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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 pair-produced vector-like top and bottom partners in events with large missing transverse momentum in pp collisions with the ATLAS detector. Eur. Phys. J. C, 83(8), 719–31pp.
Abstract: A search for pair-produced vector-like quarks using events with exactly one lepton (e or mu), at least four jets including at least one b-tagged jet, and large missing transverse momentum is presented. Data from proton-proton collisions at a centre-of-mass energy of root s =13 TeV, recorded by the ATLAS detector at the LHC from 2015 to 2018 and corresponding to an integrated luminosity of 139 fb(-1), are analysed. Vector-like partners T and B of the top and bottom quarks are considered, as is a vector-like X with charge +5/3, assuming their decay into a W, Z, or Higgs boson and a third-generation quark. No significant deviations from the Standard Model expectation are observed. Upper limits on the production cross-section of T and B quark pairs as a function of their mass are derived for various decay branching ratio scenarios. The strongest lower limits on the masses are 1.59 TeV assuming mass-degenerate vector-like quarks and branching ratios corresponding to the weak-isospin doublet model, and 1.47 TeV (1.46 TeV) for exclusive T -> Zt (B/X -> Wt) decays. In addition, lower limits on the T and B quark masses are derived for all possible branching ratios.
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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 leptonic charge asymmetry in t tbar W production in final states with three leptons at root s=13 TeV. J. High Energy Phys., 07(7), 033–54pp.
Abstract: A search for the leptonic charge asymmetry (A(c)(l)) of top-quark-antiquark pair production in association with a W boson (t (t) over barW) is presented. The search is performed using final states with exactly three charged light leptons (electrons or muons) and is based on root s = 13TeV proton-proton collision data collected with the ATLAS detector at the Large Hadron Collider at CERN during the years 2015-2018, corresponding to an integrated luminosity of 139 fb(-1). A profile-likelihood fit to the event yields in multiple regions corresponding to positive and negative differences between the pseudorapidities of the charged leptons from top-quark and top-antiquark decays is used to extract the charge asymmetry. At reconstruction level, the asymmetry is found to be -0.12 +/- 0.14 (stat.) +/- 0.05 (syst.). An unfolding procedure is applied to convert the result at reconstruction level into a charge-asymmetry value in a fiducial volume at particle level with the result of -0.11 +/- 0.17 (stat.) +/- 0.05 (syst.). The Standard Model expectations for these two observables are calculated using Monte Carlo simulations with next-to-leading-order plus parton shower precision in quantum chromodynamics and including next-to-leading-order electroweak corrections. They are -0.084(-0.003)(+0.005) (scale) +/- 0.006 (MC stat.) and -0.063(-0.004)(+0.007) (scale) +/- 0.004 (MC stat.) respectively, and in agreement with the measurements.
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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 displaced photons produced in exotic decays of the Higgs boson using 13 TeV pp collisions with the ATLAS detector. Phys. Rev. D, 108(3), 032016–33pp.
Abstract: A search is performed for delayed and nonpointing photons originating from the displaced decay of a neutral long-lived particle (LLP). The analysis uses the full run 2 dataset of proton-proton collisions delivered by the LHC at a center-of-mass energy of pffisffi 1/4 13 TeV between 2015 and 2018 and recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The capabilities of the ATLAS electromagnetic calorimeter are exploited to precisely measure the arrival times and trajectories of photons. The results are interpreted in a scenario where the LLPs are pair produced in exotic decays of the 125 GeV Higgs boson, and each LLP subsequently decays into a photon and a particle that escapes direct detection, giving rise to missing transverse momentum. No significant excess is observed above the expectation due to Standard Model background processes. The results are used to set upper limits on the branching ratio of the exotic decay of the Higgs boson. A model-independent limit is also set on the production of photons with large values of displacement and time delay.
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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). Measurements of Higgs boson production by gluon-gluon fusion and vector-boson fusion using H -> WW* -> eνμν decays in pp collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 108(3), 032005–41pp.
Abstract: Higgs boson production via gluon-gluon fusion and vector-boson fusion in proton-proton collisions is measured in the H & RARR; WW* & RARR; ev & mu;v decay channel. The Large Hadron Collider delivered proton-proton collisions at a center-of-mass energy of 13 TeV between 2015 and 2018, which were recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The total cross sections for Higgs boson production by gluon-gluon fusion and vector-boson fusion times the H & RARR; WW* branching ratio are measured to be 12.0 1 1.4 and 0.75 thorn 0.19 -0.16 pb, respectively, in agreement with the Standard Model predictions of 10.4 1 0.6 and 0.81 1 0.02 pb. Higgs boson production is further characterized through measurements of Simplified Template Cross Sections in a total of 11 kinematic fiducial regions.
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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). Strong Constraints on Jet Quenching in Centrality-Dependent p plus Pb Collisions at 5.02 TeV from ATLAS. Phys. Rev. Lett., 131(7), 072301–21pp.
Abstract: Jet quenching is the process of color-charged partons losing energy via interactions with quark-gluon plasma droplets created in heavy-ion collisions. The collective expansion of such droplets is well described by viscous hydrodynamics. Similar evidence of collectivity is consistently observed in smaller collision systems, including pp and p+ Pb collisions. In contrast, while jet quenching is observed in Pb + Pb collisions, no evidence has been found in these small systems to date, raising fundamental questions about the nature of the system created in these collisions. The ATLAS experiment at the Large Hadron Collider has measured the yield of charged hadrons correlated with reconstructed jets in 0.36 nb-1 of p+ Pb and 3.6 pb-1 of pp collisions at 5.02 TeV. The yields of charged hadrons with p(T)(ch) > 0.5 GeV near and opposite in azimuth to jets with p(t)(je) T > 30 or 60 GeV, and the ratios of these yields between p+ Pb and pp collisions, IpPb, are reported. The collision centrality of p+ Pb events is categorized by the energy deposited by forward neutrons from the struck nucleus. The IpPb values are consistent with unity within a few percent for hadrons with p(T )(ch)> 4 GeV at all centralities. These data provide new, strong constraints that preclude almost any parton energy loss in central p+ Pb collisions.
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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 Heavy Neutral Leptons in Decays of W Bosons Using a Dilepton Displaced Vertex in root s=13 TeV pp Collisions with the ATLAS Detector. Phys. Rev. Lett., 131(6), 061803–23pp.
Abstract: A search for a long-lived, heavy neutral lepton (N) in 139 fb(-1) of vs = 13 TeV pp collision data collected by the ATLAS detector at the Large Hadron Collider is reported. The N is produced via W ?N μor W -Ne and decays into two charged leptons and a neutrino, forming a displaced vertex. The N mass is used to discriminate between signal and background. No signal is observed, and limits are set on the squared mixing parameters of the N with the left-handed neutrino states for the N mass range 3 GeV < m(N) < 15 GeV. For the first time, limits are given for both single-flavor and multiflavor mixing scenarios motivated by neutrino flavor oscillation results for both the normal and inverted neutrino-mass hierarchies.
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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 Z γ γ production in pp collisions at √s=13 TeV with the ATLAS detector. Eur. Phys. J. C, 83(6), 539–30pp.
Abstract: Cross-sections for the production of a Z boson in association with two photons are measured in proton-proton collisions at a centre-of-mass energy of 13 TeV. The data used correspond to an integrated luminosity of 139 fb(-1) recorded by the ATLAS experiment during Run 2 of the LHC. The measurements use the electron and muon decay channels of the Z boson, and a fiducial phase-space region where the photons are not radiated from the leptons. The integrated Z(-> ll)gamma gamma cross-section is measured with a precision of 12% and differential cross-sections are measured as a function of six kinematic variables of the Z gamma gamma system. The data are compared with predictions from MC event generators which are accurate to up to next-to-leading order in QCD. The cross-section measurements are used to set limits on the coupling strengths of dimension-8 operators in the framework of an effective field theory.
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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). Exclusive dielectron production in ultraperipheral Pb+Pb collisions at √s_NN=5.02 TeV with ATLAS. J. High Energy Phys., 06(6), 182–42pp.
Abstract: Exclusive production of dielectron pairs, gamma gamma -> e(+) e(-), is studied using L-int = 1.72 nb(-1) of data from ultraperipheral collisions of lead nuclei at root s(NN) = 5.02TeV recorded by the ATLAS detector at the LHC. The process of interest proceeds via photon-photon interactions in the strong electromagnetic fields of relativistic lead nuclei. Dielectron production is measured in the fiducial region defined by following requirements: electron transverse momentum p(T)(e) > 2.5 GeV, absolute electron pseudorapidity |eta(e)| < 2.5, dielectron invariant mass m(ee) > 5 GeV, and dielectron transverse momentum p(T)(ee) < 2 GeV. Differential cross-sections are measured as a function of mee, average peT, absolute dielectron rapidity |y(ee)|, and scattering angle in the dielectron rest frame, | cos theta* |, in the inclusive sample, and also with a requirement of no activity in the forward direction. The total integrated fiducial cross-section is measured to be 215 +/- 1(stat.) (+23)(-20)(syst.) +/- 4(lumi.) μb. Within experimental uncertainties the measured integrated cross-section is in good agreement with the QED predictions from the Monte Carlo programs Starlight and SuperChic, confirming the broad features of the initial photon fluxes. The differential cross-sections show systematic differences from these predictions which are more pronounced at high |y(ee)| and | cos theta* | values.
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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). Searches for exclusive Higgs and Z boson decays into a vector quarkonium state and a photon using 139 fb-1 of ATLAS √s=13 TeV proton-proton collision data. Eur. Phys. J. C, 83(9), 781–33pp.
Abstract: Searches for the exclusive decays of Higgs and Z bosons into a vector quarkonium state and a photon are performed in the mu(+)mu(-) gamma final state with a proton-proton collision data sample corresponding to an integrated luminosity of 139 fb(-1) collected at root s = 13 TeV with the ATLAS detector at the CERN Large Hadron Collider. The observed data are compatible with the expected backgrounds. The 95% confidence-level upper limits on the branching fractions of the Higgs boson decays into J/psi gamma, psi(2S)gamma, and Upsilon(1S, 2S, 3S)gamma are found to be 2.0 x 10(-4), 10.5x10(-4), and (2.5, 4.2, 3.4) x10(-4), respectively, assuming Standard Model production of the Higgs boson. The corresponding 95% CL upper limits on the branching fractions of the Z boson decays are 1.2 x 10(-6), 2.4 x 10(-6), and (1.1, 1.3, 2.4) x10(-6). An observed 95% CL interval of (-133, 175) is obtained for the kappa(c)/kappa(gamma) ratio of Higgs boson coupling modifiers, and a 95% CL interval of (-37, 40) is obtained for kappa(b)/kappa(gamma).
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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 boosted diphoton resonances in the 10 to 70 GeV mass range using 138 fb-1 of 13 TeV pp collisions with the ATLAS detector. J. High Energy Phys., 07(7), 155–42pp.
Abstract: A search for diphoton resonances in the mass range between 10 and 70 GeV with the ATLAS experiment at the Large Hadron Collider (LHC) is presented. The analysis is based on pp collision data corresponding to an integrated luminosity of 138 fb(-1) at a centre-of-mass energy of 13TeV recorded from 2015 to 2018. Previous searches for diphoton resonances at the LHC have explored masses down to 65 GeV, finding no evidence of new particles. This search exploits the particular kinematics of events with pairs of closely spaced photons reconstructed in the detector, allowing examination of invariant masses down to 10 GeV. The presented strategy covers a region previously unexplored at hadron colliders because of the experimental challenges of recording low-energy photons and estimating the backgrounds. No significant excess is observed and the reported limits provide the strongest bound on promptly decaying axion-like particles coupling to gluons and photons for masses between 10 and 70 GeV.
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