Study of Z → llγ decays at √s = 8 TeV with the ATLAS detector

This paper presents a study of Z → llγ decays with the ATLAS detector at the Large Hadron Collider. The analysis uses a proton–proton data sample corresponding to an integrated luminosity of 20.2 fb−1 collected at a centre-ofmass energy √s = 8 TeV. Integrated fiducial cross-sections together with normalised differential fiducial cross-sections, sensitive to the kinematics of final-state QED radiation, are obtained. The results are found to be in agreement with stateof-the-art predictions for final-state QED radiation. First measurements of Z → llγ γ decays are also reported

Constraints on spin-0 dark matter mediators and invisible Higgs decays using ATLAS 13 TeV pp collision data with two top quarks and missing transverse momentum in the final state

This paper presents a statistical combination of searches targeting final states with two top quarks and invisible particles, characterised by the presence of zero, one or two leptons, at least one jet originating from a b-quark and missing transverse momentum. The analyses are searches for phenomena beyond the Standard Model consistent with the direct production of dark matter in pp collisions at the LHC, using 139 fb−1 of data collected with the ATLAS detector at a centre-of-mass energy of 13 TeV. The results are interpreted in terms of simplified dark matter models with a spin-0 scalar or pseudoscalar mediator particle. In addition, the results are interpreted in terms of upper limits on the Higgs boson invisible branching ratio, where the Higgs boson is produced according to the Standard Model in association with a pair of top quarks. For scalar (pseudoscalar) dark matter models, with all couplings set to unity, the statistical combination extends the mass range excluded by the best of the individual channels by 50 (25) GeV, excluding mediator masses up to 370 GeV. In addition, the statistical combination improves the expected coupling exclusion reach by 14% (24%), assuming a scalar (pseudoscalar) mediator mass of 10 GeV. An upper limit on the Higgs boson invisible branching ratio of 0.38 (0.30+0.13−0.09) is observed (expected) at 95% confidence level

Software performance of the ATLAS track reconstruction for LHC run 3

Charged particle reconstruction in the presence of many simultaneous proton–proton (pp) collisions in the LHC is a challenging task for the ATLAS experiment’s reconstruction software due to the combinatorial complexity. This paper describes the major changes made to adapt the software to reconstruct high-activity collisions with an average of 50 or more simultaneous pp interactions per bunch crossing (pileup) promptly using the available computing resources. The performance of the key components of the track reconstruction chain and its dependence on pile-up are evaluated, and the improvement achieved compared to the previous software version is quantified. For events with an average of 60 pp collisions per bunch crossing, the updated track reconstruction is twice as fast as the previous version, without significant reduction in reconstruction efficiency and while reducing the rate of combinatorial fake tracks by more than a factor two

Observation of four-top-quark production in the multilepton final state with the ATLAS detector

This paper presents the observation of four-top-quark (tt¯tt¯) production in proton-proton collisions at the LHC. The analysis is performed using an integrated luminosity of 140 fb−1 at a centre-of-mass energy of 13 TeV collected using the ATLAS detector. Events containing two leptons with the same electric charge or at least three leptons (electrons or muons) are selected. Event kinematics are used to separate signal from background through a multivariate discriminant, and dedicated control regions are used to constrain the dominant backgrounds. The observed (expected) significance of the measured tt¯tt¯ signal with respect to the standard model (SM) background-only hypothesis is 6.1 (4.3) standard deviations. The tt¯tt¯ production cross section is measured to be 22.5+6.6−5.5 fb, consistent with the SM prediction of 12.0±2.4 fb within 1.8 standard deviations. Data are also used to set limits on the three-top-quark production cross section, being an irreducible background not measured previously, and to constrain the top-Higgs Yukawa coupling and effective field theory operator coefficients that affect tt¯tt¯ production

Search for heavy Majorana or Dirac neutrinos and right-handed W gauge bosons in final states with charged leptons and jets in pp collisions at √s = 13 TeV with the ATLAS detector

A search for heavy right-handed Majorana or Dirac neutrinos NR and heavy right-handed gauge bosons WR is performed in events with energetic electrons or muons, with the same or opposite electric charge, and energetic jets. The search is carried out separately for topologies of clearly separated final-state products (“resolved” channel) and topologies with boosted final states with hadronic and/or leptonic products partially overlapping and reconstructed as a large-radius jet (“boosted” channel). The events are selected from pp collision data at the LHC with an integrated luminosity of 139 fb−1 collected by the ATLAS detector at √s = 13 TeV. No significant deviations from the Standard Model predictions are observed. The results are interpreted within the theoretical framework of a left-right symmetric model, and lower limits are set on masses in the heavy righthanded WR boson and NR plane. The excluded region extends to about m(WR) = 6.4 TeV for both Majorana and Dirac NR neutrinos at m(NR) < 1 TeV. NR with masses of less than 3.5 (3.6) TeV are excluded in the electron (muon) channel at m(WR) = 4.8 TeV for the Majorana neutrinos, and limits of m(NR) up to 3.6 TeV for m(WR) = 5.2 (5.0) TeV in the electron (muon) channel are set for the Dirac neutrinos. These constitute the most stringent exclusion limits to date for the model considered

Deep generative models for fast photon shower simulation in ATLAS

The need for large-scale production of highly accurate simulated event samples for the extensive physics programme of the ATLAS experiment at the Large Hadron Collider motivates the development of new simulation techniques. Building on the recent success of deep learning algorithms, variational autoencoders and generative adversarial networks are investigated for modelling the response of the central region of the ATLAS electromagnetic calorimeter to photons of various energies. The properties of synthesised showers are compared with showers from a full detector simulation using geant4. Both variational autoencoders and generative adversarial networks are capable of quickly simulating electromagnetic showers with correct total energies and stochasticity, though the modelling of some shower shape distributions requires more refinement. This feasibility study demonstrates the potential of using such algorithms for ATLAS fast calorimeter simulation in the future and shows a possible way to complement current simulation techniques

Search for doubly charged Higgs boson production in multi-lepton final states using 139 fb−1 of proton–proton collisions at s√ = 13 TeV with the ATLAS detector

A search for pair production of doubly charged Higgs bosons (H±± ), each decaying into a pair of prompt, isolated, and highly energetic leptons with the same electric charge, is presented. The search uses a proton–proton collision data sample at a centre-of-mass energy of 13 TeV corresponding to an integrated luminosity of 139 fb−1 recorded by the ATLAS detector during Run 2 of the Large Hadron Collider (LHC). This analysis focuses on same-charge leptonic decays, H±±→ℓ±ℓ′± where ℓ,ℓ′=e,μ,τ, in two-, three-, and four-lepton channels, but only considers final states which include electrons or muons. No evidence of a signal is observed. Corresponding upper limits on the production cross-section of a doubly charged Higgs boson are derived, as a function of its mass m(H±±), at 95% confidence level. Assuming that the branching ratios to each of the possible leptonic final states are equal, B(H±±→e±e±)=B(H±±→e±μ±)=B(H±±→μ±μ±)=B(H±±→e±τ±)=B(H±±→μ±τ±)=B(H±±→τ±τ±)=1/6, the observed (expected) lower limit on the mass of a doubly charged Higgs boson is 1080 GeV (1065 GeV) within the left-right symmetric type-II seesaw model, which is the strongest limit to date produced by the ATLAS Collaboration. Additionally, this paper provides the first direct test of the Zee–Babu neutrino mass model at the LHC, yielding an observed (expected) lower limit of m(H±±) = 900 GeV (880 GeV)

Mineralogical investigations of the interaction between iron corrosion products and bentonite from the NF-PRO experiments (Phase 2)

This report describes the findings of a second programme of work (Phase 2) undertaken by the British Geological Survey (BGS) on behalf of Svensk Kärnbränslehantering AB (SKB), to characterise the mineralogical alteration of samples of compacted bentonite from experiments that SKB have co-funded in a study by Serco Assurance (Culham Laboratory, Oxfordshire, United Kingdom) to investigate the interaction of iron and bentonite, within the EU Framework 6 NFPRO Project (Smart et al., 2006). Reacted bentonite residues from four NF-PRO Experiments – NFC1, NFC4, NFC7 and NFC13 were examined by BGS using; X-ray diffraction analysis (XRD); petrographical analysis with backscattered scanning electron microscopy (BSEM) with energy-dispersive X-ray microanalysis (EDXA) techniques, cation exchange capacity (CEC) and exchangeable cation analysis; and sequential chemical extraction. In addition, background chemical analysis of altered and background bentonite were also obtained by X-ray fluorescence spectrometry (XRFS). Bentonite immediately adjacent to corroding steel wires was found to have interacted with Fe released from the corroding metal. This resulted in the formation of narrow haloes of altered bentonite around the corroding steel wires, in which the clay matrix was significantly enriched in Fe. Similar observations were observed in bentonite around corroded iron coupons (observed in experiments NFC4 and NFC7 only), although the alteration zones were not as well developed in comparison to those around corroded steel wires. Detailed petrographical observation found no evidence for the formation discrete iron oxide or iron oxyhydroxide phases within the clay matrix but appeared to show that the clay particles themselves had become enriched in Fe. However, data from sequential chemical extraction suggests that a significant proportion (26 to 68 %) of the iron in the altered bentonite is present as amorphous iron oxide or crystalline iron oxides (15 to 33 % of the total iron). Some of the crystalline iron is present as primary magnetite and ilmenite present from the original MX-80 bentonite but part of this will also probably be secondary magnetite formed as a corrosion product of the steel. Nevertheless, sequential chemical extraction analyses also suggest that a large proportion of the iron (11-38 %) may be present within the silicate/clay mineral lattice. The implication of this would be that there has been significant conversion of the original montmorillonite to an Fe-rich clay mineral within these alteration haloes. Although XRD does not detect very much change in clay mineralogy, and suggests that the smectite in the altered bentonite is dioctahedral, it is likely that the subsampling for XRD analysis was on too coarse a scale to be able to resolve the alteration within these very narrow reaction zones around the corroded wires. The alteration observed around the corroded steel wires in experiments NFC4, NFC7 and NFC13 is more complex than that in NFC1 or earlier experiments studied in Phase 1 (Milodowski et al., 2007) or previously by Smart et al. (2006). The reacted bentonite from these experiments exhibited the formation of a Mg-Fe-rich clay mineral or aluminosilicate alteration product. This was formed within the Fe-enriched alteration halo but appears to have formed relatively early and was subsequently partially overprinted or replaced by more Fe-rich aluminosilicate. EDXA microchemical mapping did suggest some slight Mg enhancement in the reacted bentonite from NFC1 but no discrete Mg-rich phase was detected. Whilst Mg may potentially have been derived from the “Allard” reference water used in experiment NFC4, in the case of NFC7 and NFC13 it could only have been derived from the breakdown of the bentonite itself since the porefluid only contained NaCl in these two experiments. XRD observations indicated a slight increase in d002/d003 peak ratio, which could possibly be accounted for by a small amount of substitution of Fe into the octahedral layers of the smectite. This is not supported by exchangeable cation analyses, which show very little exchangeable Fe to be present within the altered bentonite. The cation exchange capacity (CEC) and exchangeable cation chemistry of the bentonite show very little difference in properties between reacted and background bentonite. However, it is also possible that the subsampling for exchangeable cation analysis was also on too coarse a scale to be able to resolve such changes within the fine alteration haloes. Fe released from the corroding steel was also observed to displace Ca2+ from the interlayer cation sites in the montmorillonite component. This was manifested by the marked concentration of Ca at the interface with the corroding metal and along the leading edges of ‘fronts’ of Fe diffusing into the bentonite matrix. The displaced Ca was seen to have reprecipitated as aragonite. The petrographical observations show that the bentonite within the alteration zone, that has reacted with and is enriched by Fe, has a tendency to show significantly reduced shrinkage on sample drying than the unaltered bentonite. Conversely, this would suggest that the reacted and altered clay will also have less ability to swell on hydration with water. This behaviour might be consistent with the partial conversion of the montmorillonite to an iron rich dioctahedral smectite such as nontronite. If this is the case, then this may have important implications for the longterm behaviour of bentonite seals around radioactive waste canisters made of iron or steel