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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). Combination of searches for invisible decays of the Higgs boson using 139 fb-1 of proton-proton collision data at root s=13 TeV collected with the ATLAS experiment. Phys. Lett. B, 842, 137963–19pp.
Abstract: Many extensions of the Standard Model predict the production of dark matter particles at the LHC. Sufficiently light dark matter particles may be produced in decays of the Higgs boson that would appear invisible to the detector. This Letter presents a statistical combination of searches for H & RARR; invisible decays where multiple production modes of the Standard Model Higgs boson are considered. These searches are performed with the ATLAS detector using 139 fb-1 of proton-proton collisions at a centre-of-mass energy of & RADIC;s = 13 TeV at the LHC. In combination with the results at & RADIC;s = 7 TeV and 8 TeV, an upper limit on the H & RARR; invisible branching ratio of 0.107 (0.077) at the 95% confidence level is observed (expected). These results are also interpreted in the context of models where the 125 GeV Higgs boson acts as a portal to dark matter, and limits are set on the scattering cross-section of weakly interacting massive particles and nucleons.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Search for flavour-changing neutral-current couplings between the top quark and the photon with the ATLAS detector at root s=13 TeV. Phys. Lett. B, 842, 137379–22pp.
Abstract: This letter documents a search for flavour-changing neutral currents (FCNCs), which are strongly sup-pressed in the Standard Model, in events with a photon and a top quark with the ATLAS detector. The analysis uses data collected in pp collisions at & RADIC;s =13 TeV during Run 2 of the LHC, corresponding to an integrated luminosity of 139 fb-1. Both FCNC top-quark production and decay are considered. The final state consists of a charged lepton, missing transverse momentum, a b-tagged jet, one high-momentum photon and possibly additional jets. A multiclass deep neural network is used to classify events either as signal in one of the two categories, FCNC production or decay, or as background. No significant ex-cess of events over the background prediction is observed and 95% CL upper limits are placed on the strength of left-and right-handed FCNC interactions. The 95% CL bounds on the branching fractions for the FCNC top-quark decays, estimated (expected) from both top-quark production and decay, are B(t & RARR; u & gamma; ) < 0.85 (0.88+0.37 -0.25) x 10-5 and B(t & RARR; c & gamma; ) < 4.2 (3.40+1.35-0.95) x 10-5 for a left-handed tq & gamma; cou-pling, and B(t & RARR; u & gamma; ) < 1.2 (1.20+0.50 -0.33) x10-5 and B(t & RARR; c & gamma; ) < 4.5 (3.70+1.47 -1.03) x10-5 for a right-handed coupling.
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ATLAS Collaboration(Aad, G. et al), Amos, K. R., Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cantero, J., et al. (2023). Constraints on the Higgs boson self-coupling from single- and double-Higgs production with the ATLAS detector using pp collisions at & RADIC;s=13 TeV. Phys. Lett. B, 843, 137745–20pp.
Abstract: Constraints on the Higgs boson self-coupling are set by combining double-Higgs boson analyses in the bb over bar bb over bar , bb over bar & tau;+& tau;- and bb over bar & gamma; & gamma; decay channels with single-Higgs boson analyses targeting the & gamma;& gamma;, Z Z*, W W *, & tau;+& tau;- and bb over bar decay channels. The data used in these analyses were recorded by the ATLAS detector at the LHC in proton-proton collisions at & RADIC;s = 13 TeV and correspond to an integrated luminosity of 126-139 fb-1. The combination of the double-Higgs analyses sets an upper limit of & mu;HH < 2.4 at 95% confidence level on the double-Higgs production cross-section normalised to its Standard Model prediction. Combining the single-Higgs and double-Higgs analyses, with the assumption that new physics affects only the Higgs boson self-coupling (& lambda;HHH), values outside the interval -0.4 < & kappa;& lambda; = (& lambda;HHH/& lambda;SM H H H ) < 6.3 are excluded at 95% confidence level. The combined single-Higgs and double-Higgs analyses provide results with fewer assumptions, by adding in the fit more coupling modifiers introduced to account for the Higgs boson interactions with the other Standard Model particles. In this relaxed scenario, the constraint becomes -1.4 < & kappa;& lambda; < 6.1 at 95% CL.
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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 polarisation of W bosons produced in top-quark decays using dilepton events at √s=13TeV with the ATLAS experiment. Phys. Lett. B, 843, 137829–22pp.
Abstract: A measurement of the polarisation of Wbosons produced in top-quark decays is presented, using proton-proton collision data at a centre-of-mass energy of v s= 13TeV. The data were collected by the ATLAS detector at the Large Hadron Collider and correspond to an integrated luminosity of 139fb(-1). The measurement is performed selecting t tevents decaying into final states with two charged leptons (electrons or muons) and at least two b-tagged jets. The polarisation is extracted from the differential cross-section distribution of the cos theta* variable, where theta(*) is the angle between the momentum direction of the charged lepton from the Wboson decay and the reversed momentum direction of the b-quark from the top-quark decay, both calculated in the Wboson rest frame. Parton-level results, corrected for the detector acceptance and resolution, are presented for the cos theta* angle. The measured fractions of longitudinal, left- and right-handed polarisation states are found to be f(0) = 0.684 +/- 0.005 (stat.)+/- 0.014 (syst.), f(L)= 0.318 +/- 0.003 (stat.)+/- 0.008 (syst.) and f(R)=-0.002 +/- 0.002 (stat.)+/- 0.014 (syst.), in agreement with the Standard Model prediction.
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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 gauge boson joint-polarisation states in W± Z production from pp collisions at √s=13 TeV with the ATLAS detector. Phys. Lett. B, 843, 137895–27pp.
Abstract: Measurements of joint-polarisation states of Wand Zgauge bosons in W +/- Z production are presented. The data set used corresponds to an integrated luminosity of 139fb(-1) of proton-proton collisions at a centre-of-mass energy of 13TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The W +/- Z candidate events are reconstructed using leptonic decay modes of the gauge bosons into electrons and muons. The simultaneous pair-production of longitudinally polarised vector bosons is measured for the first time with a significance of 7.1 standard deviations. The measured joint helicity fractions integrated over the fiducial region are f(00)= 0.067 +/- 0.010, f(0T) = 0.110 +/- 0.029, f(T0) = 0.179 +/- 0.023 and f(TT) = 0.644 +/- 0.032, in agreement with the next-to-leading-order Standard Model predictions. Individual helicity fractions of the Wand Zbosons are also measured and found to be consistent with joint helicity fractions within the expected amounts of correlation. Both the joint and individual helicity fractions are also measured separately in W+Z and W-Z events. Inclusive and differential cross sections for several kinematic observables sensitive to polarisation are presented.
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