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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2023). Measurement of the top-quark mass using a leptonic invariant mass in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 019–56pp.
Abstract: A measurement of the top-quark mass (m(t)) in the t (t) over bar -> lepton+ jets channel is presented, with an experimental technique which exploits semileptonic decays of b-hadrons produced in the top-quark decay chain. The distribution of the invariant mass m(l mu) of the lepton, l (with l = e, mu), from the W-boson decay and the muon, mu, originating from the b-hadron decay is reconstructed, and a binned-template profile likelihood fit is performed to extract mt. The measurement is based on data corresponding to an integrated luminosity of 36.1 fb(-1) of root s = 13 TeV pp collisions provided by the Large Hadron Collider and recorded by the ATLAS detector. The measured value of the top-quark mass is mt = 174.41 +/- 0.39 (stat.) +/- 0.66 (syst.) +/- 0.25 (recoil) GeV, where the third uncertainty arises from changing the Pythia8 parton shower gluon-recoil scheme, used in top-quark decays, to a recently developed setup.
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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 long-lived charginos based on a disappearing-track signature in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 022–48pp.
Abstract: This paper presents a search for direct electroweak gaugino or gluino pair production with a chargino nearly mass-degenerate with a stable neutralino. It is based on an integrated luminosity of 36.1 fb(-1) of pp collisions at root s = 13 TeV collected by the ATLAS experiment at the LHC. The final state of interest is a disappearing track accompanied by at least one jet with high transverse momentum from initial-state radiation or by four jets from the gluino decay chain. The use of short track segments reconstructed from the innermost tracking layers significantly improves the sensitivity to short chargino lifetimes. The results are found to be consistent with Standard Model predictions. Exclusion limits are set at 95% confidence level on the mass of charginos and gluinos for different chargino lifetimes. For a pure wino with a lifetime of about 0.2 ns, chargino masses up to 460 GeV are excluded. For the strong production channel, gluino masses up to 1.65 TeV are excluded assuming a chargino mass of 460 GeV and lifetime of 0.2 ns.
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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). Evidence for the H -> b(b)over-bar decay with the ATLAS detector. J. High Energy Phys., 12(12), 024–71pp.
Abstract: A search for the decay of the Standard Model Higgs boson into a b (b) over bar pair when produced in association with a W or Z boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 36.1 fb(-1), were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Final states containing zero, one and two charged leptons (electrons or muons) are considered, targeting the decays Z -> vv, W -> lv and Z -> ll. For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0.90 +/- 0.18(stat.)(-0.19)(+0.21)(syst.). Assuming the Standard Model production cross-section, the results are consistent with the value of the Yukawa coupling to b-quarks in the Standard Model.
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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 heavy resonances decaying into WW in the e nu μnu final state in pp collisions at root s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 78(1), 24–34pp.
Abstract: A search for neutral heavy resonances is performed in the WW -> e nu μnu decay channel using pp collision data corresponding to an integrated luminosity of 36.1 fb(-1), collected at a centre-of-mass energy of 13 TeV by the ATLAS detector at the Large Hadron Collider. No evidence of such heavy resonances is found. In the search for production via the quark-antiquark annihilation or gluon-gluon fusion process, upper limits on sigma(X) x B(X -> WW) as a function of the resonance mass are obtained in the mass range between 200 GeV and up to 5 TeV for various benchmark models: a Higgs-like scalar in different width scenarios, a two-Higgs-doublet model, a heavy vector triplet model, and a warped extra dimensions model. In the vector-boson fusion process, constraints are also obtained on these resonances, as well as on a Higgs boson in the Georgi-Machacek model and a heavy tensor particle coupling only to gauge bosons.
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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). A search for resonances decaying into a Higgs boson and a new particle X in the XH -> qqbb final state with the ATLAS detector. Phys. Lett. B, 779, 24–45.
Abstract: A search for heavy resonances decaying into a Higgs boson (H) and a new particle (X) is reported, utilizing 36.1 fb(-1) of proton-proton collision data at root s = 13 TeV collected during 2015 and 2016 with the ATLAS detector at the CERN Large Hadron Collider. The particle Xis assumed to decay to a pair of light quarks, and the fully hadronic final state XH -> q (q) over bar 'b (b) over bar is analysed. The search considers the regime of high XH resonance masses, where the X and H bosons are both highly Lorentz-boosted and are each reconstructed using a single jet with large radius parameter. A two-dimensional phase space of XH mass versus X mass is scanned for evidence of a signal, over a range of XH resonance mass values between 1 TeV and 4 TeV, and for X particles with masses from 50 GeV to 1000 GeV. All search results are consistent with the expectations for the background due to Standard Model processes, and 95% CL upper limits are set, as a function of XH and X masses, on the production cross-section of the XH -> q (q) over bar 'b (b) over bar resonance.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., et al. (2019). Search for pair production of Higgs bosons in the b(b)over-barb(b)over-bar final state using proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 01(1), 030–49pp.
Abstract: A search for Higgs boson pair production in the bbbb final state is carried out with up to 36.1 fb(-1) of LHC proton-proton collision data collected at s=13 TeV with the ATLAS detector in 2015 and 2016. Three benchmark signals are studied: a spin-2 graviton decaying into a Higgs boson pair, a scalar resonance decaying into a Higgs boson pair, and Standard Model non-resonant Higgs boson pair production. Two analyses are carried out, each implementing a particular technique for the event reconstruction that targets Higgs bosons reconstructed as pairs of jets or single boosted jets. The resonance mass range covered is 260-3000 GeV. The analyses are statistically combined and upper limits on the production cross section of Higgs boson pairs times branching ratio to bbbb are set in each model. No significant excess is observed; the largest deviation of data over prediction is found at a mass of 280 GeV, corresponding to 2.3 standard deviations globally. The observed 95% confidence level upper limit on the non-resonant production is 13 times the Standard Model prediction.
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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 the Higgs boson produced in association with a vector boson and decaying into two spin-zero particles in the H -> aa -> 4b channel in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 10(10), 031–48pp.
Abstract: A search for exotic decays of the Higgs boson into a pair of spin-zero particles, H -> aa, where the a-boson decays into b-quarks promptly or with a mean proper lifetime c tau(a) up to 6 mm and has a mass in the range of 20-60GeV, is presented. The search is performed in events where the Higgs boson is produced in association with a W or Z boson, giving rise to a signature of one or two charged leptons (electrons or muons) and multiple jets from b-quark decays. The analysis is based on the dataset of proton-proton collisions at root s = 13TeV recorded in 2015 and 2016 by the ATLAS detector at the CERN Large Hadron Collider, corresponding to an integrated luminosity of 36: 1 fb(-1). No significant excess of events above the Standard Model background prediction is observed, and 95% confidence-level upper limits are derived for the production cross-sections for pp -> WH, ZH and their combination, times the branching ratio of the decay chain H -> aa -> 4b. For a-bosons which decay promptly, the upper limit on the combination of cross-sections for WH and ZH times the branching ratio of H -> aa -> 4b ranges from 3.0 pb for m(a) = 20 GeV to 1.3 pb for m(a) = 60 GeV, assuming that the ratio of WH to ZH cros-ssections follows the Standard Model prediction. For a-bosons with longer proper lifetimes, the most stringent limits are 1.8 pb and 0.68 pb, respectively, at c tau(a) similar to 0.4 mm.
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ATLAS and CMS Collaborations(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2018). Combination of inclusive and differential t(t)over-bar charge asymmetry measurements using ATLAS and CMS data at root S =7 and 8 TeV. J. High Energy Phys., 04(4), 033–68pp.
Abstract: This paper presents combinations of inclusive and differential measurements of the charge asymmetry (A(C)) in top quark pair (t(t)over-bar) events with a lepton+jets signature by the ATLAS and CMS Collaborations, using data from LHC proton-proton collisions at centre-of-mass energies of 7 and 8 TeV. The data correspond to integrated luminosities of about 5 and 20 fb(-1) for each experiment, respectively. The resulting combined LHC measurements of the inclusive charge asymmetry are A(C)(LHC7) = 0.005 +/- 0.007 (stat) +/- 0.006 (syst) at 7 TeV and A(C)(LHC8) = 0.0055 +/- 0.0023 (stat) +/- 0.0025 (syst) at 8 TeV. These values, as well as the combination of A(C )measurements as a function of the invariant mass of the t(t)over-bar system at 8 TeV, are consistent with the respective standard model predictions.
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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). Measurement of jet-substructure observables in top quark, W boson and light jet production in proton-proton collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 33–47pp.
Abstract: A measurement of jet substructure observables is presented using data collected in 2016 by the ATLAS experiment at the LHC with proton-proton collisions at root s= 13 TeV. Large-radius jets groomed with the trimming and soft-drop algorithms are studied. Dedicated event selections are used to study jets produced by light quarks or gluons, and hadronically decaying top quarks and W bosons. The observables measured are sensitive to substructure, and therefore are typically used for tagging large-radius jets from boosted massive particles. These include the energy correlation functions and the N-subjettiness variables. The number of subjets and the Les Houches angularity are also considered. The distributions of the substructure variables, corrected for detector effects, are compared to the predictions of various Monte Carlo event generators. They are also compared between the large-radius jets originating from light quarks or gluons, and hadronically decaying top quarks and W bosons.
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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. (2021). Measurements of top-quark pair single- and double-differential cross-sections in the all-hadronic channel in pp collisions at root s=13 TeV using the ATLAS detector. J. High Energy Phys., 01(1), 033–76pp.
Abstract: Differential cross-sections are measured for top-quark pair production in the all-hadronic decay mode, using proton-proton collision events collected by the ATLAS experiment in which all six decay jets are separately resolved. Absolute and normalised single- and double-differential cross-sections are measured at particle and parton level as a function of various kinematic variables. Emphasis is placed on well-measured observables in fully reconstructed final states, as well as on the study of correlations between the top-quark pair system and additional jet radiation identified in the event. The study is performed using data from proton-proton collisions at root s = 13 TeV collected by the ATLAS detector at CERN's Large Hadron Collider in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb(-1). The rapidities of the individual top quarks and of the top-quark pair are well modelled by several independent event generators. Significant mismodelling is observed in the transverse momenta of the leading three jet emissions, while the leading top-quark transverse momentum and top-quark pair transverse momentum are both found to be incompatible with several theoretical predictions.
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