ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for a right-handed gauge boson decaying into a high-momentum heavy neutrino and a charged lepton in pp collisions with the ATLAS detector at root s=13 TeV. Phys. Lett. B, 798, 134942–23pp.
Abstract: A search for a right-handed gauge boson W-R, decaying into a boosted right-handed heavy neutrino N-R, in the framework of Left-Right Symmetric Models is presented. It is based on data from proton-proton collisions with a centre-of-mass energy of 13 TeV collected by the ATLAS detector at the Large Hadron Collider during the years 2015, 2016 and 2017, corresponding to an integrated luminosity of 80 fb(-1). The search is performed separately for electrons and muons in the final state. A distinguishing feature of the search is the use of large-radius jets containing electrons. Selections based on the signal topology result in smaller background compared to the expected signal. No significant deviation from the Standard Model prediction is observed and lower limits are set in the W-R and N-R mass plane. Mass values of the W-R smaller than 3.8-5 TeV are excluded for N-R in the mass range 0.1-1.8 TeV.
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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. (2018). Search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a Z boson in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 776, 318–337.
Abstract: A search for an invisibly decaying Higgs boson or dark matter candidates produced in association with a leptonically decaying Z boson in proton-proton collisions at root s = 13 TeV is presented. This search uses 36.1 fb(-1) of data collected by the ATLAS experiment at the Large Hadron Collider. No significant deviation from the expectation of the Standard Model backgrounds is observed. Assuming the Standard Model ZH production cross-section, an observed (expected) upper limit of 67% (39%) at the 95% confidence level is set on the branching ratio of invisible decays of the Higgs boson with mass m(H) = 125 GeV. The corresponding limits on the production cross-section of the ZH process with the invisible Higgs boson decays are also presented. Furthermore, exclusion limits on the dark matter candidate and mediator masses are reported in the framework of simplified dark matter models.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2018). Search for charged Higgs bosons decaying into top and bottom quarks at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 11(11), 085–55pp.
Abstract: A search for charged Higgs bosons heavier than the top quark and decaying via H-+/- tb is presented. The data analysed corresponds to 36.1 fb(-1) of pp collisions at TeV and was recorded with the ATLAS detector at the LHC in 2015 and 2016. The production of a charged Higgs boson in association with a top quark and a bottom quark, pp tbH(+/-), is explored in the mass range from m(H)+/- = 200 to 2000 GeV using multi-jet final states with one or two electrons or muons. Events are categorised according to the multiplicity of jets and how likely these are to have originated from hadronisation of a bottom quark. Multivariate techniques are used to discriminate between signal and background events. No significant excess above the background-only hypothesis is observed and exclusion limits are derived for the production cross-section times branching ratio of a charged Higgs boson as a function of its mass, which range from 2.9 pb at m(H)+/- = 200 GeV to 0.070 pb at m(H)+/- = 2000 GeV. The results are interpreted in two benchmark scenarios of the Minimal Supersymmetric Standard Model.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo, F. L., et al. (2018). Search for dark matter in events with a hadronically decaying vector boson and missing transverse momentum in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 180–58pp.
Abstract: A search for dark matter (DM) particles produced in association with a hadronically decaying vector boson is performed using pp collision data at a centre-of-mass energy of TeV corresponding to an integrated luminosity of 36.1 fb(-1), recorded by the ATLAS detector at the Large Hadron Collider. This analysis improves on previous searches for processes with hadronic decays of W and Z bosons in association with large missing transverse momentum (mono-W/Z searches) due to the larger dataset and further optimization of the event selection and signal region definitions. In addition to the mono-W/Z search, the as yet unexplored hypothesis of a new vector boson Z produced in association with dark matter is considered (mono-Z search). No significant excess over the Standard Model prediction is observed. The results of the mono-W/Z search are interpreted in terms of limits on invisible Higgs boson decays into dark matter particles, constraints on the parameter space of the simplified vector-mediator model and generic upper limits on the visible cross sections for W/Z+DM production. The results of the mono-Z search are shown in the framework of several simplified-model scenarios involving DM production in association with the Z boson.
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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 dark matter in association with a Higgs boson decaying to two photons at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 96(11), 112004–31pp.
Abstract: A search for dark matter in association with a Higgs boson decaying to two photons is presented. This study is based on data collected with the ATLAS detector, corresponding to an integrated luminosity of 36.1 fb(-1) of proton-proton collisions at the LHC at a center-of-mass energy of 13 TeV in 2015 and 2016. No significant excess over the expected background is observed. Upper limits at 95% confidence level are set on the visible cross section for beyond the Standard Model physics processes, and the production cross section times branching fraction of the Standard Model Higgs boson decaying into two photons in association with missing transverse momentum in three different benchmark models. Limits at 95% confidence level are also set on the observed signal in two-dimensional mass planes. Additionally, the results are interpreted in terms of 90% confidence-level limits on the dark-matternucleon scattering cross section, as a function of the dark-matter particle mass, for a spin-independent scenario.
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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 dark matter in association with an energetic photon in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 02(2), 226–46pp.
Abstract: A search for dark matter is conducted in final states containing a photon and missing transverse momentum in proton-proton collisions at root s = 13 TeV. The data, collected during 2015-2018 by the ATLAS experiment at the CERN LHC, correspond to an integrated luminosity of 139 fb(-1). No deviations from the predictions of the Standard Model are observed and 95% confidence-level upper limits between 2.45 fb and 0.5 fb are set on the visible cross section for contributions from physics beyond the Standard Model, in different ranges of the missing transverse momentum. The results are interpreted as 95% confidence-level limits in models where weakly interacting dark-matter candidates are pair-produced via an s-channel axial-vector or vector mediator. Dark-matter candidates with masses up to 415 (580) GeV are excluded for axial-vector (vector) mediators, while the maximum excluded mass of the mediator is 1460 (1470) GeV. In addition, the results are expressed in terms of 95% confidence-level limits on the parameters of a model with an axion-like particle produced in association with a photon, and are used to constrain the coupling g(aZ gamma) of an axion-like particle to the electroweak gauge bosons.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for Higgs boson decays into a pair of light bosons in the bb μμfinal state in pp collision at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 790, 1–21.
Abstract: A search for decays of the Higgs boson into a pair of new spin-zero particles, H -> aa, where the a-bosons decay into a b-quark pair and a muon pair, is presented. The search uses 36.1 fb(-1) of proton-proton collision data at root s = 13 TeV recorded by the ATLAS experiment at the LHC in 2015 and 2016. No significant deviation from the Standard Model prediction is observed. Upper limits at 95% confidence level are placed on the branching ratio (sigma(H)/sigma(SM)) x B(H -> aa -> bb μmu), ranging from 1.2 x 10(-4) to 8.4 x 10(-4) in the a-boson mass range of 20-60 GeV. Model-independent limits are set on the visible production cross-section times the branching ratio to the bb μμfinal state for new physics, sigma(vis)(X) x B(X -> bb μmu), ranging from 0.1 fb to 0.73 fb for m(mu mu) between 18 and 62 GeV.
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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. (2020). Search for dijet resonances in events with an isolated charged lepton using root s=13 TeV proton-proton collision data collected by the ATLAS detector. J. High Energy Phys., 06(6), 151–42pp.
Abstract: A search for dijet resonances in events with at least one isolated charged lepton is performed using 139 fb(-1) of root s = 13 TeV proton-proton collision data recorded by the ATLAS detector at the LHC. The dijet invariant-mass (m(jj)) distribution constructed from events with at least one isolated electron or muon is searched in the region 0.22 < m(jj) < 6.3 TeV for excesses above a smoothly falling background from Standard Model processes. Triggering based on the presence of a lepton in the event reduces limitations imposed by minimum transverse momentum thresholds for triggering on jets. This approach allows smaller dijet invariant masses to be probed than in inclusive dijet searches, targeting a variety of new-physics models, for example ones in which a new state is produced in association with a leptonically decaying W or Z boson. No statistically significant deviation from the Standard Model background hypothesis is found. Limits on contributions from generic Gaussian signals with widths ranging from that determined by the detector resolution up to 15% of the resonance mass are obtained for dijet invariant masses ranging from 0.25 TeV to 6 TeV. Limits are set also in the context of several scenarios beyond the Standard Model, such as the Sequential Standard Model, a technicolor model, a charged Higgs boson model and a simplified Dark Matter model.
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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. (2020). Search for a scalar partner of the top quark in the all-hadronic t(t)over-bar plus missing transverse momentum final state at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 80(8), 737–44pp.
Abstract: A search for direct pair production of scalar partners of the top quark (top squarks or scalar third-generation up-type leptoquarks) in the all-hadronic t (t) over bar plus missing transverse momentum final state is presented. The analysis of 139 fb(-1) of root s = 13 TeV proton-proton collision data collected using the ATLAS detector at the LHC yields no significant excess over the Standard Model background expectation. To interpret the results, a supersymmetric model is used where the top squark decays via (t) over tilde -> t(*) (chi) over tilde (0)(1), with t(*) denoting an on-shell (off-shell) top quark and (chi) over tilde (0)(1) the lightest neutralino. Three specific event selections are optimised for the following scenarios. In the scenario where m((t) over tilde) > m(t) + m((chi) over tilde 10), top squark masses are excluded in the range 400-1250 GeV for (chi) over tilde (0)(1) masses below 200 GeV at 95% confidence level. In the situation where m((t) over tilde) similar to m(t) + m((chi) over tilde 10), top squarkmasses in the range 300-630 GeV are excluded, while in the case where m((t) over tilde) < m(W) + m(b) + m(<(chi)over tilde>10) (with m((t) over tilde) – m((chi) over tilde 10) >= 5 GeV), considered for the first time in an ATLAS all-hadronic search, top squark masses in the range 300-660 GeV are excluded. Limits are also set for scalar third-generation up-type leptoquarks, excluding leptoquarks with masses below 1240 GeV when considering only leptoquark decays into a top quark and a neutrino.
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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. (2018). Search for top squarks decaying to tau sleptons in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 98(3), 032008–33pp.
Abstract: A search for direct pair production of top squarks in final states with two tau leptons, b-jets, and missing transverse momentum is presented. The analysis is based on proton-proton collision data at root s = 13 TeV corresponding to an integrated luminosity of 36.1 fb(-1) recorded with the ATLAS detector at the Large Hadron Collider in 2015 and 2016. Two exclusive channels with either two hadronically decaying tau leptons or one hadronically and one leptonically decaying tau lepton are considered. No significant deviation from the Standard Model predictions is observed in the data. The analysis results are interpreted in terms of model-independent limits and used to derive exclusion limits on the masses of the top squark (t) over tilde (1) and the tau slepton (tau) over tilde (1) in a simplified model of supersymmetry with a nearly massless gravitino. In this model, masses up to m((t) over tilde (1)) = 1.16 TeV and m ((tau) over tilde (1)) = 1.00 TeV are excluded at 95% confidence level.
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