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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., et al. (2022). Measurements of differential cross-sections in top-quark pair events with a high transverse momentum top quark and limits on beyond the Standard Model contributions to top-quark pair production with the ATLAS detector at root s=13 TeV. J. High Energy Phys., 06(6), 063–73pp.
Abstract: Cross-section measurements of top-quark pair production where the hadronically decaying top quark has transverse momentum greater than 355 GeV and the other top quark decays into l nu b are presented using 139 fb(-1) of data collected by the ATLAS experiment during proton-proton collisions at the LHC. The fiducial cross-section at root s = 13 TeV is measured to be sigma = 1.267 +/- 0.005 +/- 0.053 pb, where the uncertainties reflect the limited number of data events and the systematic uncertainties, giving a total uncertainty of 4.2%. The cross-section is measured differentially as a function of variables characterising the t (t) over bar system and additional radiation in the events. The results are compared with various Monte Carlo generators, including comparisons where the generators are reweighted to match a parton-level calculation at next-to-next-to-leading order. The reweighting improves the agreement between data and theory. The measured distribution of the top-quark transverse momentum is used to search for new physics in the context of the effective field theory framework. No significant deviation from the Standard Model is observed and limits are set on the Wilson coefficients of the dimension-six operators O-tG and O-tq((8)), where the limits on the latter are the most stringent to date.
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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. (2020). Measurement of long-range two-particle azimuthal correlations in Z-boson tagged pp collisions at root s=8 and 13 TeV. Eur. Phys. J. C, 80(1), 64–32pp.
Abstract: Results are presented from the measurement by ATLAS of long-range (|Delta eta|>2) dihadron angular correlations in root s=8 and 13 TeV pp collisions containing a Z boson. The analysis is performed using 19.4 fb-1 of root s=8 TeV data recorded during Run 1 of the LHC and 36.1 fb-1 of root s=13 TeV data recorded during Run 2. Two-particle correlation functions are measured as a function of relative azimuthal angle over the relative pseudorapidity range 2<|Delta eta|<5 for different intervals of charged-particle multiplicity and transverse momentum. The measurements are corrected for the presence of background charged particles generated by collisions that occur during one passage of two colliding proton bunches in the LHC. Contributions to the two-particle correlation functions from hard processes are removed using a template-fitting procedure. Sinusoidal modulation in the correlation functions is observed and quantified by the second Fourier coefficient of the correlation function, v2,2, which in turn is used to obtain the single-particle anisotropy coefficient v2. The v2 values in the Z-tagged events, integrated over 0.5<pT<5 GeV, are found to be independent of multiplicity and <mml:msqrt>s</mml:msqrt>, and consistent within uncertainties with previous measurements in inclusive pp collisions. As a function of charged-particle pT, the Z-tagged and inclusive v2 values are consistent within uncertainties for pT<3 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). 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(Aad, G. et al), Alvarez Piqueras, D., Aparisi Pozo, J. A., Bailey, A. J., Barranco Navarro, L., Cabrera Urban, S., et al. (2019). Search for high-mass dilepton resonances using 139 fb(-1) of pp collision data collected at root s=13 TeV with the ATLAS detector. Phys. Lett. B, 796, 68–87.
Abstract: A search for high-mass dielectron and dimuon resonances in the mass range of 250 GeV to 6 TeV is presented. The data were recorded by the ATLAS experiment in proton-proton collisions at a centre-ofmass energy of root s = 13 TeV during Run 2 of the Large Hadron Collider and correspond to an integrated luminosity of 139 fb(-1). A functional form is fitted to the dilepton invariant-mass distribution to model the contribution from background processes, and a generic signal shape is used to determine the significance of observed deviations from this background estimate. No significant deviation is observed and upper limits are placed at the 95% confidence level on the fiducial cross-section times branching ratio for various resonance width hypotheses. The derived limits are shown to be applicable to spin-0, spin-1 and spin-2 signal hypotheses. For a set of benchmark models, the limits are converted into lower limits on the resonance mass and reach 4.5 TeV for the E-6-motivated Z(psi)' boson. Also presented are limits on Heavy Vector Triplet model couplings.
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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). Measurements of Zγ plus jets differential cross sections in pp collisions at √s=13 TeV with the ATLAS detector. J. High Energy Phys., 07(7), 072–57pp.
Abstract: Differential cross-section measurements of Z gamma production in association with hadronic jets are presented, using the full 139 fb(-1) dataset of root s = 13 TeV proton-proton collisions collected by the ATLAS detector during Run 2 of the LHC. Distributions are measured using events in which the Z boson decays leptonically and the photon is usually radiated from an initial-state quark. Measurements are made in both one and two observables, including those sensitive to the hard scattering in the event and others which probe additional soft and collinear radiation. Different Standard Model predictions, from both parton-shower Monte Carlo simulation and fixed-order QCD calculations, are compared with the measurements. In general, good agreement is observed between data and predictions from MATRIX and MiNNLOPS, as well as next-to-leading-order predictions from MadGraph5_aMC@NLO and Sherpa.
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