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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). Search for the Zy decay mode of new high-mass resonances in pp collisions at √s=13 TeV with the ATLAS detector. Phys. Lett. B, 848, 138394–23pp.
Abstract: This letter presents a search for narrow, high-mass resonances in the Zy final state with the Z boson decaying into a pair of electrons or muons. The root s = 13 TeV pp collision data were recorded by the ATLAS detector at the CERN Large Hadron Collider and have an integrated luminosity of 140 fb-1. The data are found to be in agreement with the Standard Model background expectation. Upper limits are set on the resonance production cross section times the decay branching ratio into Zy. For spin-0 resonances produced via gluon-gluon fusion, the observed limits at 95% confidence level vary between 65.5 fb and 0.6 fb, while for spin-2 resonances produced via gluon-gluon fusion (or quark-antiquark initial states) limits vary between 77.4 (76.1) fb and 0.6 (0.5) fb, for the mass range from 220 GeV to 3400 GeV.
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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). Search for the decay of the Higgs boson to a Z boson and a light pseudoscalar particle decaying to two photons. Phys. Lett. B, 850, 138536–24pp.
Abstract: A search for the decay of the Higgs boson to a Z boson and a light, pseudoscalar particle, a, decaying respectively to two leptons and to two photons is reported. The search uses the full LHC Run 2 proton-proton collision data at root s = 13 TeV, corresponding to 139 fb-1 collected by the ATLAS detector. This is one of the first searches for this specific decay mode of the Higgs boson, and it probes unexplored parameter space in models with axion-like particles (ALPs) and extended scalar sectors. The mass of the a particle is assumed to be in the range 0.1-33 GeV. The data are analysed in two categories: a merged category where the photons from the a decay are reconstructed in the ATLAS calorimeter as a single cluster, and a resolved category in which two separate photons are detected. The main background processes are from Standard Model Z boson production in association with photons or jets. The data are in agreement with the background predictions, and upper limits on the branching ratio of the Higgs boson decay to Za times the branching ratio a -> yy are derived at the 95% confidence level and they range from 0.08% to 2% depending on the mass of the a particle. The results are also interpreted in the context of ALP models.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2024). Probing the CP nature of the top-Higgs Yukawa coupling in t(t)over-bar H and tH events with H → b(b)over-bar decays using the ATLAS detector at the LHC. Phys. Lett. B, 849, 138469–25pp.
Abstract: The CP properties of the coupling between the Higgs boson and the top quark are investigated using 139 fb(-1) of proton-proton collision data recorded by the ATLAS experiment at the LHC at a centre-of-mass energy of root s = 13 TeV. The CP structure of the top quark-Higgs boson Yukawa coupling is probed in events with a Higgs boson decaying into a pair of b-quarks and produced in association with either a pair of top quarks, t (t) over barH or a single top quark, tH. Events containing one or two electrons or muons are used for the measurement. Multivariate techniques are used to select regions enriched in t (t) over barh and tH events, where dedicated SP-sensitive observables are exploited. In an extension of the Standard Model (SM) with a CP-odd admixture in the top-Higgs Yukawa coupling, the mixing angle between CP-even and CP-odd couplings is measured to be alpha = 11 degrees(+52 degrees)(-73 degrees) compatible with the SM prediction corresponding to alpha = 0.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Search for nonresonant pair production of Higgs bosons in the b b-bar b b-bar final state in pp collisions at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 108(5), 052003–38pp.
Abstract: A search for nonresonant Higgs boson pair production in the b (b) over barb (b) over bar final state is presented. The analysis uses 126 fb(-1) of pp collision data at root s = 13 TeV collected with the ATLAS detector at the Large Hadron Collider, and targets both the gluon-gluon fusion and vector-boson fusion production modes. No evidence of the signal is found and the observed (expected) upper limit on the cross section for nonresonant Higgs boson pair production is determined to be 5.4 (8.1) times the Standard Model predicted cross section at 95% confidence level. Constraints are placed on modifiers to the HHH and HHVV couplings. The observed (expected) 2 sigma constraints on the HHH coupling modifier, kappa(lambda), are determined to be [-3.5, 11.3] ([-5.4, 11.4]), while the corresponding constraints for the HHVV coupling modifier, kappa(2V), are [-0.0, 2.1] ([-0.1, 2.1]). In addition, constraints on relevant coefficients are derived in the context of the Standard Model effective field theory and Higgs effective field theory, and upper limits on the HH production cross section are placed in seven Higgs effective field theory benchmark scenarios.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Measurement of the Sensitivity of Two-Particle Correlations in pp Collisions to the Presence of Hard Scatterings. Phys. Rev. Lett., 131(16), 162301–21pp.
Abstract: A key open question in the study of multiparticle production in high-energy pp collisions is the relationship between the “ridge”-i.e., the observed azimuthal correlations between particles in the underlying event that extend over all rapidities-and hard or semihard scattering processes. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event. To address this question, two-particle correlations are measured in pp collisions at collected by the ATLAS experiment at the LHC, with an integrated luminosity of 15.8 pb-1, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, v2, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge. p= 13 TeV using data ffiffi s
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Integrated and differential fiducial cross-section measurements for the vector boson fusion production of the Higgs boson in the H → WW* → eνμν decay channel at 13 TeV with the ATLAS detector. Phys. Rev. D, 108(7), 072003–52pp.
Abstract: The vector-boson production cross section for the Higgs boson decay in the H -> WW* -> e nu μnu channel is measured as a function of kinematic observables sensitive to the Higgs boson production and decay properties as well as integrated in a fiducial phase space. The analysis is performed using the proton-proton collision data collected by the ATLAS detector in Run 2 of the LHC at root s = 13 TeV center-of-mass energy, corresponding to an integrated luminosity of 139 fb(-1). The different flavor final state is studied by selecting an electron and a muon originating from a pair of W bosons and compatible with the Higgs boson decay. The data are corrected for the effects of detector inefficiency and resolution, and the measurements are compared with different state-of-the-art theoretical predictions. The differential cross sections are used to constrain anomalous interactions described by dimension-six operators in an effective field theory.
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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. (2023). Search for a new pseudoscalar decaying into a pair of muons in events with a top-quark pair at √s=13 TeV with the ATLAS detector. Phys. Rev. D, 108(9), 092007–26pp.
Abstract: A search for a new pseudoscalar a-boson produced in events with a top-quark pair, where the a-boson decays into a pair of muons, is performed using root s=13 TeV pp collision data collected with the ATLAS detector at the LHC, corresponding to an integrated luminosity of 139 fb(-1). The search targets the final state where only one top quark decays to an electron or muon, resulting in a signature with three leptons e μμand μμmu. No significant excess of events above the Standard Model expectation is observed and upper limits are set on two signal models: pp -> tta and pp -> tt with t -> H(perpendicular to)b, H-perpendicular to -> W(perpendicular to)a, where a ->mu mu, in the mass ranges 15 GeV<m(a)<72 GeV and 120 GeV <= m(H +/-)<= 160 GeV.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Bouchhar, N., Cabrera Urban, S., et al. (2023). Observation of an Excess of Dicharmonium Events in the Four-Muon Final State with the ATLAS Detector. Phys. Rev. Lett., 131(15), 151902–22pp.
Abstract: A search is made for potential ccc over bar c over bar tetraquarks decaying into a pair of charmonium states in the four ffiffi muon final state using proton-proton collision data at p s = 13 TeV, corresponding to an integrated luminosity of 140 fb-1 recorded by the ATLAS experiment at LHC. Two decay channels, J lig +J lig -4 μand J lig + ig(2S) -4 mu, are studied. Backgrounds are estimated based on a hybrid approach involving Monte Carlo simulations and data-driven methods. Statistically significant excesses with respect to backgrounds dominated by the single parton scattering are seen in the di-J lig channel consistent with a narrow resonance at 6.9 GeV and a broader structure at lower mass. A statistically significant excess is also seen in the J lig + ig(2S) channel. The fitted masses and decay widths of the structures are reported.
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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. (2023). Search for Dark Photons in Rare Z Boson Decays with the ATLAS Detector. Phys. Rev. Lett., 131(25), 251801–23pp.
Abstract: A search for events with a dark photon produced in association with a dark Higgs boson via rare decays of the standard model Z boson is presented, using 139 fb(-1) of root p 1/4 13 TeV proton-proton collision data recorded by the ATLAS detector at the Large Hadron Collider. The dark boson decays into a pair of dark photons, and at least two of the three dark photons must each decay into a pair of electrons or muons, resulting in at least two same-flavor opposite-charge lepton pairs in the final state. The data are found to be consistent with the background prediction, and upper limits are set on the dark photon's coupling to the dark Higgs boson times the kinetic mixing between the standard model photon and the dark photon, alpha(D)epsilon(2), in the dark photon mass range of [5, 40] GeV except for the gamma mass window [8.8, 11.1] GeV. This search explores new parameter space not previously excluded by other experiments.
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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. (2023). Combined Measurement of the Higgs Boson Mass from the H → γγ and H → ZZ* → 4l Decay Channels with the ATLAS Detector Using √s=7, 8, and 13 TeV pp Collision Data. Phys. Rev. Lett., 131(25), 251802–21pp.
Abstract: A measurement of the mass of the Higgs boson combining the H -> ZZ* -> 4l and H -> gamma gamma decay channels is presented. The result is based on 140 fb(-1) of proton-proton collision data collected by the ATLAS detector during LHC run 2 at a center-of-mass energy of 13 TeV combined with the run 1 ATLAS mass measurement, performed at center-of-mass energies of 7 and 8 TeV, yielding a Higgs boson mass of 125.11 +/- 0.09(stat) +/- 0.06(syst) = 125.11 +/- 0.11 GeV. This corresponds to a 0.09% precision achieved on this fundamental parameter of the Standard Model of particle physics.
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