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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). Measurement of Higgs boson decay into b-quarks in associated production with a top-quark pair in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 097–63pp.
Abstract: The associated production of a Higgs boson and a top-quark pair is measured in events characterised by the presence of one or two electrons or muons. The Higgs boson decay into a b-quark pair is used. The analysed data, corresponding to an integrated luminosity of 139 fb(-1), were collected in proton-proton collisions at the Large Hadron Collider between 2015 and 2018 at a centre-of-mass energy of A root s = 13 TeV. The measured signal strength, defined as the ratio of the measured signal yield to that predicted by the Standard Model, is 0.35(-0.34)(+0.36). This result is compatible with the Standard Model prediction and corresponds to an observed (expected) significance of 1.0 (2.7) standard deviations. The signal strength is also measured differentially in bins of the Higgs boson transverse momentum in the simplified template cross-section framework, including a bin for specially selected boosted Higgs bosons with transverse momentum above 300 GeV.
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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). Modelling and computational improvements to the simulation of single vector-boson plus jet processes for the ATLAS experiment. J. High Energy Phys., 08(8), 089–61pp.
Abstract: This paper presents updated Monte Carlo configurations used to model the production of single electroweak vector bosons (W, Z/gamma*) in association with jets in proton-proton collisions for the ATLAS experiment at the Large Hadron Collider. Improvements pertaining to the electroweak input scheme, parton-shower splitting kernels and scale-setting scheme are shown for multi-jet merged configurations accurate to next-to-leading order in the strong and electroweak couplings. The computational resources required for these set-ups are assessed, and approximations are introduced resulting in a factor three reduction of the per-event CPU time without affecting the physics modelling performance. Continuous statistical enhancement techniques are introduced by ATLAS in order to populate low cross-section regions of phase space and are shown to match or exceed the generated effective luminosity. This, together with the lower per-event CPU time, results in a 50% reduction in the required computing resources compared to a legacy set-up previously used by the ATLAS collaboration. The set-ups described in this paper will be used for future ATLAS analyses and lay the foundation for the next generation of Monte Carlo predictions for single vector-boson plus jets production.
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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). Study of B-c(+) -> J/psi D-s(+) and B-c(+)-> J/psi D-s*(+)decays in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 087–42pp.
Abstract: A study of B-c(+) -> J/psi D-s(+) and B-c(+)-> J/psi D-s*(+) decays using 139 fb(-1) of in- tegrated luminosity collected with the ATLAS detector from root s = 13 TeV pp collisions at the LHC is presented. The ratios of the branching fractions of the two decays to the branching fraction of the B-c(+) -> J/psi pi(+) decay are measured: B(B-c(+) -> J/psi D-s(+))/B(B-c(+) -> J/psi pi(+)) = 2.76 +/- 0.47 and B(B-c(+)-> J/psi D-s*(+))/B(B-c(+) -> J/psi pi(+)) = 5.33 +/- 0.96. The ratio of the branching fractions of the two decays is found to be B(B-c(+)-> J/psi D-s*(+))/B(B-c(+) -> J/psi D-s(+)) = 1.93 +/- 0.26. For the B-c(+)-> J/psi D-s*(+) decay, the transverse polarization fraction, Gamma(+/-+/-)/Gamma, is measured to be 0.70 +/- 0.11. The reported uncertainties include both the statistical and systematic components added in quadrature. The precision of the measurements exceeds that in all previous studies of these decays. These results supersede those obtained in the earlier ATLAS study of the same decays with root s = 7 and 8 TeV pp collision data. A comparison with available theoretical predictions for the measured quantities is presented.
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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 the Higgs boson inclusive and differential fiducial cross-sections in the diphoton decay channel with pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 027–96pp.
Abstract: A measurement of inclusive and differential fiducial cross-sections for the production of the Higgs boson decaying into two photons is performed using 139 fb(-1) of proton-proton collision data recorded at root s = 13 TeV by the ATLAS experiment at the Large Hadron Collider. The inclusive cross-section times branching ratio, in a fiducial region closely matching the experimental selection, is measured to be 67 +/- 6 fb, which is in agreement with the state-of-the-art Standard Model prediction of 64 +/- 4 fb. Extrapolating this result to the full phase space and correcting for the branching ratio, the total cross-section for Higgs boson production is estimated to be 58 +/- 6 pb. In addition, the cross-sections in four fiducial regions sensitive to various Higgs boson production modes and differential cross-sections as a function of either one or two of several observables are measured. All the measurements are found to be in agreement with the Standard Model predictions. The measured transverse momentum distribution of the Higgs boson is used as an indirect probe of the Yukawa coupling of the Higgs boson to the bottom and charm quarks. In addition, five differential cross-section measurements are used to constrain anomalous Higgs boson couplings to vector bosons in the Standard Model effective field theory framework.
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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). Search for invisible Higgs-boson decays in events with vector-boson fusion signatures using 139 fb(-1) of proton-proton data recorded by the ATLAS experiment. J. High Energy Phys., 08(8), 104–66pp.
Abstract: A direct search for Higgs bosons produced via vector-boson fusion and subsequently decaying into invisible particles is reported. The analysis uses 139 fb(-1) of pp collision data at a centre-of-mass energy of root s =13 TeV recorded by the ATLAS detector at the LHC. The observed numbers of events are found to be in agreement with the background expectation from Standard Model processes. For a scalar Higgs boson with a mass of 125 GeV and a Standard Model production cross section, an observed upper limit of 0.145 is placed on the branching fraction of its decay into invisible particles at 95% confidence level, with an expected limit of 0.103. These results are interpreted in the context of models where the 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. Invisible decays of additional scalar bosons with masses from 50 GeV to 2 TeV are also studied, and the derived upper limits on the cross section times branching fraction decrease with increasing mass from 1.0 pb for a scalar boson mass of 50 GeV to 0.1 pb at a mass of 2 TeV.
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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 Higgs boson production cross-sections in the H ->tau(+) tau(-) decay channel in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 08(8), 175–81pp.
Abstract: Measurements of the production cross-sections of the Standard Model (SM) Higgs boson (H) decaying into a pair of tau -leptons are presented. The measurements use data collected with the ATLAS detector from pp collisions produced at the Large Hadron Collider at a centre-of-mass energy of p root s = 13TeV, corresponding to an integrated luminosity of 139 fb-1. Leptonic ( tau -> l upsilon(l)upsilon(tau)) and hadronic ( tau -> hadrons upsilon tau) decays of the tau -lepton are considered. All measurements account for the branching ratio of H -> tau tau and are performed with a requirement |yH| < 2.5, where yH is the true Higgs boson rapidity. The cross-section of the pp -> H -> tau tau process is measured to be 2.94 +/- 0.21(stat)+ 0.37 – 0.32(syst) pb, in agreement with the SM prediction of 3.17 +/- 0.09 pb. Inclusive cross-sections are determined separately for the four dominant production modes: 2.65 +/- 0.41(stat)+ 0.91 – 0.67(syst) pb for gluon-gluon fusion, 0.197 +/- 0.028(stat)+ 0.032 – 0.026(syst) pb for vectorboson fusion, 0.115 +/- 0.058(stat)+ 0.042 – 0.040(syst) pb for vector-boson associated production, and 0.033 +/- 0.031(stat)+ 0.022 – 0.017(syst) pb for top-quark pair associated production. Measurements in exclusive regions of the phase space, using the simplified template cross-section framework, are also performed. All results are in agreement with the SM predictions.
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ATLAS Collaboration(Aad, G. et al), Aparisi Pozo, J. A., Bailey, A. J., Cabrera Urban, S., Cardillo, F., Castillo Gimenez, V., et al. (2022). Measurement of the polarisation of single top quarks and antiquarks produced in the t-channel at root s=13 TeV and bounds on the tWb dipole operator from the ATLAS experiment. J. High Energy Phys., 11(11), 040–62pp.
Abstract: A simultaneous measurement of the three components of the top-quark and top-antiquark polarisation vectors in t-channel single-top-quark production is presented. This analysis is based on data from proton-proton collisions at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb(-1), collected with the ATLAS detector at the LHC. Selected events contain exactly one isolated electron or muon, large missing transverse momentum and exactly two jets, one being b-tagged. Stringent selection requirements are applied to discriminate t-channel single-top-quark events from the background contributions. The top-quark and top-antiquark polarisation vectors are measured from the distributions of the direction cosines of the charged-lepton momentum in the top-quark rest frame. The three components of the polarisation vector for the selected top-quark event sample are P-x' = 0.01 +/- 0.18, P-y' = -0.029 +/- 0.027, P-z' = 0.91 +/- 0.10 and for the top-antiquark event sample they are P-x' = -0.02 +/- 0.20, P-y' = -0.007 +/- 0.051, P-z' = -0.79 +/- 0.16. Normalised differential cross-sections corrected to a fiducial region at the stable-particle level are presented as a function of the charged-lepton angles for top-quark and top-antiquark events inclusively and separately. These measurements are in agreement with Standard Model predictions. The angular differential cross-sections are used to derive bounds on the complex Wilson coefficient of the dimension-six O-tW operator in the framework of an effective field theory. The obtained bounds are C-tW is an element of[-0.9, 1.4] and C-itW is an element of [-0.8, 0.2], both at 95% confidence level.
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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. (2023). Search for heavy, long-lived, charged particles with large ionisation energy loss in pp collisions at root s=13 TeV using the ATLAS experiment and the full Run 2 dataset. J. High Energy Phys., 06(6), 158–60pp.
Abstract: This paper presents a search for hypothetical massive, charged, long-lived particles with the ATLAS detector at the LHC using an integrated luminosity of 139 fb(-1) of proton-proton collisions at root s = 13 TeV. These particles are expected to move significantly slower than the speed of light and should be identifiable by their high transverse momenta and anomalously large specific ionisation losses, dE/dx. Trajectories reconstructed solely by the inner tracking system and a dE/dx measurement in the pixel detector layers provide sensitivity to particles with lifetimes down to O(1) ns with a mass, measured using the Bethe-Bloch relation, ranging from 100 GeV to 3 TeV. Interpretations for pair-production of R-hadrons, charginos and staus in scenarios of supersymmetry compatible with these particles being long-lived are presented, with mass limits extending considerably beyond those from previous searches in broad ranges of lifetime.
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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). Differential t(t)over-tilde cross-section measurements using boosted top quarks in the all-hadronic final state with 139 fb(-1) of ATLAS data. J. High Energy Phys., 04(4), 080–108pp.
Abstract: Measurements of single-, double-, and triple-differential cross-sections are presented for boosted top-quark pair-production in 13 TeV proton-proton collisions recorded by the ATLAS detector at the LHC. The top quarks are observed through their hadronic decay and reconstructed as large-radius jets with the leading jet having transverse momentum (p(T)) greater than 500 GeV. The observed data are unfolded to remove detector effects. The particle-level cross-section, multiplied by the t (t) over bar branching fraction and measured in a fiducial phase space defined by requiring the leading and second-leading jets to have p(T)> 500 GeV and p(T)> 350 GeV, respectively, is 331 +/- 3(stat.) +/- 39(syst.) fb. This is approximately 20% lower than the prediction of 398(-49)(+48) fb by Powheg+Pythia 8 with next-to-leading-order (NLO) accuracy but consistent within the theoretical uncertainties. Results are also presented at the parton level, where the effects of top-quark decay, parton showering, and hadronization are removed such that they can be compared with fixed-order next-to-next-to-leading-order (NNLO) calculations. The parton-level cross-section, measured in a fiducial phase space similar to that at particle level, is 1.94 +/- 0.02(stat.) +/- 0.25(syst.) pb. This agrees with the NNLO prediction of 1.96(-0.17)(+0.02) pb. Reasonable agreement with the differential cross-sections is found for most NLO models, while the NNLO calculations are generally in better agreement with the data. The differential cross-sections are interpreted using a Standard Model effective field-theory formalism and limits are set on Wilson coefficients of several four-fermion operators.
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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. (2023). Search for light long-lived neutral particles that decay to collimated pairs of leptons or light hadrons in pp collisions at root s=13 TeV with the ATLAS detector. J. High Energy Phys., 06(6), 153–55pp.
Abstract: A search for light long-lived neutral particles with masses in the O(MeV-GeV) range is presented. The analysis targets the production of long-lived dark photons in the decay of a Higgs boson produced via gluon-gluon fusion or in association with a W boson. Events that contain displaced collimated Standard Model fermions reconstructed in the calorimeter or muon spectrometer are selected in 139 fb(-1) of p s = 13TeV pp collision data collected by the ATLAS detector at the LHC. Background estimates for contributions from Standard Model processes and instrumental effects are extracted from data. The observed event yields are consistent with the expected background. Exclusion limits are reported on the production cross-section times branching fraction as a function of the mean proper decay length c tau of the dark photon, or as a function of the dark-photon mass and kinetic mixing parameter that quantifies the coupling between the Standard Model and potential hidden (dark) sectors. A Higgs boson branching fraction above 1% is excluded at 95% CL for a Higgs boson decaying into two dark photons for dark-photon mean proper decay lengths between 10 mm and 250 mm and dark photons with masses between 0.4 GeV and 2 GeV.
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