detector. Physical Review D, 88(7), 072001–22pp.
Abstract: A search is presented for microscopic black holes in a like-sign dimuon final state in proton-proton collisions at root s = 8 TeV. The data were collected with the ATLAS detector at the Large Hadron Collider in 2012 and correspond to an integrated luminosity of 20.3 fb(-1). Using a high track multiplicity requirement, 0.6 +/- 0.2 background events from Standard Model processes are predicted and none observed. This result is interpreted in the context of low-scale gravity models and 95% C.L. lower limits on microscopic black hole masses are set for different model assumptions.
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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 production of WW/WZ resonances decaying to a lepton, neutrino and jets in pp collisions at root s=8 TeV with the ATLAS detector. Eur. Phys. J. C, 75(5), 209–20pp.
Abstract: A search is presented for narrow diboson resonances decaying to WW or WZ in the final state where one W boson decays leptonically (to an electron or a muon plus a neutrino) and the other W/Z boson decays hadronically. The analysis is performed using an integrated luminosity of 20.3 fb(-1) of pp collisions at root s = 8 TeV collected by the ATLAS detector at the large hadron collider. No evidence for resonant diboson production is observed, and resonance masses below 700 and 1490 GeV are excluded at 95 % confidence level for the spin-2 Randall-Sundrum bulk graviton G* with coupling constant of 1.0 and the extended gauge model W' boson respectively.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for new phenomena in final states with large jet multiplicities and missing transverse momentum at root s=8 TeV proton-proton collisions using the ATLAS experiment. J. High Energy Phys., 10(10), 130–50pp.
Abstract: A search is presented for new particles decaying to large numbers (7 or more) of jets, with missing transverse momentum and no isolated electrons or muons. This analysis uses 20.3 fb(-1) of p p collision data at root s = 8 TeV collected by the ATLAS experiment at the Large Hadron Collider. The sensitivity of the search is enhanced by considering the number of b-tagged jets and the scalar sum of masses of large-radius jets in an event. No evidence is found for physics beyond the Standard Model. The results are interpreted in the context of various simplified supersymmetry-inspired models where gluinos are pair produced, as well as an mSUGRA/CMSSM model.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2014). Search for a multi-Higgs-boson cascade in W(+)W(-)b(b)over-bar events with the ATLAS detector in pp collisions at root s=8 TeV. Phys. Rev. D, 89(3), 032002–23pp.
Abstract: A search is presented for new particles in an extension to the Standard Model that includes a heavy Higgs boson (H-0), an intermediate charged Higgs-boson pair (H-+/-), and a light Higgs boson (h(0)). The analysis searches for events involving the production of a single heavy neutral Higgs boson which decays to the charged Higgs boson and a W boson, where the charged Higgs boson subsequently decays into a W boson and the lightest neutral Higgs boson decaying to a bottom-antibottom-quark pair. Such a cascade results in a W-boson pair and a bottom-antibottom-quark pair in the final state. Events with exactly one lepton, missing transverse momentum, and at least four jets are selected from a data sample corresponding to an integrated luminosity of 20.3 fb(-1), collected by the ATLAS detector in proton-proton collisions at root s = 8 TeV at the LHC. The data are found to be consistent with Standard Model predictions, and 95% confidence-level upper limits are set on the product of cross section and branching ratio. These limits range from 0.065 to 43 pb as a function of H-0 and H-+/- masses, with m(h)o fixed at 125 GeV.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo, F. L., et al. (2021). Search for phenomena beyond the Standard Model in events with large b-jet multiplicity using the ATLAS detector at the LHC. Eur. Phys. J. C, 81(1), 11–29pp.
Abstract: A search is presented for new phenomena in events characterised by high jet multiplicity, no leptons (electrons or muons), and four or more jets originating from the fragmentation of b-quarks (b-jets). The search uses 139fb(-1)of s root = 13 TeV proton-proton collision data collected by the ATLAS experiment at the Large Hadron Collider during Run 2. The dominant Standard Model background originates from multijet production and is estimated using a data-driven technique based on an extrapolation from events with low b-jet multiplicity to the high b-jet multiplicities used in the search. No significant excess over the Standard Model expectation is observed and 95% confidence-level limits that constrain simplified models of R-parity-violating supersymmetry are determined. The exclusion limits reach 950 GeV in top-squark mass in the models considered.
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